Calculating size of heat pump required

[rowan_bradley]

I am considering fitting an Air Source Heat Pump to replace my ageing oil boiler. In order to evaluate this, I need to be able to calculate accurately how much electricity the heat pump will use to keep our house heated to the temperatures that we need. As I understand it, the efficiency of a heat pump (i.e. the kW of heat generated for each kW of electricity input) depends on the difference in the temperature between the outside air and the water circulating in my radiators. I presume that for any heat pump available in the UK I can get a table showing the Coefficient of Performance (CoP) for various outdoor temperatures and various circulation temperatures. I then need detailed weather statistics for my area, showing the number of hours per year that the outdoor temperature is at each temperature between the minimum ever seen, and the maximum. Where can I get these statistics from? I also need data about my radiators, which tell me how much heat output they produce at each combination of room temperature and water temperature. Where can I get this data from?

Does the CoP of the heat pump depend on other things? For example, does it depend on how rapidly the outdoor temperature is changing? For example, does it use the same amount of energy if the outdoor temperature is at -10C for 10 hours, and then at 0C for ten hours, as it uses if the outdoor temperature is at -10C for one hour, then 0C for one hour, repeated ten times? Does the amount of energy it uses depend on humidity, wind speed and direction, insolation, or any other environmental variables? If so, where can I get this weather data for my location from?

 

Thanks - Rowan

[Jeremy Harris]

Welcome.

 

The COP of an ASHP depends on the humidity as well as temperature, and although outside air temperature does make a difference, it's not as big a difference as many assume, as the amount of heat in the outside air is relative, with zero heat being absolute zero (so ~-273°C).  This means that a reduction in the outside air temperature of, say, 10°C from +10°C to 0°C only drops the amount of heat available in the air by about 3.5%.

 

In practice, the biggest factor that impacts on COP is the output flow temperature of the ASHP.  If this is kept low, say 40°C, then the COP can be pretty high, between 3.5 and 4.0 as a rough estimate.  Increase the flow temperature of the ASHP up to 50°C and the COP will drop back to something like 3.0 to 3.5.  Add in cool, damp, weather, which may well induce icing in the ASHP, that lowers COP further, and then you could see the COP drop down to well below 3.0.

 

The reality is that the average outside temperature during the heating season is well above 0°C, so if the flow temperature from the ASHP is kept fairly low the COP will be pretty good.

 

We run our ASHP at a flow temperature of 40°C all the time, as we don't use it for hot water, only running underfloor heating, and find that the COP averages between 3.5 and 4.0 over the whole heating season.

 

Running radiators from an ASHP usually means fitting larger ones to allow for the lower flow temperature.  An oil boiler may well be running the radiators at 50 to 60°C, and that's higher than an ASHP will run at efficiently.  To get the same heat output to the rooms with a lower flow temperature usually means increasing the size of the radiators.

 

In general, ASHPs tend to work a bit better when it's windy, aren't particularly affected by insolation at all, but will tend to run at a lower COP when the RH is high.

[Hobbiniho]

jsharris has a heat loss calculation spreadsheet that is very good, i used it to calculate the heat loss for the property that i am refurbing to give me an idea of what i was dealing with and it was almost exactly the same as what the plumber got when he specced my system, i used this website to give me my u values http://www.vesma.com/tutorial/uvalue01/uvalue01.htm 

 

the real plus point about ASHP is the RHI payments and is the reason i am fitting an ASHP rather than oil, i am changing from storage heaters run on total heat total (no) control. The capital cost for me to install both the systems is the same but as i live in scotland i am getting an interest free loan to install the ASHP and the RHI payments i will receive will basically pay for the system over the 7 year payment term, RHI is good for older properties with poorer insulation and higher heating needs as the more energy you need to heat the property the more money you get back, if you are not looking to get RHI payments then it might not be worth it as you will have to change all the radiators with an ASHP and this can be costly, i had to have 3 fan assisted radiators specced as it was not possible to get the heat output i required from a traditional radiator in the small amount of wall space i had availiable

 

there is a topic below this one about oil vs ASHP which is worth a read ? 

[Cpd]

5 hours ago, Hobbiniho said:

, i had to have 3 fan assisted radiators specced

 

@Hobbiniho By looking at your house photo I could take a guess that we gave similar vintage houses, my one was built in 1903 I think.....  I am going to go down the self install on a mega budget route a bit like @ProDave I have the ASHP in storage and when I eventually.....; get round to installation it will run the UFH, out of interest can you tell me what radiators you are going to use as I have not discounted using some upstairs.  

[Hobbiniho]

17 hours ago, Cpd said:

 

@Hobbiniho By looking at your house photo I could take a guess that we gave similar vintage houses, my one was built in 1903 I think.....  I am going to go down the self install on a mega budget route a bit like @ProDave I have the ASHP in storage and when I eventually.....; get round to installation it will run the UFH, out of interest can you tell me what radiators you are going to use as I have not discounted using some upstairs.  

 

my house was built in about 1880 ish its made from beachstone concrete, i am having a panasonic aquarea system installed, the radiators in the 2 upstairs bedrooms are just normal triple panel 900x600 radiators, the living room/ kitchen downstairs has 2 fan assisted radiators and the back bedroom/ walkin wardrobe has 1 fan assisted radiator, to be honest i could have got panel radiators that would have done the job but it was on the limits and the living/ kitchen area is 9mx 3.7m with 3 outside walls and the kitchen half has no wall space so it was more a case of being able to "move" the heat around more efficiently plus the fan assisted rads are designed for the lower flow temps, i am aware that the panasonic rads are quite expensive but im sure they will do the job well, i will just have to wait and see 

[ProDave]

Have you heard one of these fan radiators running?  I would not want a fan that I can hear running for much of the time.

[Hobbiniho]

no i havent heard one running but they quote 40dba when they are going flat out, and 19dba on the low setting, they are over specced so should run at a low speed, i will just have to wait and see what they are like. i have a feeling the fridge and dishwasher will annoy me more than the radiators