Loopcad Vs CIBSE UFH outputs - Same parameters, one 50W/m2 one 70.3W/m2...

[Ewan]

I've got a Wunda 16mm UFH kit installed at 100m pipe centres in Screed after Wunda told me it would put out 45W/m2 with my floor covering at 35 deg flow for 20 deg air temp. On later enquiry it turns out the sales assistant used the wrong info, they mistook the thermal conductivity of my 7mm SPC LVT flooring @ 0.151W/m-K to be thermal resistance, when it actually works out to 0.05 m2K/W.

 

Turns out it will actually put out 70.3W/m2, which would have caused me to say "no thank you, I'll take 150mm PCs instead" (house is well insulated).

 

So I'm now down a bunch of cash and have a system that is way overpowered for our ASHP design temp due to their error. I'll likely have to turn off half of the loops completely (any problems with that?).

 

I put the floorplan into Loopcad out of interest, but can't for the life of me get anywhere near their figures; it gives 50W/m2 for the same parameters.

Wunda say they use the CIBSE Underfloor heating design and installation guide for outputs, fair enough.

Which is closer to reality??

[JohnMo]

Won't worry over it, just run at a lower temperature - problem fixed.

[Ewan]

Can’t go lower than 35deg without changing all the upstairs rads which I sized for 35.
 

I figure best I can do is if I close off 3 of the 6 loops in the open plan room, and 1 of the 3 in the lounge.. even then it looks like 35 deg will see us down to -10 DOT 

 

I guess that’s not such a bad problem to have 😄

Edited January 25 by Ewan

[Mike]

3 hours ago, Ewan said:

I'll likely have to turn off half of the loops completely (any problems with that?).

I'd turn down the flow rate on everything instead.

[Ewan]

Can you balance away over provision of 140-170% on 10 out of 11 loops though?

Heating engineer on twitter said I may "strangle the flow" with that much restriction.

Edited January 26 by Ewan

[Iceverge]

Can you not just lower the flow temp? Nothing bad will happen to the rads upstairs other than they'll feel colder to touch.

 

You should be able to get a better COP too. For this reason I think I would spec closely spaced UFH if i was to install an ASHP.  

[JohnMo]

9 hours ago, Ewan said:

Can’t go lower than 35deg without changing all the upstairs rads which I sized for 35

It would be unusual to have a mixed system without a mixer on the UFH. Get a suitable mixer for for low flow temps, ESBE electronic or IVAR low temp mixer and you can flow any temps you like elsewhere.  Something like the ESBE mixing doesn't need any mixing so for me works better. I flow between 30 and 36 to a fan coil and 28 into the floor without issue using the ESBE. You zero need for a buffer.

 

If you are well insulated your upstairs rads may never come on anyway.

[JohnMo]

50 minutes ago, Iceverge said:

. For this reason I think I would spec closely spaced UFH if i was to install an ASHP.

Really doesn't make a huge difference once flow temps get below 30. I am on 300mm centres and have the flow set at 28 in to the UFH via a mixer. If I was at 100mm centres I think my flow would have been 26.

 

The other issue not talked about is you cannot get the heat pump to run reliably below 30 anyway with UFH (or I can't). Reason is the heat pump does cycle on off at low loads, it requires around 8 degree temp difference between target and return temp to initiate a restart. House is at 20.5 degs, floor is slightly warmer.  So if I run the heat pump at 28, I need the return flow at below 20, which is cooler than the floor and cannot happen.

 

First time I tried, heat pump ran for an hour, got to target temp and stopped, then sat idle for the next 12 hrs as the floor was never going to drop to 20 degs or below as the house was warmer than that.  So I run a slightly elevated temperature and mix down to 28. Or you batch charge the floor at an increased temperature and use as a storage heater.

[Ewan]

I was concerned about short cycling too, but that not coming back on at all due to DT is now also a worry!

The initial plan was to open loop the whole house and run both rads and UFH at the same temp, no hydraulic separation, no buffer (we're short on space. perhaps a volumiser, but I've got 200l in the UFH alone so not sure), full system volume available for defrosts, which a mixing valve would interfere with. Admittedly, this plan has not been gone over yet by an actual installer. I've got a heat geek lined up for that a bit later in the year, and this was what they suggested an approach could look like (pending them going over the install choices I've made so far!)

 

2 hours ago, Iceverge said:

Can you not just lower the flow temp? Nothing bad will happen to the rads upstairs other than they'll feel colder to touch.

 

You should be able to get a better COP too. For this reason I think I would spec closely spaced UFH if i was to install an ASHP.  

In theory yes but then according to the heat loss those rooms are under powered.

[JohnMo]

1 hour ago, Ewan said:

but I've got 200l in the UFH alone

That's a lot of pipe and a lot of loops. My 6kW heat pump circulation pump has a duty point 16L/min, which my 7 loops and fan coil use the rate up. Your issue will be having the flow rate available to satisfy lots of loops and radiators. Mixing down from 35 to 26 may actually get you out of hole, as there will be lots of water recycling within the UFH system instead of all going through the heat pump, which I think would start to cause issues requiring a buffer and additional pumps.

 

Defrost don't seem to an issue, with a mixer, because it takes the heat required from the return water, I believe.

[Ewan]

1600m of pipe, accidentally mis-sold to me off the back of their incorrect calcs. 11 loops. Makes me think that knocking a few loops off would help with the pump too.

I'll keep mixing in mind then when it comes to it, thanks.

 

[Ewan]

Just to clarify, you've sacrificed Weather Comp control of your UFH floor temp as the mixer is always set at 28deg?

Edited January 26 by Ewan

[JohnMo]

11 minutes ago, Ewan said:

Just to clarify, you've sacrificed Weather Comp control of your UFH floor temp as the mixer is always set at 28deg

 

Sort of, but it runs in the background weather compensated.

 

Heat pumps cycle at low loads, so the average flow temp drops anyway. So although mixer is trying to control 28,if the flow is below 28 because it on an off cycle it gets what it gets. So mean flow temp through the UFH drops.

 

The ESBE mixer has two set points, T and T2. So have a house thermostat set at 20, above 20 the ESBE runs at T and targets 28 deg, if the house drops below 20 (sub zero temps), it goes to T2, which is set to 36, so it can take whatever the flow temp from the HP which is based on WC curve. The WC is set for the fan coil.

[Ewan]

Sounds great! a smarter mixer than a TRV style one would certainly make a difference here.

[JohnMo]

ESBE make a few versions, I got an ESBE CUA110, super cheap of eBay and the three port valve and actuator. Just run Salus auto balance actuators on each loop, always powered on to give a fixed dT. Removed all the room thermostats as they just added nothing to control of room temps. Just have a single thermostat in the hall.