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Our Air Source Heat Pump

We moved house a year ago (April 2021) and one of the things I was keen to do was move off our gas combi boiler to an Air Source Heat Pump which we did early January 2022. Ours is an EcoForest EcoAir 3-12 Pro providing heat for a 1960s detached 4-bedroom house with poor insulation and a dodgy conservatory. We installed a 250 litre hot water tank and around half the radiators were upgraded too. Due to the rear of the house being all low windows and doors the ASHP needed to go part way up the garden 7.5m away from neighbouring bedroom windows.

Our neighbour on the other side of the fence say they can just about hear the hum if they stand outside and listen for it and the neighbour on our other side (around 15 meters away) asked if it had been commissioned yet. No one can hear it from indoors.

I'd already installed a Tado for the combi-boiler so we used that again for the ASHP. Some say you should run a heat pump as a continuous 'background' heating but we're very used to 15 to 16 degrees at night and between 19 and 20 during the day time and often slightly warmer in the evening. With the heating schedule set in the Tado app the system has worked well for the past three months. I've set the schedule to start to warm up 30 minutes earlier than we had with the combi boiler knowing there's a longer warm up time and we've not really noticed any difference. The only difference we do notice is returning home if we've been out for the day and manually turned it right down - the combi boiler was great at rapidly heating but that's about the only 'benefit' I've missed.

Our Tado thermostat turns the circulation pump on/off when calling for heat rather than instructing the heat pump control panel itself. Therefore once the heat pump buffer tank temperature is low the heat pump starts up. We're not using the Tado to determine when the hot water tank is heated although we may update the wiring to do so. Instead currently the hot water tank has higher priority than heating meaning after showers in the morning (or evening) the system will re-heat the tank. In the morning that can sometimes mean the home cools a little whilst it's heating water but we've only noticed because I've been intently monitoring how it works. As modern hot water tanks are very well insulated then there's very little loss if it were heated overnight and consumed the next day - we have an electric vehicle so we're on one of the Go Faster tariffs which would be ideal for hot water production.

We quite like being back on a hot water tank. Personally I always hated hearing the combi-boiler fire up every time a hot water tap was turned on; you could feel the wasted gas warming everything up - and worse if you turn a tap on and off causing the combi to power up and down. A constant reminder of burning stuff.

We were fortunate in having space in the utility room for the hot water tank as the loft pipework was removed years ago and the airing cupboard repurposed as storage. Hot water tanks are bulky ugly things though and look worse with the maze of pipework wrapped round them. I'll write about Sunamp and other alternatives in a future update as the squareness and size of a kitchen cabinet size (or combi-boiler size) seems to make these devices far easier to re-install than a hot water tank.

The ASHP is set to 45 degrees for heating and 55 for the hot water so I've been watching the Coefficient of Performance (COP) value when heating and producing hot water. Our home had very poor loft insulation and the conservatory roof is very out of date tubular plastic sheets offering about as much insulation as cling film but even days below freezing hasn't caused us any heating issues.



Hot Water

Jan/Feb days below freezing

3.0 to 3.5

2.8 to 3.0

March days zero to 10 degrees

3.5 to 4.5

3.0 to 4.0

Days above 10 degrees

4.5 to 5.0

4.0 to 4.5

The COP value is the magic of a heat pump; for each kWh of electricity consumed you get the equivalent of 2.8 to 5.0 times as much heating or hot water. For comparison a kettle is less than 1.0 (heat escapes from the kettle as it heats) and a gas boiler is well below 1.0. The COP doesn't include the energy consumption of the circulation pump nor the frost-cycle so you have to allow for that in any electricity consumption calculation. Frost-cycle: when the weather is below 5 degrees the rear slowly builds up a layer of frost and every few hours the heat pump pushes warm water through the heat exchanger to melt the ice resulting in a trickle of water from the base and some steam towards the end.

Our radiators were seriously out of date so worth changing. Larger radiators may sometimes be needed as the flow temperature is lower so you need a larger surface area to emit the same amount of heat. A decent survey is essential which involves measuring the room, windows and insulation to calculate the energy needed and therefore the correct radiator size. A couple of ours were incorrectly sized due to an extension added without upgrading the radiators so our house is now more consistently warm throughout than before. Nice bonus.

We were worried that larger radiators meant them taking up more wall space but this isn't always the case. Radiators come as types referred to as 11, 21, 22 or 33 and rather than saying eleven, twenty-one, it's more common to say one-one, two-one, etc. The first digit is the number of panels and the second digit indicates the fins. We had a couple of very old radiators that were just a 1 - i.e. a panel and no fins. So a 'three-three' has three panels and there's a set of fins attached to each panel. You can see where I'm going here - taking out a 11 and replacing it with a 22 (2 panels and 2 fins) increases the surface area emitting heat by just increasing the thickness (i.e. protrusion from the wall) not the width and height.

We have one 22 replaced with a 33 radiator that's installed on the side of a kitchen peninsular in the conservatory which is around 15cm depth but doesn't protrude too excessively into the room.

Between 3/11/21 and 21/1/22 we consumed 7,069kWh of gas and 946kWh of electricity; that's around 350kWh electricity per month. With the gas supply removed late Jan (no gas cooking) our electricity has jumped to 3,176kWh up to the 21st March; so we now average 1,400kWh electricity per month in a fairly comparable pair of winter months.

I.e. in the winter we're using 4 times the amount of electricity with the heat pump than previously.

Basing off the daytime rate (i.e. without breaking down the off peak use) for Go, Go Faster & Intelligent Octopus tariffs at around 35p/kWh then we're breaking even when the gas rate is above 8.75p/kWh (35p/4). Gas is currently around 13p/kWh.

And of course we're now off-gas which also means no gas daily standing charge.

In summary, so far so good.


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