Underfloor Heating vs Traditional Combi in New Build

[Johnny Jekyll]

May I ask for your educated opinions on whether we should go for underfloor heating or a traditional combi system.

 

First to paint the picture.

 

We’ve started our first self-build in South London, a small 3 bed detached house (total of 130 square metres across two floors), which includes a family bathroom, a master bedroom en suite bathroom and a downstairs toilet.

 

Foundations are already laid (before the pandemic hit which then paused the project), so we are out of the ground at least.

 

The floor make-up is essentially the 375mm reinforced raft foundation outlined above, to 1200 gauge DPM to 150mm PIR T&G insulation to separation DPM to 75mm screed to hopefully vinyl floor finish. The walls will be 100mm medium dense blocks (both inner and outer to render outside) with 100mm PIR Cavity Therm full fill insulation. The loft insulation will be 300mm of wool rolls at ceiling level.

 

Although we are not strictly going for Passivhaus, we are aiming for high levels of air tightness using membranes and tapes and installing an MVHR system.

 

So overall a decent spec.

 

We are stuck on the heating system though. We originally brought into the idea of downstairs under floor heating throughout (installed within the 75mm screed) and upstairs radiators. And to potentially use an air source heat pump and even 4kw PV Solar on the south roof. But after leaving the project alone for a while, we are now thinking is that really the way to go?

 

Pros of ASHP and UFH include no radiators, even heat and warm underfoot.

 

Cons of ASHP and UFH include more planning and specialist installation (time we don’t really have), more space for all the system bits (space we don’t really have), greater costs (even with the upcoming 2022 £4k clean heat grant after RHI), noise from the ASHP, time for the slab to heat up and cool down, likely greater maintenance due to more system bits.

 

The question is should we go with a traditional high quality smart controlled combi boiler instead e.g. Viessmann with nice rads? Even the best stuff will be quite a bit cheaper. We can hide all the pipework within the walls and posi-joists. The installation is more straight forward with no water tanks or peripherals and saves space. And we don’t need to spend the time understanding the technologies or asking dozens of questions.

 

Your experiences and independent opinions would be greatly appreciated.

 

Also, we do like the look of the new Marley SolarTile integrated PV solar roof system (don’t know if anyone has used or seen them). We would need PV Solar to get through the SAP Calcs anyway, and the installation costs of such a system would be offset by not needing to install all the additional clay tiles and associated labour on the south roof.

 

Thanks very much.

[nod]

There isn’t really a comparison 

The heat from UFH is a move even and comfortable heat 

We've found running costs to be cheap 

and haven’t used the rads in the bedrooms yet 

[Johnny Jekyll]

5 hours ago, nod said:

There isn’t really a comparison 

The heat from UFH is a move even and comfortable heat 

We've found running costs to be cheap 

and haven’t used the rads in the bedrooms yet 

Thanks for replying. Any thoughts from others with UFH systems? We are also considering using UFH with a traditional gas boiler system? 

[Trw144]

I have UFH with a gas system boiler and works very well. I certainly wouldn't revert back to radiators having lived in two houses now with UFH (on all floors). My only concern with a combi is whether it would modulate down enough for a low energy house - I'm sure others here will have experience/a view on this and chip in shortly.

Edited March 25, 2021 by Trw144

[Johnny Jekyll]

3 minutes ago, Trw144 said:

I have UFH with a gas system boiler and works very well. I certainly wouldn't revert back to radiators having lived in two houses now with UFH (on all floors). My only concern with a combi is whether it would modulate down enough for a low energy house - I'm sure others here will have experience/a view on this and chip in shortly.

Thank you, sorry what do you mean 'modulate down enough for a low energy house'?

[Trw144]

6 minutes ago, Johnny Jekyll said:

Thank you, sorry what do you mean 'modulate down enough for a low energy house'?

Combi boilers have to have a high output to produce hot water quickly for your hot water (around 30kW), which is going to be much higher than the output you need for your heating your house (which I suspect will be less than 5kW even in the cold weather). If it can't modulate (reduce its power) down this low then it will cause the boiler to cycle on/off/on/off which reduces its efficiency and increases wear on the boiler.

