Calculating heating and thermal transferrance per room

[Jeremy]

I'm working on planning a heating system for a Victorian remodel. For the heating system, I'll be putting in an ASHP, so I'm in the process of calculating the kW I'll need for the pump, and where I might need new radiators. I'm planning to fully draught+damp-proof the house as well as bulk up insulation (floor + loft + in-wall)to get u-value of 0.3 or so and replace windows with high-end double glazing (centre pane U-Value of 1.2). I'm planning to work with a qualified heating engineer to get exact kW ratings for rooms and house overall, but would like to work out ballpark figure myself in the meantime. Here's how I'm calculating my "ballpark" so far - any corrections would be most welcome:

 

(1) Room air volume in metres * air changes per hr * temperature (where temperature = difference between desired room temp and outdoor temp of -1C) +

(2) Surface area in metres of external walls (excluding surface area of windows) * u-value of insulated walls / 1000 (for kW) +

(3) Surface area in metres of windows * u-value of windows / 1000 (for kW)

(4) Surface area of ceiling * u-value of roof with insulation / 1000 (for kW)

 

I'm not quite sure how to sort out thermal inputs (woodstoves, kitchen appliances, human bodies, heat passing up through floor from room below), so I've just left that out. Also, I'm just estimating air changes, as this house won't have MHVR or anything fancy like that. Using 2/hr for kitchen and bathrooms, 1.5/hr for hallways and 1/hr for bedrooms. Ideas welcome on both of these details. For the sake of the math, I'm using figures of 20C for living room, 18C for bedrooms and 16C for hallways.

 

So, to give an example of how this is working out in practice for me:

 

An upstairs bedroom is 4m by 4m with 2.7m ceiling with 1.92m of window surface and 5.79m of (2) external walls. I'm estimating air changes/hr in those bedrooms will be 1.

This gives me:

 

4*4*2.7*1*21/1000

(5.79-1.92)*0.30/1000

1.92*1.2/1000

4*4*0.30/1000

 

This gives me an overall of 915w (or 0.915kW).

 

In terms of measuring for radiators, I gather than BTU/kW ratings for radiators is based on flow-temp of 70-80C, so with an ASHP running between 40-65C will be going for radiators which will give me +33% or so, in this case a radiator specified at 1.216kW.

 

I suspect something is missing here. Happy to be corrected by the proper experts!

[SteamyTea]

28 minutes ago, Jeremy said:

I suspect something is missing here. Happy to be corrected by the proper experts!

You need to know the transfer between rooms as well, unless you are modelling the place with only 1 internal temperature.

[Mr Punter]

Because the ASHP work best with low temperature you may want to look at underfloor heating on the ground floor, with lots of insulation.  Agree to over-sizing the upstairs rads.

 

Yes, the heat losses are the fabric losses plus ventilation losses.

[Mr Punter]

1 minute ago, SteamyTea said:

You need to know the transfer between rooms as well, unless you are modelling the place with only 1 internal temperature.

 

If the rooms are both heated there probably won't be much in it.

[SteamyTea]

2 minutes ago, Mr Punter said:

If the rooms are both heated there probably won't be much in it.

Incidental heating may make a difference, you can work out solar gain easily enough using PVGIS.  Just set the window area angle to vertical, then see what the table shows.

[Mr Punter]

7 minutes ago, SteamyTea said:

Incidental heating may make a difference, you can work out solar gain easily enough using PVGIS.  Just set the window area angle to vertical, then see what the table shows.

 

Again, is there any point for this exercise, as they will need to have a heating system sized sufficiently to heat when there is no solar gain?

[SteamyTea]

Just now, Mr Punter said:

Again, is there any point for this exercise, as they will need to have a heating system sized sufficiently to heat when there is no solar gain?

One of those "Yes and No".

For sizing radiators/UFH, then the maximum load is what matters.

To get an efficient system, then the most likely loads is needed.

The truth is somewhere between the two (I may have to look at that as it is an interesting statistical problem).

[AliG]

1 hour ago, Jeremy said:

16C for hallways.

That is unpleasantly cold. I have a spare room that the heating is never on in and gets little solar gain, it sits at around 17.5C and feels freezing compared to the rest of the house.

 

Does the house have access to mains gas? If you have to heat the water above 45C in the ASHP the COP is going to suffer and it will be materially more expensive to run than a gas boiler.

 

I would also be concerned that with those levels of insulation you are going to have problems dealing with sudden changes in outside temperature. One of the things I notice in a well insulated house with 3G windows is you cannot really tell what the outside temperature is, as it impacts the temperature inside the house quite slowly. I fear that you will not be in this position, an ASHP is good for providing a steady modest amount of heat, but if the sun goes down and the wind gets up and it suddenly gets much colder outside, it might be that you struggle to get enough heat into the house quickly to keep it comfortable.

