Trying to understand Heat Loss and UFH options

[yellowbert]

Hi everyone,

 

1st Post so hopefully it all makes sense 🙂

I'm currently in the design phase for our extension / renovation project and am trying to understand more about heat loss and UFH design so that I know what questions to ask and what possible issues to look for when we start getting our UFH quotes and designs back.

I've downloaded and completed the "Heat Loss Calculator" put together by the amazing Jeremy Harris and calculated a Total heat loss of 3.55kW with -4^oC Outside and 21^oC Inside.

 

To move forward and start designing the UFH layout:

1. Do I need to do detailed individual room heat loss calcs? if so any guidance on how to get started with those.
2. I've read that I should try to achieve UFH pipe centres as close as possible to minimise flow temps. How do I calculate whether 150mm or 200mm is appropriate?
3. With our design meeting Enerphit - I've assumed that we would have no UFH upstairs - Is this realistic or should we include a 2nd zone for first floor heating?
4. Is our ground floor U-value of 0.13 good enough for efficient UFH with low flow temps and will it be economical to run using an ASHP or do we need a lower U-value?

 

For info regarding the thermal envelope:

We are aiming to reach Enerphit and PHPP shows we can do this but we do have a few constraints - particularly with the ground floor construction, which needs to be a suspended floor due to clay soil and ground heave.

Location: West Sussex

Internal ground floor area = 132m²

Floor U-value = 0.13 (TETRIS beam and block with 75mm screed - UFH to be 16mm pipe within screed)

Wall U-values = 0.12 - 0.14

Roof U-value = 0.12

Windows and Doors Triple glazed with U-value = 0.76 average

 

Thanks everyone - feeling a bit lost and need some direction to get progressing again - Hopefully not too much info in 1 post.

[JohnMo]

51 minutes ago, yellowbert said:

Do I need to do detailed individual room heat loss calcs? if so any guidance on how to get started with those.

 

Welcome, ideal is to size room array to suit it's heat loss.

52 minutes ago, yellowbert said:

With our design meeting Enerphit - I've assumed that we would have no UFH upstairs - Is this realistic or should we include a 2nd zone for first floor heating?

 

to have no heating upstairs you have to borrow heat from downstairs, so the UFH will need to give enough energy to satisfy the whole house requirements. 

 

52 minutes ago, yellowbert said:

I've read that I should try to achieve UFH pipe centres as close as possible to minimise flow temps. How do I calculate whether 150mm or 200mm is appropriate?

Closer pipe centres do reduce flow temperature to a certain point, but you will always have a minimum flow temperature acceptable to the heat pump.

 

So heat output will be 3550÷132, so average output is 27W/m².

 

53 minutes ago, yellowbert said:

Is our ground floor U-value of 0.13 good enough for efficient UFH

In simple terms yes, if you can improve even better. Wouldn't reduce screed depth, as that gives you options thin screed doesn't.

54 minutes ago, yellowbert said:

I've read that I should try to achieve UFH pipe centres as close as possible to minimise flow temps. How do I calculate whether 150mm or 200mm is appropriate

Simple graph that shows UFH output, just draw a straight line pipe centres and 27W/m² and project to mean flow temp. Mean flow temp is the average of flow and return temperature. So a flow of 30 return of 26 would be a mean of 28.

 

[SteamyTea]

All that @JohnMo says, he is very, very, really wrong.

 

Have you looked at overheating though.  Simple, room by room, heat losses at low outside air temperatures is a rare occurrence, and usually happens at night, use it to just set the maximum size of the heat source (allowing for CoP reducing if using and ASHP and DHW heating as well).

 

Overheating is much harder to calculate as you have to take into account OAT (outside air temperature), ventillation rates, window and wall orientations .  You can get a lot of useful data from PVGIS.

 

Just knowing the U-Values of each element is not really enough, you need to know the effective surface areas and how they interact with the local weather.

