Air-to-air heat pumps

• Air-to-air heat pumps
  • Available models
  • Model results
    • All rooms heated (home)
      • Heater powers W
    • 3 heat pumps + existing radiators at 55°C (home3HP_R55)
      • Heater powers W
      • Room temperatures °C
    • 3 heat pumps + study heater (home3HP_1PE)
      • Heater powers W
      • Room temperatures °C
    • 3 heat pumps + 2 portable heaters (home3HP_2PE)
      • Heater powers W
      • Room temperatures °C
    • 2 heat pumps + 2 portable electric heaters (home2HP_2PE)
      • Heater powers W
      • Room temperatures °C
    • 2 heat pumps + 3 portable electric heaters (home2HP_3PE)
      • Heater powers W
    • • Room temperatures °C
    • 
      
    • 1 heat pump + 3 portable electric heaters (home1HP_3PE)
      • Heater powers W
      • Room temperatures °C
    • 1 heat pump + 4 portable electric heaters (home1HP_4PE)
      • Heater powers W
    • • Room temperatures °C
    • 
      
    • 4 portable electric heaters (home4PE)
      • Heater powers W
      • Room temperatures °C
    • 3 heat pumps + unlimited heaters (home3HP_RU)
      • Heater powers W
      • Room temperatures °C
    • Existing radiators at 80°C (homeR80)
      • Heater powers W
      • Room temperatures °C
  • Electriq IQOOL-SMART15HP performance
    • Power and noise
    • Vibration
    • Spectroid app
  • Hot water cylinder
    • Heating radiators from immersion
    • Horizontal cylinders
    • Vertical dual coil cylinders
    • Vertical direct cylinders
    • Installation cost of unvented cylinder
  • DHW heating
    • • Energy consumption
  • Off-peak electricity
  • Existing radiators
  • COP
    • Theoretical limit
    • Actual COPs
    • Electriq 12000

Available models

The fixing bracket holes for the Electriq are approximately 56mm below the top of the unit (based on this image of the installation template, which doesn't ppear to be to scale!). There appears to be a horizontal timber extending about 30mm below the plastic strip under the lounge side window. If we want to hang the heat pump from this horizontal timber, we would have to add an extra horizontal timber below it, with thickness around 2*(56-30) = 52mm (so the screws would go roughly in the middle of the added timber. We would have to remove part of the panel (or possibly a cladding plank) to install the additional timber.

Model results

Average winter temperature at Odiham is about 8°C.

All rooms heated (home)

This assumes heaters with adequate power in all rooms, providing 20°C in living rooms and bathroom, 18°C in bedrooms, kitchen and hall.

Heater powers W

Horizontal cylinders

The landing cupboard is 2.4m wide, so we have 1.2m available width to the middle of the doors. This seems to preclude all but the Telford cylinders. The Telford cylinders also have the advantage that no pipes attach at the ends (this is also true of Kingspan cylinders, but they do have the immersion cap at the end).

I suspect that all horizontal cylinders have poor heat distribution when heated from the immersion, so much (or most) of the coil will probably be in cooler water. They probably don't have have such a well defined thermocline as vertical cylinders (i.e. no clear separation between hot water above and colder water below), so not so good for storing hot water when some has been used.

So forget about horizontal cylinders!

Vertical dual coil cylinders

These cylinders will not fit in the landing cupboard without removing the block wall to the left of the cupboard doors. They only need about 90mm extra cupboard depth, so simply replacing the block wall with plasterboard would provide most of the extra space.

The cylinders have 2 coils. The lower one is a renewable coil with 3.3m² surface area, so could be used with an A2W heat pump if we ever found a suitable one.The upper coil is a standard one, intended for use with a pumped supply from a boiler. This appears to extend from about half-way down the cylinder almost to the top, and so would sit in the hot-water part of the cylinder and be well positioned for feeding radiators.

The immersion heater is almost half-way up the cylinder, so would quickly heat only just over half of the cylinder. The rest should eventually heat up via conduction, hopefully within the 4 hour low-tariff period.

Ideally I would wire the immersion with 3 alternative routes to turn it on:

1. Via the timer, to turn it on during the low-tariff period. The thermostat in the immersion would be set to about 65°C. This would provide maximum heat storage, and also provide Legionella protection.
2. Via a thermostat in the upper part of the cylinder with a setpoint of around 45°C. This should ensure we always have hot enough water for the sink, basins and showers/baths.
3. Via a contactor driven from a thermostat in one of the bedrooms. This would increase the cylinder water temperature depending on how much room heating is required. The thermostat should be programmable and ideally wireless, so that it could be moved between the bedrooms and study. A wireless thermostat would presumably be supplied with a receiver/contactor, although this might not handle the immersion current.

The simplest installation would be to have the immersion on all the time (with a setpoint of around 65°C).

An intermediate version would use 1 and 2, and use larger radiators in the bedrooms and study to compensate for the lower temperature. A setpoint of 45°C would probably result in a flow temperature of about 40°C, and an average radiator temperature of 35°C - so probably very big radiators.

The ideal installation would probably result in a flow temperature of about 60°C, and an average radiator temperature of 55°C.

The ideal and intermediate installations would both require the timer to be moved to the landing.

In all installation versions a room thermostat would be needed to control the radiator circulation pump.

A bypass valve might be needed to provide a circuit for the pump if all TRVs are closed, or I could simply remove the TRV from the towel rail in the bathroom. Since the pump can be run in constant-pressure mode (and we wouldn't have a boiler to be kept happy) I'm not sure we need any form of bypass. See this article about setting up the pump.

Vertical direct cylinders

These are only relevant if we don't want to heat radiators from the immersion heater, and don't want to use the boiler to heat DHW.

