Frequently Asked Questions
As a Kent boiler installer, we get asked a wide range of questions. Read our comprehensive guide of our most frequently asked questions by customers.
Now, householders are looking more closely at their energy use, and the Government has brought out Building Regulations (gas fired in 2005, oil fired in 2007) making the use of high efficiency boilers mandatory replacements in domestic use.
In order to achieve the level of efficiency mandated by the Government, it is necessary for the new boilers to partially condense the fuel. Water vapour (always present in burning fuel) is encouraged to condense against the receptacle (called a heat exchanger) containing your central heating water. An extra 10-15% of energy can be extracted that would otherwise be wasted in a normal boiler. Because of the water vapour, boiler outside flues emit a ‘steam like’ plume, and the new boilers require a permanent drain attached to them. These factors can occasionally make replacement of normal boilers difficult in a few locations.
We often get asked this by householders who have got a little confused, perhaps it is because they do not fit boilers every day.
It’s best to think of ‘High efficiency boilers’ as a better description of ‘condensing boilers’. Therefore the various types of boiler have not changed, you can still get a;
- Conventional boiler (just heats water, you need a separate hot water cylinder, pump, etc)
- System boiler (as per conventional but has a pump built in and a few other bits)
- Combination boiler (heats hot water as you use it, one box solution, no cylinder etc
Nowadays, they all have to be a High Efficiency version to meet Building Regulations. Therefore you can get ‘condensing’ derivates of all the above.
The Government have made some exceptions to the rule which are listed on their web site; however, very, very few houses will be exempt.
Any bona fide installer will refuse to fit a central heating system that does not comply with Building Regulations, because of the penalties for non compliance. If you convince someone to fit a non-condensing boiler without meeting the exception criteria, they will be operating outside the law and you will have a job selling your house, when the time comes.
In general, condensing boilers are made of better materials than their forebears, but like any product ,it depends which one you buy.
I've heard about an innovative new combination boiler that doesn't need a diverter valve, secondary heat exchanger etc and has far less moving parts. Sounds good to me!
The boiler you refer to does make great claims for the perceived advantages of not requiring these components. Time will tell whether they are right.
However time also tells us that similar designs (using what the trade calls a bithermic heat exchanger) were launched in the last 20yrs by Potterton, Worcester-Bosch and Ferroli. None of these manufacturers repeated the design in subsequent models and have reverted to using diverter valves. History repeating itself again!
I've heard about a clever box that I can install on my heating system, and it will reduce my heating bills by X%
The cheekiest one of these at the moment is a little white water header box; it was designed in Ireland around 10yrs ago, cost around £600 and didn’t sell very well. It was a good product though. A UK company has now renamed it and is trying to sell them for around £6000, including installation! They typically cold call you, make various claims and then send a salesman round. In the heating trade, these are known as mixing headers and have been around for around 100yrs.
There are also various timers that are sold with great claims of savings, typically these are designed to limit the amount of times a boiler will cycle on and off in an hour.
I guess you have to be careful. Generally, modern condensing boilers for domestic applications are very sophisticated, and most of the clever controls like weather compensation are best when you use the manufacturer’s tested version.
The best savings are normally obtained by not heating rooms in the house up to an occupational comfort level when you are not in them. if there are two people living in a house with 15 rooms, there are always going to be 13 rooms empty. A clever heating control system (eg: Evohome) will heat the rooms at the times of the day you are likely to be using them. This is the best way to save energy; not on little white boxes full of water. Can’t say too much, don’t want to be sued.
We’ve all been forced to pay for additional courses in energy efficiency.
We now have to notify the Government every time we fit a gas or oil appliance, at our cost. And we have to coerce customers into fitting products which meet the new regulations.
Given the choice, Hot Water & Central Heating would have left things just as they were, and left it to you to decide whether you wanted to be energy efficient based upon the ‘whole life’ arguments.
This is normally due to air – or more exactly hydrogen, which is released by corrosion occurring within the system. This can usually be cured by bleeding the radiator using a key. Have a cloth ready to catch the drips; the water is usually dirty.
This can be more of a challenge. This can occur on older systems and is often due to a build up of sludge (called magnetite) in the bottom of the radiator. Excessive magnetite deposits in a central heating system indicate that the water has not been treated with a corrosion inhibitor over the life of the installation, or that fresh water is being introduced, or large quantities of air are being sucked in.