As an example, my house is small, terraced and faces NE-SW.  There is a lot of glass compared to wall, so when the morning and evening sun hit, the places is bright and warm (so bright in the mornings I close the curtains as I cannot read the laptop screen.

During the day though, the sun is higher in the sky, so hits the roof mainly, and less on the windows, so the peak IAT (inside air temperature) is generally below the OAT, makes it quite nice.

 

There is often a lot of talk about natural solar energy getting absorbed by the mass of the building and released later.  Don't bother to calculate that, mainly because when you want those advantages i.e. winter and summer, the day lengths are working against you.  A lot of nonsense is spoken about 'thermal mass', but very little evidence is shown that it is effective.

 

Regarding UFH upstairs.  While it is probably not needed, if you sell the house, most people will not believe that you do not need it, and it can be used for cooling, a bit.

Edited Tuesday at 18:13 by SteamyTea

[JohnMo]

1 hour ago, SteamyTea said:

All that @JohnMo says, he is very, very, really wrong.

Is "really" the correct word?

[JohnMo]

1 hour ago, SteamyTea said:

Regarding UFH upstairs.  While it is probably not needed, if you sell the house, most people will not believe that you do not need it, and it can be used for cooling, a bit

I was thinking about that, you can cool the downstairs with the UFH (and blinds or curtains) even with pretty big solar gains. But it doesn't help upstairs and a need for sleep when it's hot. Fan coils are great at cooling and heating, even if you put one oversized one central in hall upstairs (or in your bedroom) - I would put electrics in the bedrooms so a panel heater can be added when or if you decide to sell.

[SteamyTea]

8 hours ago, JohnMo said:

Is "really" the correct word?

No idea, I had had a long day, had to move, by hand, 150 scaffold planks.  If I had left work on time, I would not have had to do that.

[yellowbert]

15 hours ago, JohnMo said:

Welcome, ideal is to size room array to suit it's heat loss.

Thanks @JohnMo that makes a lot of sense. So it would be better to do individual room heat loss calcs so that I can find the optimum pipe spacing for each room rather than having the same pipe spacing throughout.

 

15 hours ago, JohnMo said:

Simple graph that shows UFH output, just draw a straight line pipe centres and 27W/m² and project to mean flow temp.

Once I know the individual heat loss for each room I can then use the graph to determine pipe spacing for various mean flow temps and I need to make sure that I don't go below the min flow temp of the ASHP. e.g

 

If the min flow temp for the ASHP is say 25^oC - should I aim to get all the way down to this with an appropriate spacing? I guess these calculations are all for the worst case coldest day - so would you also do the same calcs for the shoulder months to make sure the flow temp on those days is also above the min flow temp for the ASHP or does that not matter?

 

Thanks again - Hoping to gradually understand this as its such an important part of what we are trying to work towards in terms of a relatively low energy renovation.

[JohnMo]

2 minutes ago, yellowbert said:

So it would be better to do individual room heat loss calcs so that I can find the optimum pipe spacing for each room rather than having the same pipe spacing throughout

Yes then it makes it way easier to balance the whole system as a big single zone.  It's just small tweaks to flow to get room temperature as you want. So when doing heat loss calculation for each room have the heated room figure you require. So a bedroom is unlikely to need 21 degs.

 

5 minutes ago, yellowbert said:

min flow temp for the ASHP is say 25^oC - should I aim to get all the way down to this with an appropriate spacing

Remember when you do the heat loss calculation it's for 1 or 2 days a year, the rest of the time it's warmer, so heat loss is lower, I would target design flow at about 30-32 and that gives you room to have a working weather compensation curve for when it's say 9 degs.

 

The other thing I found on my ASHP, even though I can do 25 degs, the heat pump would only do one heat cycle then get stuck. There is a restart hysterisis built in to the controller. So if I target 25 the restart hysterisis is 6.9, floor temp would need to go lower than 18.1, which will not happen as that also means in reality the room temperature is also at that. Not all ASHP are the same though.

