Wet ufh system

[RedSpottedSev]

Hello,

 

I started a project hastily and it has somewhat escalated. The information here has been amazing.

 

We have a 5 bed detached farmhouse (half/half previous extension, half solid 50cm stone walls, half insulated cavity). Very exposed to winds and rain.

 

We made the decision to put ufh downstairs and upstairs, but admittedly I really should have had a proper heat loss done and spent the extra on a heating engineer. Lack of knowledge and poor advice at the time... but here I am.

 

Build up is 70mm hardcore/blinding, DPM, 150mm concrete slab in mesh, 150mm PIR, 16mm ufh pipes at 150mm centres clipped to insulation with 75mm sand/cement dry screed to SR2.

 

Upstairs, removed old floorboards, leveled the joists, extra reinforced those that needed it, noggins etc... 5cm PIR between joists, soundproofing tape, routed floorboard for 16mm pipes at 150mm spacing. Primer, Perimeter 6mm expansion strip tape and then 1 to 3cm flow screed over the top to level, fill in boards.

 

Upstairs is 6 valves for 5 bedroom zones. The bathrooms are on electric ufh mats.

Downstairs is on 7 valves doing 4 zones.

I am likely going to add another 5 valves for 3 zones for an extension in the future.

All runs are below 100m, 16mm at 150 centres. My thought was this way I can reduce flows and wouldn't be left with a cold house (cant insulate internal or externals of the old part of the house).

 

The 3 manifolds have a manual mixing valve and each with its own pump.

Each manifold has a heatmeiser control that I am told connects to actuators that go on the valves. Then each room will have a thermostat that switches the actuators on and off when the rooms hit the set temperatures.

 

I had assumed that my current boiler/cylinder would have been enough to supply the system.

But my builder's plumber says that it is a very old boiler 10+ years and they would recommend an upgrade.

 

Finally to my question and thanks for reading so far:

My heatloss is around 15 to 18kw. I have ruled out ASHP due to only having access to cost/space and single phase electricity.

If I am going to upgrade my heating system, I would like to have weather comp option and hot water priority.

 

I have had lots of mixed messages and designs about this from different plumbers, with huge varying cost.

So here I am asking for option on what people have done with wet underfloor only set ups. Current recommendation has been a heat only system with a low loss header that supplies ufh and the cylinder...

 

In my head I was thinking a 4 pipe system boilder that has inbuilt domestic hotwater priority. 

Some kind of communication centre that means the heatmeisers can call for hotwater at a different temperature to the hot water cylinder.

Do I need a low loss header for the 3 manifolds to connect to and blend from?

 

Have I shot myself in the foot with the kit I already have (manual mixing valves and heatmeiser) to have weather comp? I am happy to invest time tweaking flow rates etc... to get a more stable heatoutput to stop the actuators constantly opening and closing.

 

Sorry if you think this is better suited to the the ufh forum

 

Thanks again

 

Edited October 27 by RedSpottedSev

[JohnMo]

For me, I would reduce to the number of zones down to a minimum. Your issue is you will have a slow system response and you room temps could fluctuate quite a lot of you are bouncing of a thermostat.

 

Next choice of mixers, IVAR or electronic (ESBE etc) only. Or do you even need a mixer anywhere? Can your system boiler provide enough flow for all loops on its own? If not, heat only boiler and keep manifold mixer and pump. Then a single thermostat per manifold. Run an almost flat WC curve of about 35 to 40 degs.

 

Low loss header - no

 

Flow needed will drive system or heat only I suspect.

 

I would start by looking at the longer loop and all pipes to and from that loop and look at pressure drop. Once you have that, it will set pressure drop for system, then add all the flows you need together all loops). Now you have a pump duty point. You can now make a sensible choice of system or heat only.

 

Thermostats, implement in each room - but don't give all active control, use for analysis for system balancing. Don't install manifold actuators. Use a manifold wiring centre to just switch pump on or off if you keep the mixers. No mixers and balanced system still leave off you add later if you have a problem room that overheats.