They come with two fitted immersion heaters, both 3kW. The lower one has a "smart" thermostat (explained in the installation instructions). We would probably use this in non-smart ("OPK") mode off a timer to come on during the overnight low-tariff period, with a setpoint of about 65°C. We would probably leave the upper one on permanently with a setpoint of about 45°C, so we always have hot water available.

The immersions would have to be on separate electrical supplies - fortunately we have two radial circuits (L3 and L4) available in the landing cupboard. L4 is currently used for the immersion heater. L3 is currently used for the water pump, boiler, GCH timer and 13A socket in the lounge by the bookcase. I would have to remove the lounge socket if we wanted to use L3 for the immersion heater. I suppose I might get away with "diversity" and just use L4: if the lower immersion is on, the water should be hot enough higher up to turn off the upper immersion - however, this would involve moving the timer to the landing.

Installation cost of unvented cylinder

Household quotes estimates 3 to 4 hours at £30 to £60 per hour to replace an unvented cylinder. They estimate £450 (two plumbers for a day) for replacing a vented cylinder with an unvented, excluding the cost of the cylinder itself, but including labour, fittings and sundries. This is presumably at £30 per hour, so we may have to pay £900 (at £60 per hour) making the total cost around 900+760 to 900+1075 = £1660 to £1975 for a 200l cylinder.

DHW heating

Energy consumption

This government survey found an average consumption for DHW of 16.8 MJ/day = 16.8*0.277778*365 = 1700 kWh/year.
According to GreenAge, average gas (hence heating+DHW) is about 12400 kWh/year.
So DHW accounts for about 1700/12400*100 = 14% of energy usage.
If heating was done by heat pumps and the DHW by immersion heater at full rate, the heating energy would be reduced to around (12400-1700)/3 = 3567 kWh/year and the proportion for DHW would increase to 1700/(3567+1700) = 32% of the total.
If all the DHW could be heated at off-peak rate, the DHW proportion would be back to around 14% in terms of cost.

Off-peak electricity

Under our current Go tariff, peak rate is 15.59 p/kWh and the off-peak rate is 5 p/kWh (32% of peak rate). Average rate is (15.59*20+5/4)/24 = 13.04 p/kWh (84% of peak rate).

From July the peak rate will be 34.86 p/kWh and the off-peak rate will be 7.5 p/kWh (22% of peak rate). Average rate will be (34.86*20+7.5*4)/24 = 30.3 p/kWh (87% of peak rate).

Existing radiators

COP

Theoretical limit

The theoretical maximum COP for a heat pump is given by Carnot's equation:

COP_max = T_high / (T_high - T_low)

where Thigh and Tlow are the temperatures on either side of the heat pump's refrigerant circuit, measured in K.

Some examples:

Actual COPs

I'm hoping that actual heat pumps will approximate:

COP = E x T_high / (T_high - T_low)

where E is  a constant efficiency relative to the theoretical limit. E will be different for different heat pumps.

Air-to-water heat pump COPs are usually specified at 35°C and 55°C flow temperature with an outside temperature of 7°C. According to my table above, the ratio of these two COPs should be 11.0/6.8 = 1.61.

For Daikin air-to-water heat pumps, the COP ratio is in the range 1.3 to 1.4, with E values from 0.44 to 0.54.

For LG the ratio is 1.8, with E values of 0.46 and 0.41. LG also provide a COP for 2°C outside 35°C, with E of 0.40.

Electriq 12000

The COP of the Electriq isn't specified, but if we divide the heating output power power by the electrical input power we get 2930/815 = 3.60. If we assume that this is at 7°C outside and 20°C inside, we get an E value of 3.6*(20-7)/(273+20) = 0.16. If we also assume that the 2.93 kW heat output is at 7/20°C, we get the following for outputs at different outside temperatures:

According to my model, we need about 210W/°C to heat the house (assuming heating in the downstairs rooms only).

11 Dec 2022:

With an outside temperature of -4°C, 3 heat pumps are only managing to get the downstairs rooms to 16.5°C - i.e. temperature difference of 20.5°C. This suggests that the total heat output from the 3 heat pumps is 20.5*210 = 4305W, which is 1435W per heat pump. Average input power is about 900W (for the 2 heat pumps I am currently monitoring), so COP is about 1435/900 = 1.6. This compares with a predicted COP (for indoor temperature of 16.5°C and temperature difference of 20.5) of 0.16*(273+16.5)/20.5 = 2.3, and a predicted output power of 815*2.3 = 1870W per heat pump.

The E value under current conditions is 1.6*(16.5+4)/(273+16.5) = 0.11. Using this E value, the above table becomes:

I've added a column for what my model predicts is the required heat output for each outside temperature. This suggests that the heat pumps require resistive heater assistance for outside temperatures below 0°C.

15 Dec 2022:

We did an experiment on 13 and 14 December: on the 13th we heated the back room using the heat pump only, and on the 14th we heated it using a resistive heater only and a schedule that attempted to reproduce the room temperatures that the heat pump had achieved. Average outside temperature was -0.1°C on the 13th and -0.3°C on the 14th. Outside temperature on the 12th was 0.9°, which may have aided the heat pump a little. Average back room temperature was 18.1°C on the 13th and 17.9°C on the 14th - so the experimental schedule worked OK. Average lounge temperature was 18.0°C on the 13th and 17.4°C on the 14th - this might have affected the results, but hopefully not by much.

Average power for the heat pump was 878W, and average power for the resistive heater was 1469W. This suggest a COP of around 1469/878 = 1.67 for the heat pump.

My model predicts that a heating output of about 993W would be needed under these test conditions, i.e. about 2/3 of the measured value.