 

I would use the JS Harris calculator to find out the total house heating requirement which takes into account heat passing between rooms and so on. You will need a room by room requirement to size radiators, but I would start off with the whole house number and look at the instantaneous requirement at below 0 to see if this can be supplied reasonably by an ASHP.

[SteamyTea]

20 minutes ago, AliG said:

I would also be concerned that with those levels of insulation you are going to have problems dealing with sudden changes in outside temperature. One of the things I notice in a well insulated house with 3G windows is you cannot really tell what the outside temperature is, as it impacts the temperature inside the house quite slowly. I fear that you will not be in this position, an ASHP is good for providing a steady modest amount of heat, but if the sun goes down and the wind gets up and it suddenly gets much colder outside, it might be that you struggle to get enough heat into the house quickly to keep it comfortable.

I know what you are saying, but that is a problem with incorrectly designed and sized systems, it is not an intrinsic problem with ASHPs.

[jamieled]

19 minutes ago, AliG said:

feels freezing compared to the rest of the house.

I think that's the nub of it though. Comfort at a particular temperature is very subjective and relative to the surroundings (to an extent). Most of our house sits at around 16-18 and we find it fine. But I completely accept that 16 would feel cold if the rest of the house was at 22.

 

[AliG]

I tried having the hall a couple of degrees cooler than the rest of the house on the grounds it is a big space that you only rarely spend time in.

 

But as you say because of the differential in temperature it was too noticeable. I find more than about 1.5C quite noticeable and so have all the rooms in the house set between 20.5 and 22C. That cold spare room no one ever goes into so I just close the door and let it get cold

Edited December 22, 2020 by AliG

[Jeremy]

6 hours ago, Mr Punter said:

Because the ASHP work best with low temperature you may want to look at underfloor heating on the ground floor, with lots of insulation.  Agree to over-sizing the upstairs rads.

 

Yes, the heat losses are the fabric losses plus ventilation losses.

I'd been thinking about this, but is timber framed on ground floor, part of which is suspended over a cellar, so seemed like rads would be simpler to install, albeit swapped for bigger models. Was thinking we might do electric ufh in the Small kitchen (4x4m). Is there a budget conscious option for timber subfloors?

[Jeremy]

3 hours ago, AliG said:

I tried having the hall a couple of degrees cooler than the rest of the house on the grounds it is a big space that you only rarely spend time in.

 

But as you say because of the differential in temperature it was too noticeable. I find more than about 1.5C quite noticeable and so have all the rooms in the house set between 20.5 and 22C. That cold spare room no one ever goes into so I just close the door and let it get cold

This is a great point. Will calculate max need based on more uniform house temp.

[Jeremy]

5 hours ago, AliG said:

That is unpleasantly cold. I have a spare room that the heating is never on in and gets little solar gain, it sits at around 17.5C and feels freezing compared to the rest of the house.

 

Does the house have access to mains gas? If you have to heat the water above 45C in the ASHP the COP is going to suffer and it will be materially more expensive to run than a gas boiler.

 

I would also be concerned that with those levels of insulation you are going to have problems dealing with sudden changes in outside temperature. One of the things I notice in a well insulated house with 3G windows is you cannot really tell what the outside temperature is, as it impacts the temperature inside the house quite slowly. I fear that you will not be in this position, an ASHP is good for providing a steady modest amount of heat, but if the sun goes down and the wind gets up and it suddenly gets much colder outside, it might be that you struggle to get enough heat into the house quickly to keep it comfortable.

 

I would use the JS Harris calculator to find out the total house heating requirement which takes into account heat passing between rooms and so on. You will need a room by room requirement to size radiators, but I would start off with the whole house number and look at the instantaneous requirement at below 0 to see if this can be supplied reasonably by an ASHP.

Ok, this is good to know - will rate radiators based on 45C flow temp then. We do have mains gas, and an old gas boiler, but not sure if I want to bother connecting it. The whole point is to get off fossil fuels.   Will probably also have solar thermal feeding in to unvented cylinder for hot water. 
 

And I hadn't thought of dynamic responsiveness of the whole system. Will have to contemplate that a bit. We do have woodburners in several key rooms, and will probably have them going during the winter for ambiance, do you think that might possibly make up for sharp thermal drops?

[Jeremy]

5 hours ago, AliG said:

That is unpleasantly cold. I have a spare room that the heating is never on in and gets little solar gain, it sits at around 17.5C and feels freezing compared to the rest of the house.

 

Does the house have access to mains gas? If you have to heat the water above 45C in the ASHP the COP is going to suffer and it will be materially more expensive to run than a gas boiler.