[JohnMo]

Forgot to mention your current mixing valves are unlikely to work or work well with WC. They may require to run at circa 50 degs to have any authority of manifold temperature.

42 minutes ago, RedSpottedSev said:

Some kind of communication centre that means the heatmeisers can call for hotwater at a different temperature to the hot water cylinder.

A boiler suitable for X or W plan or 4 pipe will have suitable control for switching between the two duties (CH and DHW).

 

Look at Viessmann, Atag, Intergas boilers. Size about 20 to 24kW.

 

If you are doing a new cylinder, buy a heat pump cylinder (3m² coil) so you get decent condensing and or rapid reheat when doing DHW.

[RedSpottedSev]

Thank you,

 

My understanding is that the current system boiler cant provide enough flow. So I am looking at a new boiler and will upgrade cylinder as we need 250L vs the 170L currently.

 

My priority is avoiding constant on/off yoyo switching of each room and making sure they stay as constant comfortable temperature as outside temperatures fluctuate. Efficiency gains would be a bonus.

 

I was sold the manifolds with pump and mixer initially before I had done any research (and hadn't planned upstairs ufh and getting rid of all radiators) so I have some of the kit ie pump, mixer, manifold, heatmeiser control and thermostats.

 

So, talking about the ideal set up (which I may have to do in stages and accept some wastage of things already ordered), I can swap out the fixed/manual mixing valve on the manifolds to ESBE. I haven't come across IVAR but will read about them.

Each manifold will then have ESBE/grundfos upm3 auto as a pump station and a hestmeiser.

 

Manifold 1 - main downstairs (can reduce to 2 zones)

Manifold 2 - kitchen and extension (high heat loss - glass sliders etc.. reduce to 3 zones)

Manifold 3 - upstairs (single zone but option to shut off guest room when not in use)

 

So... can I have a set up where if and when properly balanced, the heatmeiser and actuators close/open rooms that might not be in use, and the rooms in use will just have thermostats for info only, with the esbe running weather comp?

 

How do you decide where the call for heat comes from for each manifold? (Coldest room and then adjust other flow rates down?)

Also do I need extra wiring pulling up alongside the 5 core (heatmeiser) for the ESBE valves?

 

Which boiler and control unit is likely going to offer the easiest communication/connection set up to the heatmeisers and esbe?

 

Thanks again, I appreciate I have a lot to learn yet..

 

[JohnMo]

Not sure you really save anything by not heating rooms, the unheated rooms just suck in the heat from other rooms, so net energy input really doesn't change.

 

Would really try to move to a single zone per manifold, the loops in that manifold get balanced to get room temps you want. Having room sensors in each room allows you run all loops or none just by switching pump. So example, room 1 is targeting 20 and rooms 2,3 and 4 targets 20.5. if any room drops below there target all loops come online and later all go off. But better still balance the whole lot to run WC. If suitably balanced all should be ok.

 

Issue I found was if you are shutting down loops you can run into the situation where only one zone is calling for heat and the boiler goes into meltdown, short cycling and all the gain just get burnt with excess gas consumption.

 

Share your current boiler and mixer model/make?

[marshian]

On 27/10/2025 at 16:47, JohnMo said:

Not sure you really save anything by not heating rooms, the unheated rooms just suck in the heat from other rooms, so net energy input really doesn't change.

 

Just to add to @JohnMo's comments - Covered very well below with a bit of the maths

 

 

On 27/10/2025 at 16:47, JohnMo said:

Issue I found was if you are shutting down loops you can run into the situation where only one zone is calling for heat and the boiler goes into meltdown, short cycling and all the gain just get burnt with excess gas consumption.

 

Hell yes - Been there done that - "Smart TRV's" that can call for heat from the boiler even if only one room needs it will result in massive energy wastage as the boiler fires up and then potentially short cycles until demand increases

 

[RedSpottedSev]

Thank you @marshian and @JohnMo. I have been doing some more reading and your posts in the ufh section. That video is maths heavy, but I am working through things back to front. In that I am doing my heat loss, then using the formulas from heat geek and urban plumbers to calculate what flow I would need and use that as a starting point for each loop (work in progress).

