In-slab UFH layout and suitable ASHP?

[zzPaulzz]

Hi all.   Please critique my UFH design.

 

Background -  my house is borderline passive so has a very low calculated heating load of 2kW for 0.5º external + 20º internal.  Ground floor area is 110m2, total treated floor area is 171m2.  The house superstructure is the MBC Passive frame with wall U at 0.12 and roof at 0.1. It will sit on a 175mm reinforced concrete raft set into a tray of 350mm XPS for a U value of 0.09. Air tightness of 0.6 ACH. MVHR for ventilation.  Windows and doors average U of 0.85.

 

Based on advice here and elsewhere (Heat Geek mainly) I've gone with a zone-less UFH design that treats the slab as a massive storage heater. This layout has 6 roughly equal 83m loops of 16mm Pert-Al-Pert.  The loops will be placed on the lower reinforcement mesh 50mm above the insulation and 125mm from the top of the slab.  There is an another mesh set 50mm from the top surface but I don't want to use that for the UFH to avoid any the puncture risk from the internal stud work nails.

 

Once the slab is up to temperature I will feed heat in low and slow using weather compensation.  My current thinking is to use a Vaillant Aerotherm Plus 3.5kW ASHP with open flow UFH and no additional pumping/buffer etc.  Apart from two electric towel rails upstairs I will have no other heating.  Sensible? FYI, I'm still looking for someone to design this for me who knows what they are doing ... 

 

Also would welcome thoughts on a good brand/source for pipes and manifold please? There seems so many out there that it's hard to choose! 

 

[Conor]

A better way to run it is during overnight low rate, and let heat radiate out during the day. That's what we do, very rarely the heat is on during the day. You'd maybe want an ASHP with a slightly bigger output in that case, we've a 9kW heatpump for 253m² floor area, similar U values.

[zzPaulzz]

Thanks Conor, that's a option for sure. I'll have battery storage too so can load shift to avoid peak rate when running the ASHP all day. 

[JohnMo]

As @Conor we run generally in storage heater mode (on E7 tariff). We have a 13kWh battery, but on a typical winter day (rubbish PV) we ended up drawing in full price electric too often when doing WC.

 

We have a heat demand of about 3.5kW but at -9, and with 6kW we cannot batch charge the floor fully at off peak rates when really cold.

 

Thick floor will buffer heat very well without overheating the house. In March we charged the floor in the day on excess PV. So was getting a CoP of 5 from free electric, running heat pump at 33 to 35 degs.

 

Unlike a radiator system, thick screed allows you to oversize the heat pump with zero penalty. But use a low hysterisis thermostat +0, -0.1 is best failing that +/- 0.1.

 

Is +0.5 degs a realistic design temperature for a heating system? I would be looking at -3 as a minimum - Or are you living somewhere hot?

 

Look on eBay there are 8kW ASHP on there which looks good for less than £1500.

[JohnMo]

Forgot to mention not withstanding your wee concrete thickness I wouldn't have pipes under walls. Many would, but no chance of hitting pipes if they are not there. And easy to design out now.

 

No need for UFH in the plant room, under staircase or in cupboards.

 

Does your UFH design output enough heat to provide heat for upstairs?

[zzPaulzz]

Thanks @JohnMo I missed off the minus sign so my design temp should be -0.5º. This is what the PHPP uses based on my location so I didn't change it. I'm in Suffolk BTW. 

[LnP]

How will you heat your DHW in winter? If it's with the heat pump, what recovery time will you get with 3.5 kW?

[zzPaulzz]

3 minutes ago, JohnMo said:

Forgot to mention not withstanding your wee concrete thickness I wouldn't have pipes under walls. Many would, but no chance of hitting pipes if they are not there. And easy to design out now

No need for UFH in the plant room, under staircase or in cupboards.

Does your UFH design output enough heat to provide heat for upstairs?

I'm probably mistaken but I am assuming that with 125mm of concrete above the pipe the heat will dissipate so much by the time it reaches the surface that there's no point worrying about what's above it.  

