Air Source Heat Pumps vs. gas boilers

First the bad news for heat pumps. The Energy Saving Trust heat pump survey in 2009 found that many users were not impressed at all. The follow up in 2013 improved the results but the final average system COPs of 2.45 (air source) and 2.82 (ground source) were still way below the headline figures quoted for these machines a lot of which are claiming over 4 these days. So maybe heat pumps are intrinsically good but tricky to install?

Despite all that, what is really good about heat pumps is that they can deliver more energy than they consume in electricity.

The power multiplier
The power multiplier

So a small one would be just like this diagram; working on the power of an electric kettle but delivering the power of 3 to your hot tank – a COP (Coefficient Of Performance) of 3 then. By contrast your immersion heater delivers and also consumes the power of an electric kettle so it has a COP of 1.

Heat pumps are all sold with an industry standardised COP. This is misleading to say the least and the reason why optimism is defeated by experience. Far from being a fixed figure the COP actually swings widely depending on outside air temperature and temperature delivered in the home. The COP plots here show how a kick is engineered to give a good headline figure; that kink in the graph is exactly at the publication point.

A sneaky kink
A sneaky kink

You might buy a machine with a quoted COP of say 3.75 but while making domestic hot water on a cold night it will be working at less than 2. There are benign swings however and given a sunny winter day with some warm air to chew on an ASHP can see COPs almost up to 5. You can see this on the blue line on the graph. Delivering 35c water to the underfloor heating the COP goes over 5 as the outside air goes over 10c. Note that the pale blue line (delivery temperature 50c for radiators) still only goes to around 3 so for most of the time the average COP will only be near 2.5. So, heat pump with radiators – think carefully.

Gas per costs about a third of electrical power so after adjusting for efficiency a gas boiler is similar to a heat pump with a COP of 3. Many people in the survey would be comparing their new heat pump to a gas boiler; a formidable opponent when running on cheap gas. A gas boiler is much more powerful than most heat pumps and delivers at usefully high temperatures so a heat pump must have an overall COP of over 3 to justify a hefty purchase price.
Perhaps the performance could be lifted further?

The next bit is a bit dull – you might want to skip on to conclusions below.

To winkle out some ideas we’ll take daily temperature data for January in Guildford ( and relate that to a COP matrix made from the published data from a modern ASHP (inverter drive scroll, r410a, delivering to under floor heating at 35 degrees).
We will be looking to lift the COP by running the ASHP at the warmest ambient temperatures possible.
A look at a January temperature trace shows:
There is usually a 5 degree swing between the mean night time temperatures and the daytime mean.
Night time temperatures are flatter and longer than the sharper daytime peak at 1-2pm.
The morning transition from lows to highs is halfway there by 10am.
Temperature rises coincide with sunrise, not surprisingly.

Relating the above to the COP matrix:
Running a 7hr shift from 10am gives an average COP of 3.86 – much better than gas.
The equivalent night time shift only gives a COP of 2.92 – but almost as good as gas.
If the pump has to make hotter water for radiators these day/night figures drop to 2.7 and 2.11 and for 55 degree hot water making 2.3 and 1.85– gas beats this hands down.
Storing daytime running means that delivery temperatures probably need to be around 50 degrees leading to an average COP of under 3 although bigger storage tanks improve this.
ASHPs can be smaller if they run continuously day and night on an average COP of 3.4 – still 13% better than gas.
Direct electrical heating is often used to boost hot water making (COP = 1) and this can lower the average COP. If we can avoid this practice and run predominantly in the daytime it should theoretically be possible to get a COP of 3.35 (7hrs day, 2hrs night, 2hrs hot water).

Transmission: Put 100W/square metre through your floors and your feet will be uncomfortably hot so somewhere near half that will be a good yardstick for calculating the power you need.

A small ASHP can run a bit more efficiently than a gas boiler in a modest well insulated house. Fan-coil units in bedrooms and underfloor heating elsewhere are essential. The heat pump should run in daylight except maybe for a boost before dawn to guarantee morning showers and take the chill off the floors.

Of course, with PV panel prices falling relative to electricity prices, the time is coming when your heat pump will run for free while the sun shines. At the moment it looks like we are firmly in no brainer territory and it is certainly worth checking now to see how the sums stack up.

