Sensible target temperature on each floor when using rads and ASHP

[Berkshire_selfbuild]

Hi All,

 

in currently in end stages of my self build - at first fix stage. I need to spec rads on 1st and 2nd floor which provide wattage to match target temperature of each floor of 21deg. Ground floor we have UFH. My plumber has suggested speccing rads to work at 36deg flow. These rads are huge in places. A few options I have in brining down the rad sizes is to operate at a higher temperature (40deg or even 45) which im reluctant to due to reducing efficiency. Or I can bring down the target temperature however I don’t want to compromise on comfort. However as the house will be super insulated and is 3 storey high, would lowering the temperature by 1 degree as you go up be sensible? I.e 21deg ground floor, 20deg first and 19deg 2nd floor?

[JohnMo]

Do you need 21 in bedrooms - no, is it comfortable sleeping in 21 degs bedrooms - for me no.

 

Trouble is when you start adding radiators with a different design temperature you also need to add mixers etc to UFH.

 

If going MCS route they design bedrooms at 20 I believe. Do you actually need radiators at all - maybe not? Many on here haven't bothered when they have an upstairs and maybe added an electric point in bedrooms for a panel heater as insurance.

 

Having lived in multiple properties from old and drafty to modern and airtight we have never had heating in bedrooms. Slight lie currently have UFH in bedrooms but buried below high tog carpet, so it rubbish anyway.

 

Are you expecting any overheating in summer. If so why not fan coils. Then flow at UFH flow temps and have decent cooling when it's hot.

 

Make sure the heat pump you are getting does cooling out the box.

[Berkshire_selfbuild]

Thanks for response @JohnMo. The UFH will have a mixer regardless due to how low the UFH can actually work. 
 

Great to know that most don’t even bother with rads on uppers. I don’t think I’ll do without them completely - the mrs gets cold even in the summer! But at least I know that I can change the target temp upstairs even lower and should still be fine. 😀

 

overheating may be a slight issue on the south side but we’ll have pergolas/canopies above sliding doors to help with. Although upstairs bedrooms may still get hot. I’ll deffo find out about the cooking feature in ASHP.

[SimonD]

I think the first question has to be: what do you consider to be huge? And how are these impacting the space?

 

In my experience so far, which is now a few designs using radiators, is that it is far easier to design for 40C (45C if retrofit) on radiators than 35 and more so the higher the target room temperatures. It's basically down to the old law of diminishing returns. Remember that this is also done at the design outdoor temperature so for the majority of the year your flow temp is going to be lower. At the end of the day, if you can manage 40C on the rads, it'll still be a good efficient system, providing the rest of the system has been designed, installed and set up properly. Here's a good video illustrating savings with varing COPs 

 

 

 

If your house is going to be highly insulated, I would forget about trying to design for different temperatures as the house will even out overall. I still don't understand the logic behind the MCS/CIBSE temps at all as they don't hang together very well between design temperatures and operational temperatures in the real world.

 

In my house, I've had to design for 23 in the space my wife wants the warmth as she really feels the cold. My wife wants the bedroom warm and toastie in the winter and 18-20 doesn't cut it. I wouldn't be allowed to compromise on comfort for the sake of a few quid during the winter. I'm currently upsizing all my bedroom radiators because when I designed the system for 18C we never really anticipated the usage of these rooms would change to being more like study spaces and even though the heating works as designed, it doesn't work for sitting still for long periods for my wife and the boys.

 

In my house, which is a deep retrofit rather than new build, I've designed to 40C but managed to oversize the rads by an average of 25% which will theoretically give me something like 36-37 at design outdoor temperature. 

 

But also keep in mind that some air source heat pump controllers might not provide the granularity in the weather comp curve to set it to a specific flow temp at design outdoor temp and if the necessary curve sits too high, you'll have to use room influence - so it can be a careful balance. If you're likely to end up with solar gain in parts of the house, you also want to consider room influence controls.

 

It does sound like you need to have a good conversation with the system designer (is this your plumber or are they using a design company?) to understand and agree the strategy here

 

44 minutes ago, JohnMo said:

If going MCS route they design bedrooms at 20 I believe.

 

18 is the design indoor temp and 17-19 is the operational design temp. All defined by CIBSE

 

 

 

 

[Berkshire_selfbuild]

Thanks @SimonD. With regards to specifying varying temps across floors, my logic is more down to the fact that that as my property is being built to latest building regs with high levels of insulation etc, heat will rise and upper floors will naturally be hotter and if target temp is 21 for ground/1st/2nd floors, realistically the upper floors will get hotter than lower floors. Therefore if I spec target temp as 21/20/19 then I may stand a chance of balancing the temp throughout. But what do I know?!! It’s certainly a head spinner all of this for me.

[JohnMo]

2 hours ago, Berkshire_selfbuild said:

The UFH will have a mixer regardless due to how low the UFH can actually work. 

Not sure who is leading you down that path? But it's nonsense. Your UFH system should be manifold, floor loops and that is pretty much it. All run in weather compensation on a fully open system, certainly no buffer. A volumiser may add running time when warm outside. No actuators or thermostats are needed. All heat pump will flow at 25 some at 20 in heating mode. The way UFH works is via dispersion of heat within the screed. The further it has the travel the more the temperature drops via radial conduction.

[SimonD]

1 hour ago, Berkshire_selfbuild said:

Thanks @SimonD. With regards to specifying varying temps across floors, my logic is more down to the fact that that as my property is being built to latest building regs with high levels of insulation etc, heat will rise and upper floors will naturally be hotter and if target temp is 21 for ground/1st/2nd floors, realistically the upper floors will get hotter than lower floors. Therefore if I spec target temp as 21/20/19 then I may stand a chance of balancing the temp throughout. But what do I know?!! It’s certainly a head spinner all of this for me.

 

Yes, heat will rise. There are a good few people on here who have chosen to only have UFH on the lower floor and none upstairs - so if your house is well insulation enough, this could very well be a better solution.

 

It all depends on the heat loss of the house and what you want to design for - so unless you want to geek out on all this yourself, you need to have a good long chat with your designer.

[JohnMo]

Ultimately the floor output has to be considered when compared to the whole house demand, so for example.

 

House 200m² and heat demand of 3kW would require the floor output to be 3000W / 200, or 15W per m², if the whole house had UFH, but as you are on 3 storeys of equal floor area (assumed) on each floor you now need 3x the output from ground floor UFH. So 45W per m².

 

If on 200mm centres for the UFH loops that would be approx 40 degs flow temp on the coldest day and the UFH would provide heating for the whole house. 150mm centres would drip that be a couple of degrees.

[ProDave]

We are one of those with no heating upstairs. and being inland a bit in the Easy Highlands we know what cold winters are.  Fair enough if your bedroom must be 21 you will need heating, but we don't.

 

If you are to accurately calculate heat demand on a room by room basis you must allow for heat from downstairs coming up, so must calculate the U value of the ceiling / floor.  And again if a 3 storey house.  Intuition says your upstairs rooms will need a lot less heat than the lower floor rooms.

 

I would stick with UFH throughout, and the fact you are more likely to have carpet upstairs does not matter as you will need less heat anyway.