Review/critic UFH design

[umiq88]

Initial design has come back.

 

Can anyone spot any issues? 

[JohnMo]

Why dedicated loops in halls, they have clumped together pipes at side of halls, then added dedicated loops to halls. Just spread the pipes leading through halls to room at 200mm centres.

 

100mm centres in kitchen seems excessive?

 

Do you really need all those thermostats?

 

I would do bathrooms at 150mm centres and the rest at 200mm. I did all ours at 300mm centres - zero issues.

 

 

[ProDave]

As above DO NOT fit UFH loops in hallways.  I made that mistake (on professional advice) in my first self build over 20 years ago.  It seems the "professionals" do not learn.  The hallway has only a tiny bit of external wall / door to actually loose heat and lots and lots of internal walls which adjoin heated rooms so zero heat loss.

 

UFH pipes pass through them, they will give some heat, not that you need it.

 

How well insulated is the house?  We are in the Highlands so a cold climate and find 200mm pipe centres throughout perfectly fine so someone would have to present me with a very good reason why any of your rooms should need 100mm?

[BotusBuild]

That looks like a lazy design to me.

 

As John says, both the long thin halls can be heated by using the loops from other rooms feeding back to the manifold, so that's two loops saved straight away.

 

Have they had the heat loss per room figures to work with? The 150mm centres on the loops maybe too much for a new build well insulated house (assumption made on this being a new build).

[JohnMo]

Just looked back at some of your posts @umiq88. Looks like your house is the same size as ours around 200m².

 

Your issue with so many loops could be flow rate from heat pump. You may need more flow than a small heat pump can deliver (around 1m³/h).

 

By way of comparison we have 7 loops in total. One thermostat. Below is what our plan looked like and how it was actually laid out. Even at -9 we only need to flow at about 35 to batch charge the floor. Average room height excluding lounge is 3m, lounge is 6m.

 

 

Edited April 13 by JohnMo

[umiq88]

Thanks everyone for the replies. Yes seems to be a lazy generic design.

 

House is new build and spec is following 

206sqm

U values are

0.15 - External Walls - 110mm cavity PIR and 50mm insulated plasterboard on external walls(37.5+12.5)

0.12 - Floor - 150mm PIR 75mm screed

0.13 – Roof – 400mm Rockwool or 200mm PIR

0.7 – Windows 

1.4 – Doors

 

I'm wiring myself so won't be putting in the stats but is there a downside to the closer pipe centres?  Would turning down the flowrate have the same impact as larger spacing. 

 

I do want to make sure I have lowest flow temp possible in order to get max efficiency. Heat pump will be Daikin 8kW

 

What questions would you ask or how would you go back to supplier to improve design.

Edited April 13 by umiq88

[ProDave]

5 minutes ago, umiq88 said:

Thanks everyone for the replies. Yes seems to be a lazy generic design.

 

House is new build and spec is following 

U values are

0.15 - External Walls - 110mm cavity PIR and 50mm insulated plasterboard on external walls(37.5+12.5)

0.12 - Floor - 150mm PIR 75mm screed

0.13 – Roof – 400mm Rockwool or 200mm PIR

0.7 – Windows 

1.4 – Doors

 

I'm wiring myself so won't be putting in the stats but is there a downside to the closer pipe centres?  Would turning down the flowrate have the same impact as larger spacing. 

 

I do want to make sure I have lowest flow temp possible in order to get max efficiency. Heat pump will be Daikin 8kW

 

What questions would you ask or how would you go back to supplier to improve design.

speak to designer / architect  / builder and get a blown in beads or celulous cavity fill.  WAY more chance of actually achieving theoretical calculated U value of walls.

 

As noted several times, forget UFH in halls.

 

Use SAME spacing throughout if you are uncomfortable at 200mm use 150mm.  That will mean rooms more likely to be balanced so easier to fine tune with flow rates and use as one or 2 zones.

[JohnMo]

1 hour ago, umiq88 said:

do want to make sure I have lowest flow temp possible in order to get max efficiency

Sounds a big heat pump. But ok for batch charging.

 

Simple table for working out flow temp based on room heat loss.

Just draw a line between centres and W/m² to get mean flow temp. One you get low W/m² flow rate doesn't change dramatically with pipe centres changes. You may have to extend the W/m² line downwards.

