Retrofit UFH help - 2013 New Build

[foundationsmcr]

Hi, for context I'm extending a property that was built in 2013 to current building regs.

 

I'd like to install a wet UFH system running off our existing Combi boiler, and have ruled out digging down (cost and disruption - we have a small young family to consider!). Obviously I'd like to minimise the threshold increase in terms of floor build up as best I can.

 

We are planning to finish the downstairs with LVT flooring.

 

I'm lost with all the various options and suppliers, but need an estimate to build into our remortgage costs.

 

How do I go about narrowing down the spec and cost quickly? What info do I need and how do I find it out to help secure an estimate.

 

I understand our flooring is a concrete base, but I'm no builder and don't know where to start with this.

 

Can anyone help?  Many thanks 

[JohnMo]

  On 11/03/2025 at 16:25, foundationsmcr said:

understand our flooring is a concrete base,

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Is you current floor insulated? If so how much insulation or do you know the U value?

 

What is the motivation to move to UFH? 

 

What boiler do you have (make and model)?

 

Will you still have radiators elsewhere in the house?

Edited March 11 by JohnMo

[Andehh]

We did an overlay system in a 2006 house, 10 years ago. 25mm Insulated tray,  on tile adhesive, 16mm pipe, latex screed. 50mm build up, but some careful shallow ramps with the neighboring carpet corridors meant it worked.

 

Heat up times similar to radiators, so was controlled as one.

 

Afraid this is the only pic I have from back then!

 

[foundationsmcr]

  On 11/03/2025 at 16:27, JohnMo said:

Is you current floor insulated? If so how much insulation or do you know the U value?

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Thanks for your help.  I assume we have 100mm PIR within the floor build up, I'm trying to locate the original plans that came with the house when we bought it.  As I've said, it was built to Building Regs in 2013 so is there not a standard for that time?

  On 11/03/2025 at 16:27, JohnMo said:

 

What is the motivation to move to UFH? 

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We'd just prefer not to have rads as it restricts where we can place furniture.  We also like the concept of a warmer feeling underfoot.

  On 11/03/2025 at 16:27, JohnMo said:

What boiler do you have (make and model)?

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Baxi Duotec 2 Combi 

  On 11/03/2025 at 16:27, JohnMo said:

Will you still have radiators elsewhere in the house?

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Upstairs yes, but we're planning to remove them all downstairs.

[foundationsmcr]

  On 11/03/2025 at 17:06, Andehh said:

We did an overlay system in a 2006 house, 10 years ago. 25mm Insulated tray,  on tile adhesive, 16mm pipe, latex screed. 50mm build up, but some careful shallow ramps with the neighboring carpet corridors meant it worked.

 

Heat up times similar to radiators, so was controlled as one.

 

Afraid this is the only pic I have from back then!

 

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Thanks for sharing.  What was the overall build up from the original floor if you don't mind me asking.

[Andehh]

About 50mm overall... 10mm tile adhesive.... 25mm insulated trays...15mm thin screed then tiles on half, carpet on half. About 40sqm in total.

[JohnMo]

Be worth watching this, it's talking about heat pump but equally applicable to gas. System used is easy and doesn't take up much space, you need to screed over, but you get good heat transfer.

If you are keeping the rads as they are you will need 2 flow temps one for radiators and the other fir UFH so a mixer is needed. You can several different mixers done good some not so good. An Ivar mixer is good, link to first one that came upon Google, shop around for best price.

 

https://underfloorparts.co.uk/product/ivar-uni-mix-underfloor-heating-ufh-pump-mixing-kit-underfloor-heating-manifold-control-unimix/?srsltid=AfmBOoqyNurwUf8R844Nbc0pdaRMSUPkMqZy8wvhKLBpAY1c1CW1wiun

 

Link to discussion here

 

Problems you could have

Zones - the more thermostats you put in the system the worse its going to perform. Small zones and short cycling is an issue.

 

I would suggest you explore weather compensation, you would have TRVs only on the bedroom radiators, everything else would just tick away on weather compensation.

