Near the Ashdown Forest, in Sussex, UK, is the village of Hartfield.  Apart from being the home of Pooh Bear and featuring what might be the only Pooh Bear shop (!) it is the location for our work this week.

Hartfield location

Hartfield location

This was interesting job, our client had an electric boiler which she had, over the last 3 years, invited nearly every heating and electrical contractor within a 30 miles radius to repair.

Pilot Electric boiler and heatstore

Pilot Electric boiler and heatstore

The house was of reasonably modern construction, with water underfloor heating.  Coupled to this was a ‘Pilot’ Electric Boiler, and a Thermal store.

Unfortunately, the Pilot boiler had long since been out of production and there were some relay parts needed that were no longer available without altering the circuit board.  This, and a seized pump and two seized valve actuators made us suggest that it was time for whole new heater.

The Pilot Boiler has an interesting history.  Our client was married to the joint inventor, who had unfortunately passed away many years earlier.  He and a business partner had designed a boiler that would heat a large (approx 400 litre) insulated tank of water using electricity bought at the Economy 7 off-peak tariff.  The underfloor heating screed would be warmed up using off-peak electricity, at the same time the heatstore would be warmed up, working like a rechargeable battery, only using energy in the form of stored heat.

The idea was that during the day, the Pilot Boiler would use the stored heat from the heatstore into the underfloor heating first, before requiring energy bought at peak rate.  This meant that the heating energy was being bought at around 3.5p per Kwh (2011 prices) instead of 13p per kwh (2011).  NB:  Gas prices in 2011 are over 4p per kWh.  So it was a clever idea, and in the 80s it was sold with joint marketing of the regional electricity board.  I don’t know how many of these Pilot Boiler setups there are out there, this is the first one I have seen.  Other heat store electric boilers were available  but were not as easy to install or integrated as the Pilot.

As the house already had underfloor heating, and was well insulated, we recommended that an air source heat pump was installed.  We are agents for the Mitsubishi Ecodan system.  There are lots of air source heatpumps out there, but we have found the Mitsubishi people to be reliable, trustworthy, and they take a real pride in their products.  We have been really impressed by their ethics, you can’t install their products unless you have been on their courses, provided proof of insurance etc.

Some recent Goverment funded research has suggested that heatpumps have been mis-sold in up to 80% of installations in the UK, which is not surprising, because lots of companies have sprung up using words like ‘Green’, ‘Sustainable’, ‘Renewable’ etc which are really just sales organisations, rather than heating engineers.

The Mitsubishi system comprises of a small external unit, and an internal pumpstation.  Air Source Heat Pumps work by capturing latent heat in huge volumes of external air.  This is captured in a matrix containing refrigerant and using a ‘vapour compression cycle’ the external low level heat in the air is converted into usable heat at up to around 45C.

Ecodan external unit

Ecodan external unit

The above picture shows the external unit, mounted on noise reducing mounts.  When running, it is quieter than a quality dishwasher.

Air Source Heat Pumps use electricity, and electricity is expensive at around 13p per kWh;  this is where the Coefficient of Performance (CoP) comes in.  For most of the heating season, CoPs of between 300% – 400% should be possible.  In other words, if your heatpump uses 2kW of electricity, it should deliver over 6kW of heat into the house.  This sounds like magic, but the extra energy is the latent heat in the air being extracted.

This particular unit will continue working in external temperatures down to -20C.  The efficiency is not very good when it is -10C outside, but starts looking very good from around 5C upwards.

This all sounds too good to be true….. but it is true, but there are some things you need to be wary of.  For every 10 enquiries we get about these units, only one is viable.  The first thing to know is that this technology is only efficient for heating that needs up to 45C touch temperature.  So if you live in a house with radiators, these will have been sized expecting 75C water to be inside.  45C going through them will not be warm enough.  You can significantly upsize your rads, but then we are going to ask you difficult questions about what the insulation is like in your floors, walls and ceilings.

By far the best partner for heat pumps is water underfloor heating.  The two technologies were made for each other, because UFH wants around 45C.

Back to the install;  we took out the Pilot Boiler and heatstore, wired in the Misubishi internal controller (called FTC2), and linked the output of the external Ecodan to the existing underfloor heating.

New internal installation

New internal installation

For simplicity, we used a standard Honeywell CM907 room thermostat to control the underfloor heating;  our customer need know nothing about the Mitsubishi system because once commissioned, there are no Japanese user controls.  All the interfaces are standard, familiar UK central heating controls, the clever people at Mitsubishi made it this way!

Well, does it work?  We put this system in in early December 2010, and our customer reports that it has performed faultlessly through one of the coldest winters in recent times.

Heat Pumps do work, but they aren’t the answer to everything.  Be very wary of the sales pitch.  However, in this scenario above, it will be cheaper to run than a gas or oil fired system and require less maintenance.  Depending on how the electricity is generated, Co2 emissons will be lower too.

Finally, what is the difference between air source and ground source heatpumps?  The ground source type extract heat from the soil, boreholes or watercouses instead of air, and are usually a bit more efficient.  However, you have to dig holes, there are large quantities of pipework buried underground, and the gear costs more.  Since air source and invertor driven compressors have become more commonplace, the ground source stuff sales have plummeted……