A tip to make your heat pump installation cheaper. Use the PV panels and an energy diverter (e.g. Eddi) to heat your existing immersion heater. then you won’t need the special tank to accompany the heat pump which will just do central heating. Your installation might get a lot nearer to the £5,000 grant.

And while we are at it, don’t forget to consider a mini-split heat pump air conditioner. It’s an air to air unit and relatively so cheap it’s hard not to go and get one right away. I did.

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More on this topic in LIST OF POSTS


Heat pumps in Southern Europe (and UK) – Air or Ground?

Day COP vs. Night COP

Day COP vs. Night COP

Good COP bad COP
A ground source heat pump (GSHP) is more efficient than an air source heat pump (ASHP) but is this relevant, or even true, for central Europe where air temperatures are warmer than in England for example? Taking the daily high and low average temperatures for Perugia in Central Italy and converting them directly into COP figures for a modern ASHP gives a chart showing the performance envelope. The straight black line shows an approximation of GSHP performance. Note how the GSHP starts really well and declines over the season as the ground energy gets depleted. Even so it performs strongly throughout and has the ASHP licked overall especially at night when the air temperature falls dramatically.
That’s the simple story but can the ASHP fight back in any way?
You’ll see that the big red line at the top shows that every day there is a time when the ASHP will always be running a better COP so the ASHP could be elevated with a total commitment or even a bias towards day time running. This assumes adequate power and bigger energy storage with tanks of some 2,000 litres. A house with high thermal mass and heated concrete floors will store energy too.
This regime pulls the average COP for the ASHP a bit higher than halfway between the day and night lines so over the season the ASHP is a match for the GSHP; but the battle isn’t over yet. Time to introduce the solar shed.
The solar shed in its simplest form is a greenhouse for the ASHP and apart from adding a little sunlight to the equation it allows 2 extra sources of warmer air to be added. The ASHP fan blows air out of the shed and replacement air can come from vents leading from the house air exchange (bathrooms and kitchen for example) and also from large diameter tubes buried in the ground. The latter is old technology and has been shown to raise the air temperature by around 5 degrees C. The green dotted line on the chart shows the night time COPS being elevated by 5 degrees and as you can see the COP envelope is an easy match for the GSHP and draws ahead in the late season.
Finally some ball park economics starting with a €10,000 ASHP and the equivalent €20,000 GSHP. Assigning a typical bill of €2,000 to run the GSHP a standard ASHP would cost €2,500, at the very worst, and thus take 20 years before the total cost overtakes the GSHP – already a no brainer for the ASHP before we even start on economy measures. Increasing the storage to a 2,000 litre twin tank system would add up to €3,000 and that would be recouped in 6 years, which is useful, but the air recovery and ground heating systems don’t look feasible on top of that although eco-warriors might like to go the whole hog.
The conclusion is that Air source is the way to go in Southern Europe and big tanks are worthwhile if the quotes look reasonable.

What about England?
If you live in England you might be wondering how relevant this graph is to you. Comparing monthly average temperature highs and lows for Perugia vs. Horsham gives these results for the 6 cold months only:
Nights are only 1 or 2 degrees colder in England except for Jan and Feb when Italy is 1 degree colder so on balance there is no real difference.
The average monthly lows for both zones are just above freezing – i.e. no trick technology required for air source heat pumps.
Days are always warmer in Italy, 1 – 3 degrees. The swing between night and day is a little more and so air source heat pumps make more sense particularly if biased towards daytime running with big tanks.
For the 3 coldest months England is only a degree or two colder during the day than Italy so in COP terms they are in the same ball park.
The bottom line is that an ASHP is just as valid wherever you are, the graph holds true and it makes sense to run during the warmer day and store in bigger tanks. Running an ASHP on economy 7 at night is also valid of course and I’ll make comparisons when I’ve updated my models.
If anyone is into the nitty gritty of COPs and stuff I have prepared a COP matrix for a modern ASHP in Excel. You can select an air temperature and the target temp such as floor, rads, fancoils and read off the COP. Just contact me if you’d like a copy.
If you are starting a project and need advice on integrating wood, solar, gas and an ASHP by using a heat bank just let me know. I’ll be in England full time after Summer 2015.