What are Heat Pumps?
A Heat Pump takes a warm source of energy, and moves it to a cooler area (your house).
Using the Second Law of Thermodynamics, the movement of the heat in conjunction with a compressor pump can liberate some energy performance improvement. The performance improvement is called Coefficient of Performance, CoP for short.
Thus, it is possible to take 1Kw of electrical energy, and turn it into a useful 3Kw of heat energy – using a low temperature heat source to start off with – this would be a CoP of 3.
The low temperature heat source could be;
- your garden, using a network of pipes buried under one metre of earth
- a vertical borehole, maybe 100 metres deep (depending on water table)
- the bottom of a watersource, for instance a lake
- latent air around your property
The first three examples involve taking low level heat from the ground and involve significant earthworks; the last version uses a simple exterior fan module. These are expected to be the primary growth area in renewable energy, because they do not need a significant sized garden or an expensive borehole.
Similarly, air source heat pumps blow colder air out, having extracted the useful heat.
Coefficient of Performance:
Heat Pump manufacturers quote CoP figures, unsurprisingly most claim theirs to be better than the competition. CoP is ratio of electrical energy used, to useful heat output. So the higher the better?
What is not always made clear is that Heat Pumps are not technically suited to high temperature outputs.
With your gas boiler you are used to 60°C hot water and 75ºC radiators, just by pressing a button. Heat Pumps work most usefully in the region of 35º-45°C output, and it is around this temperature that the CoP figures are usually based on.
But your existing radiators aren’t going to be warm enough at 45ºC………..the house will be very cold in the height of winter. Sure, you can turn the HeatPump temperature up, but the CoP drops alarmingly until you get to 1. In other words, it turns into an expensive electrical heater.
With water based heatpumps it can be assumed that the ground will keep a reasonably constant temperature year round. Air source heat pumps are able to extract less heat from the air in the coldest conditions – this causes their CoP to lower.
In real terms, the weather drops below freezing comparatively rarely, and therefore the resultingloss of efficiency does not significantly impact on the year’s running cost.
Reading between the lines:
It’s common for manufacturers of heat pumps to make a number of claims that you will find attractive, such as;
“our 2012 model offers efficiencies of up to 450% due to the super inverter design”
“our latest heatpump uses XYZ gas to enable high temperature working up to 70C”
When you read these, both of which may be INDIVIDUALLY true, remember that they can’t be true AT THE SAME TIME.
Physics means that the vapour compression cycle can’t deliver high efficiency AND high temperatures in water. One or the other are possible.
So if you fit a heat pump in an older house with poor insulation values without underfloor heating, you WILL be disappointed. Many companies that made a fortune with Solar Electric PV are now trying to diversify into heat pumps. They don’t understand heating yet, but they need to survive.
Heat pumps aren’t the answer to every problem, they have been around for over 100 years.
How do I keep my CoP high?
1. Have a house heating system that uses low temperature heat emitters. Underfloor heating in other words.
2. The second rule with a Heat Pump is insulation. Scandanavians insulate their houses so that they are almost airtight, with triple glazed windows, draught seals etc. It is so cold for most of the year that if their heat pump dies, they die! If you have a poorly insulated house, a heat pump is unlikely be capital cost effective. Most of the UK housing stock bears no relation to the highly insulated homes in parts of northern Europe.
So if you are sitting in a older house with radiators and relatively poor insulation you may want to stop here and look at a condensing boiler instead. If you use Kerosene fuel oil keep reading, because that is expensive at the moment.
Heating the house:
An air system involves fans and diffuser outlets in each room, these are not very popular at the moment, but are very efficient. They require an air outlet in each room, similar to a split airconditioning unit.
The water type of heating we are all familiar with; radiators and underfloor heating (UFH). Because you are only going to achieve a high CoP at lower temperatures, UFH is the best partner for heatpumps. UFH uses water at 45C or thereabouts,and therefore needs no adaptation to work with heatpump technology.
You can use radiators with Heatpumps, they just need to be significantly oversized so that the low temperatures of the water inside are balanced by a correspondingly larger surface convection/radiated area. Typically they will need to be 30-40% bigger.
Alternatively, there are some fan convector radiators available, which are designed for low temperature heat sources like HeatPumps but take up less space. The Smiths Ecovector for example, or the more esoteric Jaga range of fan assisted low H20 radiators.