Edited March 25, 2021 by Trw144

[Johnny Jekyll]

26 minutes ago, Trw144 said:

Combi boilers have to have a high output to produce hot water quickly for your hot water (around 30kW), which is going to be much higher than the output you need for your heating your house (which I suspect will be less than 5kW even in the cold weather). If it can't modulate (reduce its power) down this low then it will cause the boiler to cycle on/off/on/off which reduces its efficiency and increases wear on the boiler.

Thanks for explaining.

[PeterW]

15 hours ago, Johnny Jekyll said:

The question is should we go with a traditional high quality smart controlled combi boiler instead e.g. Viessmann with nice rads? Even the best stuff will be quite a bit cheaper.

I don’t see how you’re making this cost comparison ..?? Decent Viessman with all the toys is just over £1500 and then needs install etc so add £1000. Rads aren’t expensive but install of pipe work, TRVs etc you’re adding say £1500 to do a decent job with quality kit. So £4k all in. 
 

You now have to service the boiler annually (£100 or so) rads will need painting and replacing at some point, and you have an ongoing rise in energy costs which you can’t offset. 
 

Flip side is UFH on 2 floors (not really necessary but for comparison) would be £700 for materials, couple of towel rails is £200, UVC (which could go in the attic) is £850. UFH is probably £1200 to install and zero maintenance thereafter. UVC is £400, annual check should be £50. So that’s £3350 all in.  If you can follow a plan then you can even self install UFH - saves you money. 
 

Now look at heat source for that and you have choices. Small ASHP would be £3500, install another £800. Potential to offset this using the carbon grants and you’re now looking cheaper than your gas boiler.
 

Also you can self consume, with summer months PV straight into the hot water tank as you’re not getting any FIT worth talking about. 

[Johnny Jekyll]

1 hour ago, PeterW said:

I don’t see how you’re making this cost comparison ..?? Decent Viessman with all the toys is just over £1500 and then needs install etc so add £1000. Rads aren’t expensive but install of pipe work, TRVs etc you’re adding say £1500 to do a decent job with quality kit. So £4k all in. 
 

You now have to service the boiler annually (£100 or so) rads will need painting and replacing at some point, and you have an ongoing rise in energy costs which you can’t offset. 
 

Flip side is UFH on 2 floors (not really necessary but for comparison) would be £700 for materials, couple of towel rails is £200, UVC (which could go in the attic) is £850. UFH is probably £1200 to install and zero maintenance thereafter. UVC is £400, annual check should be £50. So that’s £3350 all in.  If you can follow a plan then you can even self install UFH - saves you money. 
 

Now look at heat source for that and you have choices. Small ASHP would be £3500, install another £800. Potential to offset this using the carbon grants and you’re now looking cheaper than your gas boiler.
 

Also you can self consume, with summer months PV straight into the hot water tank as you’re not getting any FIT worth talking about. 

 

Thanks very much Peter. May I have your opinions about ASHP noise?

[Johnny Jekyll]

Also may I ask, in terms of UFH kits, are there any preferred manufacturers / suppliers? E.G. ProWarm / Wavin / Thermoflow / John Guest / Polypipe / Nu Heat? Or are they all pretty equal in terms quality? Are cheaper alternatives just as good?

[PeterW]

UFH - Wunda will do all this for you for 1/3rd of the price of the “specialists” for a better product. 

[jack]

2 hours ago, Johnny Jekyll said:

Thanks very much Peter. May I have your opinions about ASHP noise?

 

I have a 5kW Panasonic Aquarea that's about 6 years old. You can't hear it when it's doing UFH. There's a slight noise when it's doing DHW, but it's still pretty quiet.

 

To minimise noise, it's important that you mount it properly (ground mounting much better than wall mounting for noise) and connect it with flexible connectors to avoid vibration transmission to the house. 

[Johnny Jekyll]

1 hour ago, PeterW said:

UFH - Wunda will do all this for you for 1/3rd of the price of the “specialists” for a better product. 

Thanks Peter

[Johnny Jekyll]

1 hour ago, jack said:

 

I have a 5kW Panasonic Aquarea that's about 6 years old. You can't hear it when it's doing UFH. There's a slight noise when it's doing DHW, but it's still pretty quiet.

 

To minimise noise, it's important that you mount it properly (ground mounting much better than wall mounting for noise) and connect it with flexible connectors to avoid vibration transmission to the house. 

Thanks Jack