UFH = 46 deg? (Which comes first, ASHP or UFH / chicken & egg?)

[Great_scot_selfbuild]

Hi all,

 

I must be at a good stage in our project, because I have lots of detailed questions (that's the way I'm looking at it anyway). I feel like some of this is ground I've partly covered before, but I'm now closing in on some final supplier selection and critical decisions, but struggling to find a company/supplier that I can wholeheartedly trust for having covered the detail to the level I am wanting.

 

Which comes first - chicken or the egg / ASHP or the UFH

 

I've got a UFH quote based on the floorplan and I was pleasantly surprised to receive a detailed (to my view) LoopCAD design with spiral layout (as I requested), showing plenty of circuits (it's a single zone for the entire floor).

However, the water supply table states a temperature requirement of 46 degrees, which seems exceptionally high compared to all the discussions I have previously had with ASHP suppliers who have emphasised the preference for aiming to have a low temperature.

 

Our house will have a passive-level of thermal and airtightness performance, so I wasn't expecting such a high temperature requirement for the UFH. 

 

So, what's my question?

The UFH water temperature and flow table subsequently outputs a kW load (just over 7kW); I was planning to work through the heat loss calculator spreadsheet this weekend to do my own ASHP / Heat loss checks. How should I take this UFH table into consideration when it comes to setting the specification of the ASHP (and allowing for the DHW 300L tank)?

 

Grateful for your thoughts and experience, as always...

GREAT SCOT!-Drawing.pdf GREAT SCOT!-Water Supply Summary.pdf

[JohnMo]

13 minutes ago, Great_scot_selfbuild said:

temperature requirement of 46 degrees

But have floor output set at 50W/m² which huge, suspect you are nearer to 15 or below.

 

We are 192m² living space, on 300mm spacing, our flow temp goes up to about 34 at -9,no way you would ever need over 30.

 

Our heat loss with a very poor form factor is about 3.5kW, in fact just bought a 4kW heat pump.

 

Do your own calculations, do not over size your heat pump. 7kW sounds huge.

 

 

[MikeSharp01]

41 minutes ago, Great_scot_selfbuild said:

passive-level of thermal and airtightness performance,

So your heat demand should be 10W/m² or there abouts. Unless you have a huge house or live in the Arctic circle you probably need less than 3kW.

[Nick Laslett]

@Great_scot_selfbuild, something not quite right with the LoopCAD inputs. Ask for the Heat Loss Summary Report, that will include target room temp. I have attached mine as an example. Do you know the design outside temp?
 

My flow temp today with a 1°C outside temp, is 28°C. Room temp is 18.4°C. 
 

In my original LoopCAD design, I had target room temp of 23°C, which had a flow temp of 30°C, with a floor temp of 25°C. 
 

Looking at the attached report, the floor temp is consistent with mine, but the flow temp is 46°C. What is stopping the heat getting out of the floor? Maybe they forgot to add the insulation under the floor in LoopCAD. 

The Heat Loss Summary report will show the heat loss through walls, windows and floor. 
 

 

Edited Tuesday at 17:59 by Nick Laslett

[SimonD]

4 hours ago, Great_scot_selfbuild said:

Which comes first - chicken or the egg / ASHP or the UFH

 

Neither! It's a good room-by-room heat loss calculation that must come first which you then use to size the heat source, followed by the emitter design. Then you look at flow rate, velocity and pressure loss through your pipework.

[Great_scot_selfbuild]

23 minutes ago, SimonD said:

 

Neither! It's a good room-by-room heat loss calculation that must come first which you then use to size the heat source, followed by the emitter design. Then you look at flow rate, velocity and pressure loss through your pipework.

The simplest summary I’ve heard - TVM.

[Great_scot_selfbuild]

2 hours ago, Nick Laslett said:

@Great_scot_selfbuild, something not quite right with the LoopCAD inputs. Ask for the Heat Loss Summary Report, that will include target room temp. I have attached mine as an example. Do you know the design outside temp?
 

My flow temp today with a 1°C outside temp, is 28°C. Room temp is 18.4°C. 
 

In my original LoopCAD design, I had target room temp of 23°C, which had a flow temp of 30°C, with a floor temp of 25°C. 
 

Looking at the attached report, the floor temp is consistent with mine, but the flow temp is 46°C. What is stopping the heat getting out of the floor? Maybe they forgot to add the insulation under the floor in LoopCAD. 

The Heat Loss Summary report will show the heat loss through walls, windows and floor. 
 

 

 

Thanks - this is what I was missing! (Because I can’t use loopCAD I wasn’t aware of the inputs it required, and they never asked for any thermal performance inputs, so god knows what they’ve based their LoopCAD design on.

 

How on earth is any customer meant to navigate this industry with such levels of poor customer service and engagement (I’m sure the very expensive consultancies make their money on the basis of this ‘quality’ elsewhere in the market.

 

 

[Post and beam]

Just to throw some more real world figures into the mix. Our new build is not at Passive levels. Even at -5 degrees the other night and the day times either side of that the highest flow temp i have seen is 38 degrees.)

And that is to maintain a ground floor at 23 degrees ( dont ask). WC is currently .5, i have played around with this parameter.

 

46 degrees flow does seem very high.

[JohnMo]

2 hours ago, Post and beam said:

ground floor at 23 degrees ( dont ask)

Typical of someone looking at the numbers instead of how it feels. We have room sensors in each room, but they have no numbers to see. We sit very comfortable at 19 to 20. In fact I get complaints if it gets warmer especially at night in bed.