 

My WC curve runs from 25 flow at 20 outside to 32.8 at -9. Reality is over 10 average outside, no heat is required for the house, so the WC curve automatically inhibits ASHP at anything warmer than about 12. So over the last day and half, even though 7 overnight and 12 currently heat pump hasn't run, except for a short period yesterday on excess PV which is force operation at a higher flow temp. House is currently sitting at 21.5 degs.

[yellowbert]

15 hours ago, SteamyTea said:

Have you looked at overheating though. 

Thanks @SteamyTea - To be honest this is something we are a little worried about - PHPP calcs that the architect has done shows there won't be any problems, but it's a big leap of faith to just trust the numbers - a bit like trusting that we won't need heating upstairs!

In addition to looking at shading some windows I'm starting to think it might be worth installing either UFH or fan coils or similar upstairs so that we have a method of heating if needed but also to actively cool in the event that rooms do over heat on those really hot days.

 

13 hours ago, JohnMo said:

I would put electrics in the bedrooms so a panel heater can be added when or if you decide to sell.

That's also an excellent idea and won't cost much for the peace of mind that we can add the panel heaters if we need to in the future.

 

5 hours ago, SteamyTea said:

I had had a long day, had to move, by hand, 150 scaffold planks.

@SteamyTea - This is exactly the kind of thing I'm not looking forward to quite so much as the other work - one of the things that scares me the most is shifting several tonnes of bricks as I'll likely have to do a lot of the demolition work myself to save cost 😬

 

[yellowbert]

6 minutes ago, JohnMo said:

I would target design flow at about 30-32 and that gives you room to have a working weather compensation curve for when it's say 9 degs.

Thank You @JohnMo - this makes more sense than trying to hit the very lowest flow temp for that 1 coldest day. And just to confirm - when you say design flow - thats referring to the ASHP flow temp rather than the mean temp of the flow and return?

[yellowbert]

10 minutes ago, JohnMo said:

Yes then it makes it way easier to balance the whole system as a big single zone.

So if putting UFH or fan coils upstairs on the first floor - would you do this on the same single zone as ground floor or as a separate 2nd zone?

[JohnMo]

14 minutes ago, yellowbert said:

when you say design flow - thats referring to the ASHP flow temp rather than the mean temp of the flow and return

Yes

11 minutes ago, yellowbert said:

if putting UFH or fan coils upstairs on the first floor - would you do this on the same single zone as ground floor or as a separate 2nd zone

I would do it all as a single zone. Not a fan of UFH in bedrooms as it's too slow to respond. Fan coils change fan speed to modulate room temperature. With fan off zero heat output so may as well flow all the time.

 

When designing UFH in bathroom do quite close centres for the pipes as you don't really have much floor area to room volume. Or use a fan coil in there also. Cool Energy have them with towel hangers designed for wet rooms.

[Temp]

We have stone and tiled floors in bathrooms and it's nice to have these warm at times of the year when we don't need heating in the bedrooms. We also have more bedrooms than we need now the kids have left home. For these reasons we find having everything on separate programmable zones quite handy. 

[yellowbert]

7 hours ago, Temp said:

We have stone and tiled floors in bathrooms and it's nice to have these warm at times of the year when we don't need heating in the bedrooms.

Thanks @Temp - that is also a great point - perhaps fan coils for most of the upstairs on the same zone as the UFH on the ground floor - with some supplementary electric UFH for the bathrooms which can be linked to a timer / stat - that would give us the efficiency of a single zone for all the plumbed in heating but the flexibility of having heating in the bathrooms when everything else is off.

 

I think I'll work through the individual room heat loss calculations, which may take me some time - I'll then share my workings on here which should hopefully be helpful for anyone else trying to work through the same process of designing their own heating system. I'm sure I'll make lots more mistakes on the way.