 

Your boiler does allow weather compensation, adding an external sensor, will allow heating water temperature to change in response to changes in outside temperature. This has a big impact on gas consumption for the better.

 

 

 

[foundationsmcr]

Ok so I have some quotes now and would really value some insight/honest views of the various options.

 

I'm likely to go for an overlay system clipped into matts with Screed poured over, as I understand there is sufficient insulation in our existing floor given that I'm trying to avoid a significant build up.

 

Here are the specs I've been looking at with various contractors 

 

Option A

 

Area: 90sq m
Depth: 30mm
Compressive Strength: up to 25 N/mm
Membrane 500 gauge

System Type: Tackernail
Pipework:16mm PERT
Pipe Spacings:150 mm
Manifold Type:Warmin Mixing Manifold with Circulation Pump
Manifold Size:8 Ports
Heating Controls:4no. x Heatmiser NeoStat 230v Wired Thermostat 1no. x Heatmiser UH8 Wiring Centre

Warmin LowPro 25 UFH Panels

• Low height panel of just 20mm

 

• Allows 25mm Flowing Screed on Solid Floors*

 

• Allows 35mm Flowing Screed on Floating Floors*

 

• Tested and certified using Thermio+ Flowing Screed

 

Multi-zone Underfloor Heating System (Wired Control)

• Warmin 16mm x 2mm PERT Pipework

• Warmin Stainless Steel Manifold with flow meters, flow and return temperature gauges, water pressure gauge and auto air vent

• Warmin Mixing Valve Unit with Grundfos UPM3 Circulation Pump

• Warmin 230v Actuators

• Heatmiser NeoStat 230v Wired Thermostats

• Heatmiser UH8 Wiring Centre

 

 

 

[foundationsmcr]

Option 2

 

1x 8port manifold along mixing kit and additional ufh mod. FASTWARM

pump (Wilo/Grundfos pump, Emmeti/Giacomini Stainless Steel

Manifold),

 Supply and fit visqueen (dpm) along relative primer, adhesive(where required) 

 Run Pipework on ufh System (12mm diam, spaced 150-100mm pipe centre)

Supply and fit 2x HoneyWell (or similar) S-Plan Zone Valves,

 Run 22mm copper feeds between manifold and boiler.

Hive mini thermostat

Tarmac Gypsum based liquid screed 25mm

 

 

Please share honest views on these systems!

 

[JohnMo]

Don't like either option - Run the lot as a single zone. Single thermostat that switches pump on. No actuators needed or wiring centre. Get an Ivor mixer and only an Ivor mixer - assume your bedroom rads need a higher flow temperature. If you upsize the rads run whole lot direct from boiler with no mixer and WC.

 

I would do neither option, I would have the ground slab fully engaged instead of isolated with insulated panels. Use egg crate style panels. Only use 16mm pipe not 12mm. 12mm performance isn't great you need higher flow temperature

 

[foundationsmcr]

Hi John, thanks for the swift response.  Just help me understand that a little more.

 

Single zone I get, so no room sensors or differentials.

No actuators or wiring centre? What does this do?

We're not planning to change upstairs rads so does that mean we'll need the mixer, so what's an "Ivor" mixer specifically? 

Slab fully engaged instead of insulated panels. I thought the Option A system would be a plastic tray like the attached image, is this the same as "egg crate style"?

16mm pipe, will our boiler support this flow temp and rate?

Last question, would there be any benefit in connecting the system to our current Nest smart thermostat?

 

Thanks again, this forum is a font of knowledge and fantastic support network.

 

[foundationsmcr]

Sorry, I should add, the 12mm is probably driven by the fact we are trying to minimise build up due to laying this on existing floor.

[Nickfromwales]

Agree to only use 16mm pipes here, but also accept that the different floor coverings will yield different performance / comfort characteristics, eg one will emit more w/m2 and the other less, so heat up times will differ from the set time to be at x temp at x time etc. This can be combatted by using smart self-learning stats per room / space, as these will decide how long a zone took to get to temp and then adjust (called setback) accordingly.

