Advice required

[ChrisDL]

Evening - rattling through future issues.

 

Couple months ago I made use of @Jeremy Harris heat loss calculator to try and size a ASHP, link to that here. Moving onto UFH - from these calculations worst case scenario I may need in the depths of a very rare, dark and cold winters day 7kW per hour of heat energy to maintain our 21 deg C. 

 

Our barn conversion has a ground floor of about 300m2 and 200m2 upstairs, I have been recommended to go belt and braces and install UFH upstairs, more so because we have a concrete first floor. We are planning to have a polished concrete floor across most of the ground floor.

 

I have read that UFH concrete has, albeit maximum, heating output of 100W/sqm. We have one central room of 100 sqm which could potentially throw out 10kWh itself. Now I would like to believe my plumber will recognise this but I am not so sure. Hypothetically with UFH upstairs and downstairs we only need 14W/sqm. 

 

This is where my concern lies and I would like either reassurance or someone to find fault in my calculations/assumptions. Since otherwise is UFH upstairs unnecessary? Or is it possible to run UFH that low? Spacing the loops extra wide, very low flow temperatures or rates.

 

Appreciate any feedback/thoughts.

 

Chris

 

[Adsibob]

I will let others comment on the detail of your calcs, but on your temp question as long as you get the right mixer/manifold you should be able to run your UFH at whatever temp you want - within reason. I didn’t specify the manifold/pumps so my contractor installed fairly cheap stuff. The coldest water it will mix is 35C but that works just about ok for most of our floor surfaces. I’m sure you could get one that did 30C. That should be quite a bit cooler than 35C.
i wouldn’t space the pipes too much as otherwise you will get cold spots. Better to play with diameter of pipes and temp of water.

[ChrisDL]

Thank you Adsibob - yes I have calculated a spacing of 250mm. Going to see if I can drag @JohnMo back into this to check the sheets I have filled in below. I don't understand the circuit pressure. I assume I should be doing this on a per room/ loop basis rather than the whole house?

 

Wondering how common it is to be running such low flow temperatures...

 

[jack]

8 hours ago, ChrisDL said:

Wondering how common it is to be running such low flow temperatures...

 

It depends on your pipe spacing and power output requirements.

 

I've run my ASHP at 25 ºC with pipes at 200 mm spacing, with weather compensation pushing that up to 27 or 28 ºC in very cold weather. The house has been perfectly comfortable with only that heat output. That's with a polished concrete slab on the ground floor and no other heating anywhere else. (To be fair, the bathrooms do get a little chillier than desirable, so we've installed some electric heaters to take the chill off in the morning.)

 

The mixing valve on the UFH manifold is set at 30 ºC and is really just used as a safety cut-out in case a valve fails somewhere in the setup and the ASHP tries to deliver water intended for heating our unvented cylinder to the UFH circuit.

 

Your calculated 14 W/m² isn't that much more than the Passivhaus limitation of 10 W/m², so I can't see it being a serious issue.

[JohnMo]

My heat demand is 14 W/m2 of the top of head.  My spacing is 300mm, flow temps is about 27.  The delta T of 10 isn't realistic it's likely to 4 or lower at those flow temps.

 

High delta T means lower input of heat into the floor for a given flow temp, as the mean flow temp is lower.

 

The higher the flow temperature the more likely the room temperature will overshoot.

 

So it's all a bit of a suck and see and a balance trade off.  Also floor finish has quite a big influence.  So what correct in one room may need tweeking in another by changing the delta T.

[ChrisDL]

Thank you @jack and @JohnMo. Looks like the my calculations are not too detached from reality! Which is reassuring. My expectations are based on my folks old place which absorbed it's own mass in energy daily and was never really warm. I will plan for 250mm spacing which should give us plenty of margin for adjustment.  @JohnMo could you explain circuit pressure for me? Is this essential information to know?

[JohnMo]

It just a pressure drop (head loss) calculations, you can use to size the pump in big systems, but not really needed in domestic installs as the pump in most cases will be variable speed or automatically adjust to the pressure and flow required.

[Temp]

I'd put pipe in the floor upstairs just in case.