I’ve been asked by the head of our household to look into air source heat pumps. We are hopefully moving shortly, and she would like to have an eco friendly heating system in the new residence. I’ve done a little bit of research, and it would appear that financially they don’t financially make any sense even including the £5K government grant available.
My initial work looked at the Kw/H generated from 1 litre of oil and 1 cu ft of gas, I’ve used 60% and 90% boiler efficiency to work out how much it will cost for 10.35Kw of heating energy (equivalent to 1 litre of kerosene which my existing property uses). I’ve looked at heat air source heat pumps and used the optimistic 300% efficiency, however, my reading suggests that this drops down to say 200% (and lower) which I haven’t used) when the air is cool in Winter. The saving with todays high energy costs means that it won’t break even for say 15 years. In fact if the house is highly insulated I will be dead before it breaks even. If I look at energy prices before Ukraine, and oil is going back down in price, the payback is again ridiculous / if ever.
It seems that an efficient air source system needs to run as c40C which is too low for hot water so you need an immersion heater to top up the hot water tanks to stop legionnaires growing (which isn’t I believe included in the 300% efficiency figures quoted). The radiators need to be much larger than for a gas / oil boiler and preferably it should be underfloor if you are to attain the same room temperature. Now my wifey likes it warm (a career running hospitals has made her acclimatised to the heat before anyone suggests running the house at a lower temperature)…..around 22C which isn‘t a big temperature differential for a 40C system, I’m actually wondering in a house that may not be very thermal efficient if it will be possible to attain this room temperature.
Has anyone else looked at how cost effective moving to air source is? Am I missing something?
Whats people experiences, the good the bad and the down right ugly in installing and running such systems?
I've been in the industry for 27 years. There are many factors to consider prior to making an informed decision. Don't listen to you mate down the pub who has watched a YouTube video on ASHP. Firstly you need to do heat loss calculations for every room in the house. All 4 walls and the window(s) sizes. Are walls solid, cavity, cavity with insulation, internal, external etc. Once calculated this will give you the total heat loss of the property thru walls. Windows, are they single glazed, double glazed etc. If double glazed what's the air gap? It used to be 16mm now it's 20mm. Are the frames wood or Ali. This will give heat loss thru the windows. Once calculated this will give you the heat loss for every room.
Next look at every radiator in the house. What type are they (K1 single, K2 double, K3 triple (panels)) Then look at the relative heat output from each radiator at 80deg C, 60deg C & 40deg C. Next look at you loft insulation and floor types. Is the ground floor solid concrete, suspended, etc, first floor is usually suspended floor boards or chip board. How much loft insulation do you have. Once you have calculated all the above this will give you the total heat loss for the whole property.
Most ill informed people think that they'll have to change every radiator in the house for some super sized monstrosity. Not so. Example, if the heat loss for a room is say 700 watts and you have a 600 X 600 K2 radiator in there and the heat output for that radiator at 40deg C is 800 watts then why change it? Do this exercise first before even thinking about an ASHP.
Next, hot water cylinder. Legionella grows at temperatures below 55deg C. A typical 200litre cylinder will change it water volume around once every day in a typical household. The temperature of a cylinder only needs to be set at between 38 & 43deg C max otherwise you have to blend in cold water to get it to this temperature. Any hotter and it will burn you. For safety sake we set cylinders to achieve 61deg C once a week to eliminate legionella risk.
Next look at your heating circuit pipe sizes. If it's this new 8mm micro bore plastic forget it. You'll have to change all your pipework. Most properties have 22 & 15mm copper pipe. If that's the case then your good to go although I would add that power flushing the heating circuit is always recommended to remove any crud in the system therefore increasing efficiency. There are other options to help with efficiency, flow rates, heat distribution, headers or buffer zones, weather compensation etc but don't want to get too technical here.
Without trying to offend, your average plumber wont know or cant do the correct heat loss calculations nor will they be able to size and design a system to run at max efficiency at 40deg C. Most of them can fix a shower or a WC or install a new boiler, (all they do is look at the current boiler kW rating and install a new one of the same rating)
To sum up. It's a highly technical exercise to see if a domestic ASHP is suitable for a property. Government make all these outrageous claims that we'll be a nation of heat pumps by 2030 but in reality we need to look firstly at the correct training, sizing and installation methods before embarking on this journey.
Systems are getting better and we can get water temperature of 70+ Deg C on some systems now but those systems are not suitable for all properties. Hope this helps some of you.