Great question about the upstairs heating!  I've seen a lot of people say they never use their upstairs heating, and that the MVHR will help balance heat around the house.  I should be pumping enough heat in as the heat load calcs from the PHPP indicate I need 11W per m2 of treated floor area, so I've just take that for the whole house and divided by the ground floor area to get ~18W per m2, or ~2kW.

 

[Nick Laslett]

@zzPaulzz, I went with Wundatrade for my pipes. 
 

16mm Hdpe-Al-Pex Pipe - 100m

 

https://www.wundatrade.co.uk/shop/home/water-underfloor-heating-solutions/joist-floor-heating-pipe/16mm-hdpe-al-pex-pipe/

 

I went with Emmeti for my manifold and valves. 
 

Emmeti Topway T2 Plus Manifold.

https://www.cli-mate.co.uk/product/emmeti-topway-t2-plus-manifold/

 

Cli-mate are based in Bury St Edmunds on the Rougham industrial estate.

 

For my design I used the serpentine counterflow pattern, with 200mm spacing and tried to make each loop 100m long. 

https://www.lhs.plumbing/post/the-comprehensive-guide-to-underfloor-heating-pipework-layouts

 

Since I did my install, design opinions seem to have evolved. So basically filling the whole slab in uniform UFH pipes, ignoring walls, rooms, kitchen, etc. This is what I gathered from a recent @Nickfromwales post. The reason behind this is the whole slab is going to sit at a constant low temp, and the heat will slowly be absorbed by all the concrete as a giant heat sink. So no hot spots, or cold patches. 
 

If you want to look at any of this kit, you’re welcome to visit. I have off cuts of the pipe and you can see the manifold. 

Edited April 28 by Nick Laslett

[zzPaulzz]

1 minute ago, LnP said:

How will you heat your DHW in winter? If it's with the heat pump, what recovery time will you get with 3.5 kW?

Another great question that I just put to ChatGPT.  Roughly 3.15 hours in winter.  Need 10.5kWh to raise cold fill water at 5º to 50º DHW. Estimate output of Vaillant Aerotherm 3.5kW in winter is ~3.2kW with COP of 2.5-2.8. For summer this time drops to 2h40m.  I think that's ok in both cases given the thermal capacity of my slab. 

[Nickfromwales]

24 minutes ago, zzPaulzz said:

Another great question that I just put to ChatGPT.  Roughly 3.15 hours in winter.  Need 10.5kWh to raise cold fill water at 5º to 50º DHW. Estimate output of Vaillant Aerotherm 3.5kW in winter is ~3.2kW with COP of 2.5-2.8. For summer this time drops to 2h40m.  I think that's ok in both cases given the thermal capacity of my slab. 

I’d install a bigger UVC and plan a cheeky 2-3hr hour top up via the immersion heater(s), timing this to happen for the last 2 hour window of cheap rate nighttime juice, a-la Octopus, so you’ve got a good supply of DHW for the morning shower runs etc. 

 

This would be a winter strategy btw, but assuming solar and a base temp assumed at 50-55°C when heated off the HP, then having higher capacity can only be a benefit here as you’ll have more energy stored in the same size cylinder when heating via the immersion. Add PV and topping up the DHW at midday for the evening (at a time where an hour or two of HP > DHW won’t effect anything / anyone, this is win win.

 

I’d try to keep the HP away from doing DHW overnight, ever, during winter, especially with such a lack of headroom with a 3.5kW unit. When you do the sums and include defrost and the expedited demise of the HP it makes more sense to use cheap electricity directly (near 100% efficiency) via the immersion when running the HP just doesn’t make sense.

[zzPaulzz]

29 minutes ago, Nickfromwales said:

I’d install a bigger UVC

Sorry Nick, what is a 'UVC'?

[Nickfromwales]

1 minute ago, zzPaulzz said:

Sorry Nick, what is a 'UVC'?

Unvented hot water cylinder. 

[zzPaulzz]

That's a nice strategy.  Will mull that over. 

[Nickfromwales]

6 minutes ago, zzPaulzz said:

That's a nice strategy.  Will mull that over. 

Indeed. There’s just not enough difference to go and beat the crap out of a tiny HP. Then it’s far more justifiable for a small HP. 
 

If you steer away from this then seriously consider a 5kW HP to get headroom for DHW production. 