 

Remember a heat pump flows a min 25 degs in heating mode. Most the time the average heating temp is closer to 7 degs than design temp, so half the heat input is needed.

 

MWT is mean water temp, so mid point between flow and return temp.

 

[Nickfromwales]

6 hours ago, JohnMo said:

Your issue with so many loops could be flow rate from heat pump. You may need more flow than a small heat pump can deliver (around 1m³/h).

You’d just hydraulically separate the heat source from the UFH pump then, so very easy to do tbh. As long as the heat pump can deliver heat higher than the design kw requirements it’ll be fine.

 

I’m a big fan of putting more pipe (therefore more volume of water and cross sectional area for transferring heat energy) into the floor, and that add volume; this can often result in no need for a buffer. 
 

This all boils down to the unique circumstances, as no two scenarios are identical, so needs design input and some thought. 

[JohnMo]

1 hour ago, Nickfromwales said:

big fan of putting more pipe (therefore more volume of water and cross sectional area for transferring heat energy) into the floor, and that add volume; this can often result in no need for a buffer

But isn't bucket loads of tonnes of concrete or screed a big enough buffer already?

 

Thermal conductivity for concrete ranges from 0.8 to 2.5 W/(mK), thermal conductivity of water is 0.598 W/m·K at 20 °C. So the floor should suck heat away as quick as you can give it, as long as you have delta T between floor and the water. So why would you need or want a buffer?

 

1 hour ago, Nickfromwales said:

just hydraulically separate

Aren't you just adding inefficiency by doing that? An additional pump, possibly distortion on each side of the separation? Leading to increased flow temperature, for little or no advantage.

[Nickfromwales]

29 minutes ago, JohnMo said:

But isn't bucket loads of tonnes of concrete or screed a big enough buffer already?

All depends, has the OP stated they have a thick slab? Great if they have, but even then you have a limited amount of time to get heat into such a thing if you want to batch heat on off peak electricity.

 

31 minutes ago, JohnMo said:

Aren't you just adding inefficiency by doing that? An additional pump, possibly distortion on each side of the separation? Leading to increased flow temperature, for little or no advantage.

As above, and always case specific. The very minimum of flow temp is relative to how quickly you can inject heat energy, so low temps with a thick slab requires ‘on constant’ (long and low) vs a thinner screed which will need to be brought up to temp much quicker, and lose it just as quickly soon after. 
 

Horses for courses. 

[umiq88]

Thanks for all the advice here. Slab is 75mm Cemfloor which is cement based screed. 
 

My thoughts were you need enough volume and surface area in the system to keep the heat pump temp down and avoid it cycling. 
 

I’d prefer not have a second pump as I’m trying to keep it to a minimum of components. 
 

@JohnMo I get your approach but do you see a downside other than the. Maybe pressure loss and materials to additional pipe work? 

[JohnMo]

2 hours ago, umiq88 said:

I get your approach but do you see a downside other than the. Maybe pressure loss and materials to additional pipe work?

Pressure loss isn't an issue, it's driven by the longest single loop, rather than the accumulation of all the loops.

 

Just think it's a zero thought design - that's what we did on the last 10 jobs - cut and pasted into yours. It will not be easy to balance, because the output of each floor emitter is the max it can be for a given floor area. The analogy being designing a radiator system to fit max wall space available not the heat output required.

 

As you will see on my plan each room output is set to match the heat required, this allow very easy fine tuning of room temperature by a slight alteration of loop flow rate. It's also good for buffering the floor because the heat output matches what you need, once buffered your room temp becomes a combination of heat loss and floor temp.

 

Low temps are good for CoP, but that isn't the whole story. My average CoP over this winter is about 3.6. which isn't that great - but, I only run in cheap rate or powered by PV. Both cases I want zero cycles while there is a demand for heat. Run a very simple WC curve starting at 33 and finishes at 36. Generally when running the CoP is 5 to 6 in the day and about 4 at night. The rest of the time I use the floor as a buffer to stop a summer house from being cold, so circulation pump runs 24/7.

 

Have run full WC and generally I run just over 30 at -9, the rest of the time 28ish. But it's way cheaper to run on cheap rate or excess PV at elevated flow temperature. I get cycle times of about 6 hrs no what the outside temperature