 

I would also be concerned that with those levels of insulation you are going to have problems dealing with sudden changes in outside temperature. One of the things I notice in a well insulated house with 3G windows is you cannot really tell what the outside temperature is, as it impacts the temperature inside the house quite slowly. I fear that you will not be in this position, an ASHP is good for providing a steady modest amount of heat, but if the sun goes down and the wind gets up and it suddenly gets much colder outside, it might be that you struggle to get enough heat into the house quickly to keep it comfortable.

 

I would use the JS Harris calculator to find out the total house heating requirement which takes into account heat passing between rooms and so on. You will need a room by room requirement to size radiators, but I would start off with the whole house number and look at the instantaneous requirement at below 0 to see if this can be supplied reasonably by an ASHP.

What's the JS Harris calc?

[Jeremy]

6 hours ago, SteamyTea said:

Incidental heating may make a difference, you can work out solar gain easily enough using PVGIS.  Just set the window area angle to vertical, then see what the table shows.

Do you mean this: https://re.jrc.ec.europa.eu/pvg_tools/en ? Fab tip. Had not run across this!

[SteamyTea]

1 minute ago, Jeremy said:

Do you mean this: https://re.jrc.ec.europa.eu/pvg_tools/en ? Fab tip. Had not run across this!

Yes, in the data table it will show how much irradiance is available, by square metre.

 

5 minutes ago, Jeremy said:

The whole point is to get off fossil fuels

 

5 minutes ago, Jeremy said:

We do have woodburners in several key rooms

Better off burning gas, for so many reasons.

 

6 minutes ago, Jeremy said:

Will probably also have solar thermal feeding in to unvented cylinder for hot water. 

Don't waste your time and money, just fit some PV, it works out best.

[Jeremy]

6 hours ago, AliG said:

 

Does the house have access to mains gas? If you have to heat the water above 45C in the ASHP the COP is going to suffer and it will be materially more expensive to run than a gas boiler.I

Ok, am back with a question: I did a quick check on the tech specs for performance on the Samsung 12kw ASHP we are looking at. At -2C ambient temp, running the pump at 55C,  for 5.33kW in it will produce 8.73kW of heating. Isn't that still almost twice as efficient as gas?

[AliG]

This is the heat loss calculator provided by a previous member of the group.

 

It gives a good idea of heating requirements for a house. I personally would allow for as built performance being worse than calculated performance so add something on for this.

 

https://forum.buildhub.org.uk/applications/core/interface/file/attachment.php?id=18333

 

With your U-Values it will be very difficult to run UFH in a suspended timber floor and get enough heat into the room. I tried this in my last house which probably had around 0.25 U-value and had to run the flow at 60C in the kitchen for when it was cold outside.

 

This gives you an idea of the COP of a heat pump depending the flow temperature and the outside temperature, ideally they are run at 35C or less for heating. At 45C flow you are looking at a COP of 3 or less which means that it will cost almost twice as much as gas for heating, but with less carbon. However, if you can get registered for the RHI then you will get more than the extra cost back.

 

 

 

[AliG]

6 minutes ago, Jeremy said:

Ok, am back with a question: I did a quick check on the tech specs for performance on the Samsung 12kw ASHP we are looking at. At -2C ambient temp, running the pump at 55C,  for 5.33kW in it will produce 8.73kW of heating. Isn't that still almost twice as efficient as gas?

We cross posted there -

 

Yes, but gas costs around 2.5p/kWh and electricity around 14p, currently electricity is 5-6x the price of gas (On my tariff it is 2.14 vs 14.29, so electricity for me is over 6x the price of gas, you will lose a little bit of the benefit in the efficiency of a gas boiler being less than 100%)

[Jeremy]

9 minutes ago, AliG said:

We cross posted there -

 

Yes, but gas costs around 2.5p/kWh and electricity around 14p, currently electricity is 5-6x the price of gas (On my tariff it is 2.14 vs 14.29, so electricity for me is over 6x the price of gas, you will lose a little bit of the benefit in the efficiency of a gas boiler being less than 100%)

Gotcha. Thanks for the clarification. Definitely plan to get this all on RHI (thus solar thermal as well), and then get Solar PV installed in a few years (hopefully we get a decent subsidy regime in place soon as feed-in scheme is pretty dismal), so cost not as much of a concern overall for me. But point taken re: cheap gas!

[PeterW]

15 minutes ago, Jeremy said:

Ok, am back with a question: I did a quick check on the tech specs for performance on the Samsung 12kw ASHP we are looking at. At -2C ambient temp, running the pump at 55C,  for 5.33kW in it will produce 8.73kW of heating. Isn't that still almost twice as efficient as gas?

But at 85% efficiency, 8.73kW of heat needs 10.27kW of gas at a cost of 28.75p/hr. Using 5.33kW of electricity at an average of 14.5p would cost you 77.6p/hr or nearly 3 times the cost of gas. 
 

Efficiency and cost can be very different !