Air source heat pump performance compared to a gas boiler

One thing that has just struck me today looking at my data and thinking how I am doing compared to my gas boiler is that my Daikin Altherma 3 has the water circulation pump built into it.

Of course, when measuring my electricity consumption and heat produced to arrive at a COP, although it doesn’t matter really, in originally looking at our gas consumption no account is taken of the electricity used by the circulation pump.

Today with the heat pump it was actually quite a lot compared to the overall electricity consumption.

It looks like my Daikin heat pump uses about 150 watts to pump the water.

As I said it doesn’t really matter as it is what it is, but it does mean that the heat pump is actually even more efficient compared to my gas boiler than the figures initially suggest.

I am curious when looking at the publicly available data, how many of those installations report the full electricity use of the heating/dhw system.

Is it most of them or do many systems have circulation pumps that are not included in the electricity consumption?

As an example.

Today I have used 4.50 kWh of electricity to produce 15.00 kWh of heat but 1.20 kWh of the electricity consumed was running the pump inside the heat pump.

If my gas boiler was 90% efficient I would have used 16.67 kWh of gas to produce 15.00 kWh of heat and that would have cost me around £1.16 plus a standing charge of about £0.27 so £1.43 in total without the electricity cost of the pump.

My heat pump has consumed 3.30 kWh of electricity without the pump and my cost for that was £0.56.

So by my reckoning the heat pump is nearly a third of the price to operate today as my gas boiler would have been and we have saved £0.87 today over sticking with the gas boiler.

Not bad!

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150W is a pretty high pump consumption. The pump on the old gas boiler used 30-50W, depending on how it was setup.

The circulation pump on the new heat pump consumes 70W when running at full output but ramps down to about 10W at 50% output.

My heat pump hasn’t been running long enough to get complete data but if the COP stays near where it is now and if I can arbitrage economy 7 the cost should be about 3.5p per kWh of heat compared to 7p per kWh of gas. That would be much better than I expected.



Yes 150 watts seems high, higher than I would expect.

I am guessing what the power consumption is though.

The heat pump uses 23 watts on stand by and seems to use about a continuous 175 watts between heating runs.

I was assuming that the circulation pump is running but maybe it is something else.

No heat is being produced.

But I just had a thought, I can see my flow rate in my data so I will check what is going on.

So, I just checked and it is definately the circulation pump as I can see the flow rate remains steady at around 20 litres per minute all of the time when heat isn’t being produced.

My gas boiler would be pumping water around the radiators.

I wonder why my heat pump is doing this?

There doesn’t see to be any obvious benefit in pumping water around the radiators with no heat being produced. All of the time this is going on the flow and return temperatures are within about 0.1c of each other.

I think I have worked out the cause of my problem of electricity use with no heat output.

I have been reducing the leaving water temperature relative to my WD curve.

I have obviously found the limit!

The flow temperature is not high enough to achieve the desired room temperature. The flow and return temperatures converge as the heat generated is now matching the house heat loss. The heat pump stops heating as it should do but the pump continues to run to try to fulfil the temperature requested.

This is obviously damaging efficiency and using electricity with no benefit at all.

So I have bumped my offset back up to where it was a few days ago.

Looking at my data though I see no reason why the heat pump should stop heating before the requested room temperature is reached. It is shutting off early.

I think my problem is a little deeper in the setup as Octopus configured it with fan coils when I have radiators.

I looked at the installation manual and radiators have a fixed delta t of 8c and UFH and fan coils are variable delta t. Radiators have a maximum flow temperature of 60c whilst the others are limited to 55c.

But I did find a setting regarding the pump, there are three options. One is sampling every three minutes and is only available when control is set to lwt.

For other operation the pump can be set to continuous or request. Mine was set to continuous and there is a warning with this that it will use more electricity. The pump stays on whilst room temperature is not achieved whereas request only operates the pump whilst heating is in operation.

I have set it to request and will see what happens.

The heat pump should aim to achieve a target leaving water temperature. The leaving water temperature will be “weather compensation” (roughly the right output, based on outdoor temperature) plus "load compensation (plus or minus a bit on the base curve depending on how far the actual temperature is from the desired setpoint)

If you dial the load compensation down too much then the heat pump will run purely on the basic weather compensation curve.

Radiators need quite a lot of load compensation. Underfloor heating needs less load compensation. Fan coils don’t need any because the fans will choose whether to run or not depending on room temperature.

The unhelpful behaviour is probably because the unit has been set to fan coil mode.

Continuous pumping can be useful for evening out room temperature with underfloor heating and some rooms getting east sun / some rooms getting west sun etc. (shuffling the heat around) Unclear why you would set it to continuous.

Ask what the settings for the unit ought to be; then ask why they are as they are?

The pump is probably controleld by PWM. In fan coil mode suspect it will be a fixed flowrate and the fans either draw or don’t draw from the constant flow fan coil circuit. In leaving water temperature mode does it look at the deltaT every 3 minutes and adjust the pump speed to maintain the target deltaT of 8C? (which is a balance between the energy needed to pump water around, which doubles if deltaT is say 4C, and the efficiency increase by dropping flow temperature - 54/46 is the same as 52/48 in terms of radiator output, but the former uses more compressor power/less pump power and the latter vice versa)

Cost of electricity for boilers can be astounding. e.g. Vaillant Ecotec 824 is 10W continuous for no good reason when not operating. Pump is then fixed speed so horribly inefficient for space heating at less than maximum rated output.

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