Other indications of excessive magnetite are noisy pumps, sticking valves, poor warm up times, intermittent radiator warm up and boiler overheating.
Although it is often possible to effect an improvement by adding a chemical ‘central heating restorer’ to the system, or disconnecting the radiator and flushing it out outside with a high pressure hose, ultimately the most permanent remedy is Powerflushing.
This can be caused by an airlock. Try turning all the other radiators in the house off (using the radiator valves, noting the number of turns each time) and then see if the errant radiator works. Reset the valves to their original positions when you have finished (see below regarding ‘balancing’). Alternatively, if you have bought cheap thermostatic radiator valves from a DIY shed, or have a system suffering from sludge, the internal pins in the TRVs regulating the water flow may have become stuck down in the closed position. Remove the TRV head (not the whole valve) by unscrewing it, and pull the pin gently upwards with a pair of pliers.
Note that this tends to be a temporary fix.
Lastly, sometimes central heating systems suffer from a ‘blocked cold feed’. This will gradually cause the system to run out of water, because it isolates the water feed and expansion tank from the radiator network. Repairing a blocked cold feed is not an easy DIY proposition.
This can be achieved by adjusting the lockshield valve (usually covered with a plastic cap) to apportion flow across all the radiators in the property. Professionals usually set these up by checking the temperature differential across the inlet/outlet pipes; however a more basic balance can be achieved by impeding the flow of those radiators nearest the heat source (the boiler) and opening the valves to a greater degree on those radiators furthest away.
If your boiler is a ‘heat only’ type with the zone valves in the airing cupboard, then the airing cupboard should be viewed as the ‘start’ of the system in terms of turning down the nearest rads.
This is usually down to a failed motorized valve (on a conventional cylinder/boiler system).
This is usually a fault within the diverter valve mechanism; this is not a DIY job.
This is down to a number of variables including flow rate and system design. If the room takes longer ‘than normal’ then the pump could be faulty, or there could be an impediment in the system, for instance a stuck radiator valve or sludge.
Copper water cylinders have a finite lifespan; if the water is leaking from the bottom of the cylinder it is prudent to take immediate steps to replace it, as the situation could rapidly get worse.
Instructions for draining the cylinder are outside the scope of this section. If the water is leaking from a pipe union to the cylinder, this can normally be fixed by remaking the joint. However, your plumber will charge you less if you have already drained the system down before his (or her) arrival.
On most modern boilers, the pilot comes on automatically when the boiler detects a ‘call for heat’ from a room thermostat or hot water thermostat’. This is because in the strive for gas efficiency it is now considered wasteful to have a pilot light burning 24 hrs a day. If one of these types of boiler doesn’t light you need to call out a registered ‘Gas Safe Registered’ operative.
Some older types of boiler have a ‘permanent pilot’ light. In normal situations this will stay burning 24 hrs a day, 365 days a year. It may go out for a number of reasons, such as an interruption in the supply (someone turned it off?), a draught, or a fault with the boiler. Before calling professional help, it is usually worth attempting to re-light the pilot yourself, following the instructions with the boiler. If the manual has been lost, the manufacturer usually prints lighting instructions on the inside of the door. If the pilot goes out again soon after, professional attention is required.
t is prudent, for safety and efficiency, to have the boiler serviced annually. If your boiler has an extended warranty, the manufacturer may ask for proof of annual servicing before a breakdown callout is arranged.
Most Gas Safe Registered technicians have personal experience of the dangers of not looking after gas burning appliances. We have come across a number of instances where faults have laid undetected due to lack of maintenance and have resulted in carbon monoxide poisoning of the house occupants. In severe cases lives could be at risk. Drowsiness, nausea, headaches are all symptoms.
If you live in rented accommodation, the Landlord is legally obliged to have a safety inspection carried out annually. You can ask to see the inspection certificate at any time.
If you are a Landlord, please bear in mind that most contractors performing ‘CP12 inspections’ or ‘safety checks’ do not check the internals of the combustion chamber or clean the unit internally.
An annual CP12 is not a form of maintenance for your boiler. The best analogy is a car; no-one would expect their car to run properly by purely submitting it for the annual MOT.
For conventional boilers, during the winter the boiler thermostat should be set higher than the summer. It’s best not to run the boiler indefinitely on the highest setting – unless you have a ‘thermal store’ (see below).
Turn off the gas supply at the meter. Open windows but do not turn any electrical items on or off.
Call the emergency gas leak helpline on 0800 111 999 in the first instance.