 

I'd also recommend just clipping to the existing slab, and then screeding over with as little (suitable) product as you can get away with. There are self levelling cementitious screeds that can be laid at an inch thick or thereabouts quite reliably, but these (iirc) require a particular primer to provide the requisite 'purchase', so they don't part ways with the slab after being installed.

 

If you use clip rails and fix straight down as I did here, 

 

 

then you will also have additional points of mechanical purchase via the clip rails. These I nailed down with a Spit Pulsa gas powered nail gun, firing 22mm masonry nails directly into solid concrete. £500 gun that paid for itself twice over on this one project, but you can hire also.

 

These pipes were at 100mm oc, so that provokes the snail pattern, so the pipes only have to turn 90^o vs serpentine where the pipe turns 180^o at the end of every run; you need to not exceed the minimum bending radius of the pipe or it will kink!

 

Prepare to have to install a 50L buffer tank (aka volumizer) for a bit of buffering for the boiler, as a lot of combis of that era won't want to modulate down to where you'd want them to be, if driving UFH directly.

[JohnMo]

  On 14/03/2025 at 21:53, Nickfromwales said:

setback

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Do you actually save anything doing setbacks? Never really seen any savings especially with UFH. Can't you just balance flows to alter output via flow rate adjustment. 2 Deg change in dT is about 10% change in output.

 

This then eliminates, 8 actuators, 4 or 5 smart thermostats and a buffer. So about £400, plus electrician to install most of it.

[Nickfromwales]

  On 14/03/2025 at 22:52, JohnMo said:

Do you actually save anything doing setbacks? Never really seen any savings especially with UFH. Can't you just balance flows to alter output via flow rate adjustment. 2 Deg change in dT is about 10% change in output.

 

This then eliminates, 8 actuators, 4 or 5 smart thermostats and a buffer. So about £400, plus electrician to install most of it.

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In a real life situation, in a 2013 B regs build which will be poorly insulated and very draughty (sorry OP), I think it will be an absolute necessity to guarantee rooms achieve the desired temps and then stay there.

 

This advice is case specific to this thread and OP, and reflects the unknowns, build fabric, and different floor coverings.

 

Balancing this, room by room, via flow rates on the manifold....... is just going to be wishful thinking afaic.

 

£400 to assure you of excellent boiler longevity, and user comfort? Money very well spent IMHO.  

 

"This ain't no passive house baby!"

 

edit: closer to £800 tbh with buffer and plumbing alterations, but I still would recommend this in a heartbeat.

[foundationsmcr]

Thanks again both. 

 

Our floor covering will be LVT throughout so all the same.

 

Nick, ref clipping to the slab, is the primer used to bond the screed to the slab or this clip rails you refer to, or the pipe? I'm a little confused. 

 

Please explain the reference to the boiler modulating and the volumizer.

[Nickfromwales]

Any bare concrete will have dust / other surface contaminants so you primer that before laying any kind of screed or adhesive at a thickness of less that 50mm (normally above that you won’t need to prime but the installers would usually decide this).

 

If you pour some screed onto a dry slab you’ll likely be able to chip it back off in one piece as it’ll have just adhered to the surface dust etc. A bit like you greasing a cake tin.

 

The clip rails add to the mix as they provide a mechanical connection, but should not really be relied on as the only measure. Thin screed systems <30mm need a specialist primer which stays tacky for a long time.

 

Boilers modulate, a bit like you controlling a light with a dimmer switch, so they can match the heat demand as closely as is possible. To drive a non modern boiler into UFH you’ll likely force some short cycling, the boiler repeatedly lights, heats up, can’t get rid of enough heat so hits the internal stat set point, then waits to cool slightly, then lights again, and repeat. That’s can lead to reduced longevity and much deceased efficiency. 
 

A buffer / volumiser adds a runoff volume of water, if you think of heat as air, it’s like you blowing up a balloon quickly, and then just letting the air out slowly over the next 10 mins, then repeating, therefore taking the ‘short’ out of short cycling. That make sense?

 

[marshian]

  On 11/03/2025 at 20:34, foundationsmcr said:

Baxi Duotec 2 Combi

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Which specific model?