We haven’t discussed hot water yet in any detail.
A heatpump usually runs at lower temperatures than conventional boilers. When heating hot water it needs to get a bit warmer, so the CoP lowers during this process. Some heatpumps use mains immersion heaters to top up hot water, which is fine in countries where hydro and nuclear generated electricity is cheap, renewable and plentiful. In the UK our electricity is none of these things.
Generally speaking, a heatstore is needed inside the property. This takes the form of a large water cylinder, which is a large buffer mass of heat.
What is the heatstore for?
Firstly, to keep up the ‘degree minutes’ of the heatpump. Secondly, to provide a means of supplying hot water.
Fixed rate compressors in heatpumps are most efficient when operating for long periods. A cycling heat pump compressor will use more energy (starting surges etc) and have a significantly reduced operating life. Thus systems are designed to maximise ‘degree minutes’.
So heatpump specifiers and designers usually need to provide a large buffer store of water from which Underfloor or radiators are attached, and typically a hot water cylinder will be wholly submerged inside this tank too. This allows the heatpump to run on longer without cycling.
The heatstore is usually quite large and heavy. it would typically occupy your airing cupboard but will usually be larger than a conventional water cylinder.
Some unpalatable points about the environmental credentials of UK electric power:
In Scandanavian countries (and France, for example) electricity is genuinely renewable, because it is mainly produced (> 80%) from nuclear and hydro electric sources. Electricity in these countries is less carbon intensive to produce than than burning fossil fuels like coal, oil and gas. I have conveniently avoided the known pitfalls of nuclear energy, of course.
In the UK, 80% of our electricity currently comes from burning fossil fuels. And only about 30% of the fuel’s energy ever gets as far as a power point, the rest is lost in generation and transmission losses.
Therefore electric cars and trains in the UK may be called ‘low emission’, but the source of their energy isn’t. When/if nuclear energy is properly underway in the UK this may help reduce emissions, but at the moment electric trains and cars are just at the other end of a wire to a carbon rich generating plant.
Heatpumps use electricity of course; their efficiency is very good if used correctly. But because of the transmission losses and the electricity generating process, their use today may not be making any significant headway in reducing carbon emissions, compared to an efficient gas boiler at home.
So where will heatpumps make a difference?
In well insulated homes (eg: built in the last 20 yrs) with underfloor heating.
Homes not on the gas distribution network currently running on oil or LPG that are well insulated and small. (Larger homes will need 3 phase electricity or a larger supply).
Homes where boiler flue siting is difficult – but in a well insulated home
Remind me about the hype?
Co2 emissions: Heat pumps use electricity and we primarily generate electricity in this country from some very Co2 intensive sources; oil, gas and coal. If anyone says a heat pump has less Co2 impact in the UK, don’t just take this as the truth, ask the right questions like ‘Where is the electricity coming from’?.
Life will be the same: If you take out a gas or oil boiler to fit a heat pump, you may be surprised to learn that you can’t have a ‘combi’ and even with a Hot Water cylinder, the heating has to be turned off to create the hot water. In practice this means you have to warm up your hot water storage overnight, and if you use that up during the morning (do you have a teenager in the house?) the heating will have to be turned off if you want a shower, for a few hours.
I will get Government grants: Not if your house has, or had, a gas meter you won’t. The simple reason for this is that compared to a modern gas boiler, heat pumps don’t offer any tangible savings. The new 2014 Government Renewable Heat Incentive has all sorts of strings attached, including a Green Deal Assessment. Payments cover a smaller period and don’t recoup the capital cost of the installation.
My radiators can be reused: MCS (Government scheme managers) advise that radiators should be 300% the size used for a conventional boiler in order to create a similar amount of warmth.
It is new technology: Heat pumps were invented over 100 years ago. Improvements have been made since but it is not new in the same way as an iPhone or the Internet. I don’t think anyone could argue that Heatpumps have changed at a more advanced rate than boilers in the last 50 years, for example.
Which heatpumps do Hot Water & Central Heating install and recommend?
For more advice or a quote contact HWCH, your central heating and heat pump installer for Edenbridge and Oxted – also covering the surrounding areas.