If the leak is on your pipework/equipment (the customer side of the meter) the gas supplier will normally turn off the supply and advise you to contact a Gas Safe Register (formerly called CORGI) authorised company (such as Hot Water & Central Heating Ltd) to expedite repairs.
This is a calculation based upon a number of variables. Room size, height, and the effectiveness of insulation needs to be accounted for. Where there are limited positions available to site a radiator in a larger room, consider a ‘double’ convector unit rather than the usual single type. In kitchens where space is limited, consider an underplinth fan assisted unit, such as a Smiths Spacesaver.
The use of Thermostatic Radiator Valves prevents individual rooms being ‘overheated’. Without TRVs the only temperature monitoring and control is done by a room thermostat; this covers the whole house rather than an individual room. Some rooms tend to warm up quicker than others, therefore a TRV can maintain comfort whilst potentially aiding economy.
The building regulations strongly advise that on significant upgrades, TRVs should be fitted to sleeping areas.
If you want to accurately control the heating in each room look at the ‘Honeywell Evohome’ colour product on our website.
NB: Cheaper TRVs can get stuck over time, so it is worth buying a premium brand (eg: Drayton, Pegler or Honeywell) in preference to DIY warehouse cheapo stuff with names you do not recognise.
Flushing is where an existing sealed radiator system is flushed through with water. Some installers suggest this will be beneficial before installing a replacement boiler, however these are normally fitters aiming for the shortest installation times and maximum profits.
This practice is not recommended by boiler manufacturers because it does not remove the damaging sludge and scale deposits. Flushing a system with water is usually only carried out where soldering has recently taken place and flux and other deposits need to be removed.
PowerFlushing describes the use of a high power pump and flushing machine used in conjunction with chemicals and heat, to dislodge and dissolve accumulated sludge and scale debris. This can rejuvenate a partially blocked system, giving faster warm up times, warmer radiators, greater efficiency and prevent the untimely demise of the boiler, pump and motorized valves. Boiler manufacturers all advocate that a system is cleaned to British Standards prior to the connection of a new boiler. Powerflushing is the only way to comply.
Modern, higher efficiency boilers are far more susceptible to water borne contamination than the older, cast iron heat exchanger types. We have seen new combination boilers virtually ruined after less than a year in service because of sludge. Our advice is to have an old system Powerflushed before connecting a new boiler.
Finally, when the Powerflush is being carried out, please bear in mind that to carry out the job properly, the machine should be connected for a minimum of 4-5 hours. The operative will need to chase around the house opening one radiator at a time, or the Powerflush will be ineffective.
Since a Powerflush machine with accessories is around £1000 to buy, some central heating installers are resisting spending a sum which may be more than their van is worth. By the time the negative effects of their advice have become apparent to the householder (eg: when the boiler heat exchangers fail) your installation cheque would long have been cashed.
Some radiator systems are plumbed with microbore (eg: 10mm or 8mm pipe). It was very popular on new builds from the 70s, because the pipe was cheap and could be installed by unskilled persons. Unfortunately, a Powerflush machine does not work properly with microbore due to the flow impediment from the narrow bore pipe……..
You must have all gas appliances inspected and certificated once per year.
You (or a letting agent contracted to you) and your tenant must each hold a copy of the certificate. It would be prudent to have the boiler serviced periodically, too.
A gas safety inspection is not a form of maintenance. No warranty is given or implied regarding ongoing reliability of a gas appliance following a safety inspection – just like a car MOT.
I’m a Landlord, do I need to check my electrical appliances annually too?
At the time of writing there is no mandatory requirement of this nature. However, from a due diligence perspective it is advisable. In the worst case scenario, a Landlord would have no legal defence in Court if a tenant was electrocuted using an appliance on the inventory and no safety testing regime could be proved to have been in operation.
A number of Letting Agents are introducing Gas and Electrical appliance annual inspections as part of their policy.
An unvented system is a term commonly used to refer to a hot water system which is sealed from the atmosphere.
In most normal stored hot water systems there is a storage tank linked to the cylinder. This storage tank provides gravity pressure to the hot water outlets (eg: taps), but also allows for temperature-based expansion of the water in the cylinder. As water cannot be compressed, when quantities of HW are stored there has to be a method of allowing expansion. An unvented system does not have a header tank. Hot water is stored at mains pressure within a sealed cylinder built to withstand higher pressures than a conventional gravity cylinder. Expansion of the water is accommodated by a gas trapped in a membrane, typically Nitrogen. The cylinder is designed in conjunction with the compressible gas membrane so that under normal circumstances, the additional volume required as the stored water is heated up can be safely taken up by the gas being compressed.