[garrymartin]

If I've read this correctly, that's a 175mm slab with two sets of mesh reinforcement. Can I ask what site conditions have led to that design choice?

[Bramco]

39 minutes ago, Nickfromwales said:

 

I’d try to keep the HP away from doing DHW overnight

Our HP is only on ever on heating. If you set it to both, then hot water takes priority over heating and in the winter, if you want to only use really cheap rate electricity to charge an UFH slab, then you'll find that the HP won't give you enough heat.   

Our strategy, is to have the HP on for the full 6 hours of O Go Intelligent at 7p doing heating.  The PV diverter is programmed to heat the hot water during the same period. We also charge the batteries during this time period.  The during the day if there is PV, it goes to the batteries, then the diverter and then to export.

 

[marshian]

59 minutes ago, zzPaulzz said:

Another great question that I just put to ChatGPT.  Roughly 3.15 hours in winter.  Need 10.5kWh to raise cold fill water at 5º to 50º DHW. Estimate output of Vaillant Aerotherm 3.5kW in winter is ~3.2kW with COP of 2.5-2.8. For summer this time drops to 2h40m.  I think that's ok in both cases given the thermal capacity of my slab. 

 

Couple of points I think your water calcs are a bit pessimistic (Appreciate worst case is probably safer but......)

 

1. Mains water does vary but around 10 deg C is as cold as I've seen in Norfolk

 

2. The tank is always going to have some bleed of hot to cold so at the point of re-heat it's more likely for the bottom of the tank to be in the 20 deg area and top to be low to mid 30's as a start point

 

This mornings water heating with a gas boiler was 4.5 kW used

 

Start Temp Top 34.1 C

End Temp Top 51.7 C

 

Start Temp Bottom 26.4

End Temp Bottom 51.1

 

Tank is about 115 Litres

[zzPaulzz]

Just now, garrymartin said:

If I've read this correctly, that's a 175mm slab with two sets of mesh reinforcement. Can I ask what site conditions have led to that design choice?

Clay and lots of trees. 450mm ring all the way around and through the middle too..  Some might say the SE was hedging their bets. 

[saveasteading]

4 minutes ago, garrymartin said:

175mm slab with two sets of mesh reinforcement.

Good question. That is the spec for a warehouse with 10 tonne forklifts and 8m high racking.

But it seems to be typically specified with some kit type "raft" packages, all built on polystyrene, to spread the load over the eps.

I would need some convincing of the logic.

 

Genuine structural rafts can be optimal on low strength but not hopeless ground, to spread the load of the walls.

[Nickfromwales]

9 minutes ago, saveasteading said:

Good question. That is the spec for a warehouse with 10 tonne forklifts and 8m high racking.

But it seems to be typically specified with some kit type "raft" packages, all built on polystyrene, to spread the load over the eps.

I would need some convincing of the logic.

 

Genuine structural rafts can be optimal on low strength but not hopeless ground, to spread the load of the walls.

Especially when the ring and intermediate beams are doing the majority of the work. 175mm is over egged for a domestic residence as the slabs then distribute weight over the area so benefits become nigh on inconsequential over 100mm imho. 
 

280mm for a previous job, and then the guy wanted to put UFH on top in a screed FFS. In a 1.5 story house!!

 

I challenged, with reason, and they reduced to 230mm(!!) iirc, but agreed also to then lift the slab, inset the UFH pipes, and feck the screed off. Saved the client 5 figures in 2 sentences. And I was late to the meeting lol. 

[zzPaulzz]

Regarding the slab design, this was really stressing me but I've decided to go with my SE's design.  I don't want to reopen that debate, sorry.  

[Nickfromwales]

1 minute ago, zzPaulzz said:

Regarding the slab design, this was really stressing me but I've decided to go with my SE's design.  I don't want to reopen that debate, sorry.  

We’re just filling our boots, no need to ‘go there’ . You must go the distance with your SE, obvs, but if there’s any threat of an earthquake I’m coming to stay with you 🙂

[Conor]

Glad I didn't tell you guys about my 200mm thick slab with 300mm / 450mm thick ring beams  🤣

[zzPaulzz]

It was all great feedback, I really appreciated it.