Max Heat Output?

Do you still have the installation manual for it? (because if you do that will tell you exactly what min modulation is)

 

If I look on line for installation manuals - the range of min modulation across different version is pretty damn large - Some have min modulation of 3.7 kW on CH others are as high as 11kW depending on the Version (Peak Output)

 

IMO you will struggle if your actual boiler doesn't modulate down to a min of around 5kW - you'll almost certainly need a volumiser to keep cycling to a minimum.

 

Many Combi boilers have massive heat outputs to meet the needs of HW but this compromises the space heating side (CH) of the boiler.

 

 

[JohnMo]

  On 15/03/2025 at 10:38, marshian said:

almost certainly need a volumiser to keep cycling to a minimum

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I always thought that, my original setup had one and I zoned to death. But have found UFH in concrete/screed absorbs heat really well due to a massive density compared to air. One of the reasons to keep the existing floor engaged is it will soak up the heat well. Your issue comes with zones and not having the flow rate to keep the boiler happy, especially if you only have one tiny zone open - there just isn't any flow. Short cycling will occur - A volumiser doesn't help with flow rate either. They help with water capacity but a fully engaged floor is just way better.

 

I can quite happily run my boiler doing about 8kW or heat pump doing 5kW for hours at a time without house overheating (using 0.1 Deg hysterisis thermostat). UFH just isn't the same as a radiator system, especially when you can have plenty of concrete to work with, as the OP can.

[foundationsmcr]

  On 15/03/2025 at 10:38, marshian said:

 

Which specific model?

Max Heat Output?

Do you still have the installation manual for it? (because if you do that will tell you exactly what min modulation is)

 

If I look on line for installation manuals - the range of min modulation across different version is pretty damn large - Some have min modulation of 3.7 kW on CH others are as high as 11kW depending on the Version (Peak Output)

 

IMO you will struggle if your actual boiler doesn't modulate down to a min of around 5kW - you'll almost certainly need a volumiser to keep cycling to a minimum.

 

Many Combi boilers have massive heat outputs to meet the needs of HW but this compromises the space heating side (CH) of the boiler.

 

 

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Our boiler is a Baxi Duotec Tec 2 - 33 GA model

Max heat output is 33KW DHW and 29.6KW CH (Condensing)

I think the Min/Max is 5.1 / 29.6

CH flow rate temp 25-80 degrees (+-5 degrees)

 

I'm not sure if I'm reading the correct element of the manual.  I've snipped some tech details and attached them.  Will this require a volumizer? As mentioned, we will still be running Rads upstairs.

 

I'm reading some conflicting into on single or multi zones. We have young kids and doors are open/closing constantly, so would it therefore be better to have one single zone? 

 

If attaching pipe to the concrete, how do you prevent the screed leaking out with no DPM type barrier down to contain it?

 

Thank you all, this is a steep learning curve!

Edited March 16 by foundationsmcr

[Nickfromwales]

  On 16/03/2025 at 14:21, foundationsmcr said:

If attaching pipe to the concrete, how do you prevent the screed leaking out with no DPM type barrier down to contain it?

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A very light gauge membrane will be necessary unless you go for a dry (aka traditional) screed. The perimeter expansion skirting that you put around the room will come with a similar gauge membrane attached that drops down and the you tape the two together.

 

Depends on the slab and how many breaks there are. If none you can do what I did for the job with the pics above and just tape the expansion skirting membrane to the screed, and then no membrane is needed for the main areas. 

 

  On 16/03/2025 at 14:21, foundationsmcr said:

Will this require a volumizer? As mentioned, we will still be running Rads upstairs.

 

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The volumizer is usually only necessary when the heat circuit is short or the rate at which the energy can be consumed is low, so if the rads are always on an running whenever UFH is on you could possibly get away with it, but when designing a system you MUST always specify it for the absolute worst case scenario so it can cope (and comply with the boiler MI's which will ask you to connect the appliance to a system that compliments it sympathetically, eg removes the possibility of it ever routinely short-cycling).