In a sealed hot water system there have to be safety systems to prevent excess temperatures and pressures being reached; because of the risks involved such systems must only be installed and maintained by suitably qualified personnel (such as Hot Water & Central Heating). Suppliers of unvented products include OSO, Vaillant, Ariston, Heatrae-Sadia (also branded Santon and Main), Kingspan Group, AO Smith.
In common with most installers, no.
There are a plethora of reasons but these include missing parts and the subsequent delays, faulty and damaged units, dubious warranty and a legal requirement upon us to fit only UK approved products.
You can get as many as you like, but bear in mind every quote uses a significant amount of diesel and usually around 3hrs of someone’s time.
2 or 3 is normally sufficient to get an idea who’s who and what’s what. If you have obtained 5 quotes and are still not happy, there is something wrong.
You could, sometimes that is the best option.
But remember, if you compare prices on a best seller book, you will get the same book wherever you buy it from. Because it is just a product.
When you have a new heating system, we add our workmanship into the mix, and smaller components you won’t be aware of. The cheapest quotes often turn into the jobs with messy pipework, cheap components, insufficient system cleansing, and undersized gas pipes.
Similarly, a painter and decorator might put in the cheapest price, but if all your skirtings aren’t rubbed down and the cracks filled, it won’t matter if he’s using the same paint brand as the next more expensive tradesman, the end job will still be rubbish.
Warren Buffett said; “Price is what you pay – value is what you get”.
If you still have any questions or require more information on any of our services, including oil boiler installation for Oxted and Tonbridge areas (other localities considered) then please do get in touch on 01732 866 282 and we’ll be happy to assist.
This very much depends on the type of boiler chosen, and the quality of the system it is connected to. It is usually worth specifying the best boiler you can afford, they are not all the same inside. Be wary of your contractor trying to earn a free holiday in Spain by recommending and fitting budget boilers.
We recommend Vaillant, Viessmann, and Worcester Bosch domestic gas products (and have received no free holidays to date).
Worcester-Bosch customer service is superb, because they use all their own staff rather than 3rd parties. It pays to look at the detail!
We also carry out commercial gas and oil fired boiler replacements in larger properties and churches etc. For these we recommend Bosch, Viessmann commercial and Archie Kidd Thermal.
Combination boilers offer instantaneous domestic hot water as well as central heating, and as such are very convenient. However, their design is more complex than conventional boilers and the lifespan is typically 10-12 years.
Conventional cast iron boilers have a longer life, 20 years is not uncommon. However, basic cast iron boilers are becoming a thing of the past due to their poor efficiency performance.
Yes, if the system is run occasionally it will prolong the life of certain components, including the pump. Some boilers automatically run the pump periodically during periods of inactivity to help extend their life and prevent seizure.
A pressurized system uses an expansion vessel, containing a diaphragm and a compressible gas (typically air or nitrogen). The expansion in the water as it is heated is accommodated by the nitrogen being compressed. The main benefits are the lack of a requirement for a ‘filling and expansion’ tank in the loft, and improved efficiency.
An added benefit with a sealed radiator system is that a leak from a radiator while you are away from the property can only lose a finite amount of water. A conventional radiator system fed from a tank will continue to refill indefinitely.
Good question. It’s usually found on an internal wall, often under the sink. However some properties, typically conversions to multiple occupation, do not seem to have one! Even though they should, of course. If you can’t find an internal stopcock, an external stopcock can usually be found under a small square iron cover in the pavement. This can be very stiff – and is usually covered in mud.
A special tool on a long handle can be bought from plumbers merchants which has been designed for the task.
If you prefer baths to showers, and water the garden regularly, a water meter may prove to be the more expensive option. Remember, the water board do not have any particular incentive to reduce your bill so these meters are usually for their benefit.
It’s best to paint radiators in ‘radiator enamel’ which is less likely to discolour over time with heat. Most new radiators already come with a gloss white finish. Painting will have very little effect on performance, as a significant amount of heat in a modern radiator is by convection rather than radiation.
Yes you can, but bare/new copper should be first lightly abraded with emery cloth or wire wool, and then wiped with white spirit to remove any preservative/grease.
It will reduce them.
It can mean that the bearings are badly worn, there is an obstruction in the flow of water (sludge commonly), or there is no water in the system.
No, we believe that the parts market is adequately addressed by the retail and wholesale specialists.
The feeling of comfort and warmth, with a friendly persona.
This depends what type of cylinder you have. You can upgrade to one of the latest ‘fast recovery cylinders’ which will (boiler permitting) get a whole cylinder of cold water reheated in around 18 minutes. This is a good compromise when you need a larger cylinder but have no room!
The new ACV ‘tank in tank’ cylinders have a very fast reheat time.
Is it more expensive to run an electric immersion heater relative to using a gas boiler to heat my water?
Yes, in almost all cases.
Although if you have an oil boiler and an Economy 7 electricity meter, it may be cheaper running it overnight on the immersion than heating with oil. Those of you with gas will probably be unaware that 1M UK households heating with oil have seen their bills rise by 300% in 3 years……
If you have a Combination Boiler there is usually a hot water temperature setting. If the water is scalding there is a fault.
For normal systems comprising a hot water cylinder there is normally a ‘cylinder stat’ mounted on the side, which should be preset to 55-65 degrees. If there is no cylinder stat fitted you have an older system and the hot water temperature will be governed by the thermostat in the boiler.
Lastly, if you heat your water by immersion heater and the water temperature is far too hot, there is a high likelihood that the internal electric thermostat is stuck in the ‘on’ position. If you are competent with electricity then this is an easy job to change without the risk of getting wet. Far more serious consequences could result from a boiling hot water cylinder if your tank is plastic. So change a faulty thermostat immediately!
NB: New building regulations in 2004 stipulated that replacement immersion heaters are equipped with a resettable ‘overheat’ thermostat. This is a valuable safety feature, but has since shown to be liable to nuisance tripping.
On a Combination Boiler this is usually due to sludge and scale build up in the DHW heat exchanger. This is not a DIY job to rectify, and can be indicative of more far reaching corrosion problems in the system. Chances are your system was not Powerflushed when the boiler was put in.
In a normal, cylinder fed system, the ‘cylinder stat’ could be set too low. Note that during winter months, the boiler thermostat will need to be set nearer the maximum than in summer, due to the extra demands made by the central heating.
Sometimes this is due to system pipework design causing an airlock. A common fault is the pipework is not vented correctly where the hot water outlet comes off the top of the cylinder. Air bubbles rising from the cylinder get trapped at the top outlet causing an airlock. Once the airlock has been removed (maybe by opening a bath tap) the system will work until it has had a dormant time for the air bubbles to achieve an ‘airlock’, usually the following morning. Poor pipework layout within the property can also cause this problem.
What are the differences between ‘combination’ and ‘conventional’ boilers?
A combination boiler is where the boiler handles central heating and instantaneous hot water requirements without needing external tanks or cylinders. Just add radiators.
Conventional boilers are merely a gas/water heat exchanger, the water from which is then routed using external controls to radiators and hot water heat exchangers (eg: a cylinder).
For a more detailed explanation, see our boiler installation page.
Warm air systems are amongst the most efficient forms of space heating.
If you warm air system has developed a fault, many parts are still available. Intrinsically warm air is very reliable, but the old open flue systems can fail in a dangerous way and should be inspected every year by a competent person.
Significant improvements have been made to warm air heater design, with varispeed fans and modulating gas burners. The sole domestic UK manufacturer is Johnson & Starley. At the time of writing, Johnson & Starley had just launched a complete new WarmCair high efficiency warm air heater range.
We fit Lennox high efficiency condensing gas warm air systems, offering an efficiency of up to 95%. This product, made in the USA, in our view takes warm air to the next level, and is ahead of radiator systems in terms of efficiency. Lennox units can replace most warm air heaters of 50,000 BTu output upwards.
We also offer the Multicalor air handling units, these allow us to provide radiators, or underfloor heating in conjunction with a warm air solution.
This is a rating which tells the consumer how much of their gas is turned into useful heat output by the boiler (working in ideal conditions). This is normally stated in the manufacturers brochure.
The SEDBUK rating is a reasonable benchmark, but it does not take into account a number of other factors such as ‘standing losses’ – which is heat loss when the boiler is cycling on and off. Therefore, not all SEDBUK A boilers are equals in real world situations.
Conventional boilers emit hot exhaust gases from the flue when they are operating. Some atmospheric (open flue) boilers are incredibly inefficient, with only just over half the gas burnt being put to practical use. The rest of the heat is vented outside.
A condensing boiler transfers more of this otherwise wasted heat to the water in the boiler, before it reaches the flue outlet.
An excellent idea, which saves on fuel use and therefore operating costs. During normal operation, condensing boilers lower the temperature of their exhaust gases below 100 degrees, and plumes of water vapour can be seen coming from the flue outside. Compared to older less efficient boilers, the flue gases are more dense, meaning they do not rise skywards as quickly.
For this reason condensing boiler flues should be sited sensitively.
Whilst there are gas consumption savings to be made from a condensing boiler, their actual efficiency is dependent upon the system design of the heating circuit.
This is very dependent on the type of house. However, in order to minimize disruption, HWCH always endeavour to put more manpower on larger jobs so that the customer inconvenience is kept to a minimum.
This varies enormously between manufacturers. Generally speaking, the older the boiler the more difficult spares are to obtain.
The Gas Safe Registration implies that at least one of the operatives of a company/partnership has achieved the technical proficiency in order to handle gas safely.
Many companies (particularly in the building industry) display a Gas Safe logo but do not have any permanent Gas Safe qualified staff, reying on subcontractors or private installers working for them. These organisations are not following the Gas Safe guidelines and may be acting fraudulently. If vehicles/paperwork/websites do not list a Gas Safe registration number, you should be suspicious. You may be surprised how many companies display a Gas Safe Register logo on the web without a registration number. Ours is 196826.
It is illegal for a non-registered individual to work on any gas appliance, other than in their own residence.
A company’s Gas Safe registration does not guarantee fair trading or value for money, Gas Safe Register’s remit is promoting and maintaining gas safety and standards. The size of your repair bill is not a Gas Safe Register issue.
This is where a large quantity of preheated water is kept in a highly insulated container. This water is then used to indirectly (eg: by heat exchange) warm up cold water, on demand. If you know how an ordinary hot water cylinder works, the best way to describe a heat store is ‘its a HW cylinder working in reverse’. The water in the cylinder is boiler fed and kept hot 24 x7; cold, usually mains fed water is passed up the primary coil and emerges at 55 degrees the other end.
Because it’s mains pressure, no shower pumps etc are required. If you have a larger house one of these offers the convenience of a combi but the reliability and flow rate of a tank fed system. Disadvantages are space and cost. DPS, RCM, Albion and Gledhill are the best known suppliers.
Unfortunately, heatstores were mainly fitted in new build property and as we all know, this market collapsed in the recent recession. The part of Gledhill that made heatstores went into administration in November 2008. DPS went into administration in 2010.
ACV have an interesting thermal store/combi hybrid called the Heatmaster TC35. We have installed some of these units which can deliver a water flow of up to 41 litres per minute.
Unvented systems rely absolutely on the external water main pressure and flow rate to provide the water impetus.
A surprising number of installations do not deliver a good performance because they are either connected to a small diameter supply pipe from the road or the supplier water main has insufficient flow for the application. These matters should have been noted by your specifier prior to installation.
If you own unvented system does not deliver a satisfactory performance, the good news is that we have the answer. See the water mains boosting section.
I want an unvented hot water system in my house but my plumber has told me there is insufficient flow rate from the water board.
We can almost certainly enable you to have what you want. See water mains boosting section.
‘Pumping over’ is where water is forced up the ‘open vent’ into the Feed & Expansion tank. The tank is hot, steam/condensation is produced in the loft, sometimes the overflow will run.
This is potentially very damaging to the boiler and radiators and should be addressed in short order.
Causes can be;
- The circulator pump is incorrectly sited
- The pump is oversized
- There is a restriction in the system
- The pipes connecting the feed and expansion tank do not connect to the heating system in the right place
A positive head shower pump is used where water would normally flow from the shower head without assistance from a gravity fed system. The pump senses the flow of water and automatically starts when the shower is used, to augment the flow.
In some situations the water tank is below the level of the shower outlet. When the shower is turned on no water would therefore flow because there is no gravitational incentive. This is where the negative pressure pump is required; using diaphragms an air pressure difference is sensed when the tap is opened and the pump is switched on. Because of the extra components required, negative pressure pumps are more expensive.
Hot Water & Central Heating do not normally recommend the fitment of a shower pump unless there are no alternatives, because they tend to be noisy and vibrate excessively.