 

Old boiler is Worcester 430/i unknown age but on last service it was not looking good. I think at least 12 years old.

New boiler: will go in loft with new cylinder (heat pump coil) to be both decided on compatability of current ufh kit i have (specifically the heatmeisers and how to get weather comp working)

 

I will be doing my best to run each manifold as a single zone, only using actuators on the loops as on/off or very high/ low set back temperatures.

 

The manifolds and it's pump (Grundfos upm3 auto)/mixer is from Thermrite. X2 will be downstairs in garage, supplying the downstairs area. One upstairs for bedrooms.

 

I am assuming if I was to do the above, I would still need to change my mixer valves to ESBE electronic valves for proper weather compensation? Do they work with the heatmeisers?

 

As each of the 3 manifolds have it's own pump on different flows, do I need to do anything special? Can I get a boiler that deals with this set up without special kit? Im confused about LLH/kimbo/closed coupled Tee...

 

Right now I want to make sure I've pulled up all cables needed from each manifold as we are due to start plastering. Currently have 6mm and 2.5mm going up to loft for boiler and cylinder, and 5core from each manifold to future boiler position. Do I need more for ESBE? Do I need temperature monitors on each inflow for the manifolds?

 

Thanks again for the help and education!

 

PS the upstairs orange coloured rooms will be guest rooms which I wanted to zone off, but from what I gather it is better to continue to keep warm even when not being used?!

 

 

 

Edited October 31 by RedSpottedSev

[marshian]

16 minutes ago, RedSpottedSev said:

Thank you @marshian and @JohnMo. I have been doing some more reading and your posts in the ufh section. That video is maths heavy, but I am working through things back to front. In that I am doing my heat loss, then using the formulas from heat geek and urban plumbers to calculate what flow I would need and use that as a starting point for each loop (work in progress).

 

 

It seems that way initially but like most things but you do get you head around it eventually

 

16 minutes ago, RedSpottedSev said:

PS the upstairs orange coloured rooms will be guest rooms which I wanted to zone off, but from what I gather it is better to continue to keep warm even when not being used?!

 

Unheated rooms will steal heat from wherever they can - let me help with an example

 

My house (stupid T shape) has front bedroom above front hallway both having 3 external walls and with the added bonus of north facing - being a  "lets not be wasteful with energy sort of chap" (Mrs Alien says "tight b'stard") I didn't heat the front hall for most of the time just bringing it up to temp for the few hours we might use it (arriving and leaving). The Rad in the bedroom above had to have much higher flow rates as well as a higher set point than the other bedrooms (one because of the 3 external walls and two because some of the heat was being stolen by the hall below)

 

Now you might say but heat rises and yes it does but a better statement is that heat always moves to cold and it doesn't care if down is a direction it can move in.

 

So now I heat the front hall to 17 deg - it's cooler than the rest of the house but it does have an internal door to separate it - as a result the bedroom above needs a lower flow rate to the rad because the difference between the two rooms is much narrower

 

So by all means run rooms at lower temps but consider the impact on rooms above or below is all I would say - if you size rads or UFH for low flow temps to provide the heat needed based on a heat loss calculation you could find that the room won't reach target temp if it's having it's heat stolen by a room above or below or even beside it.

 

[RedSpottedSev]

Thank you

@JohnMo can you point me to a resource on how to do this properly:

 

"I would start by looking at the longer loop and all pipes to and from that loop and look at pressure drop. Once you have that, it will set pressure drop for system, then add all the flows you need together all loops). Now you have a pump duty point. You can now make a sensible choice of system or heat only."

 

Do you mean I calculate the distance from boiler to manifold 28/22mm and then add the longest loop which I think is 98m at 16mm. Somehow calculate the pressure drop? Then find the overall flow I would need through each zone based on my heat loss and heat output from each floor?

 

One more thing I cant get my head around...on DHW priority, when cylinder calls for heat, boiler diverts to dhwp, then do the pumps on each manifold keep circulating water around each manifold....but what happens if the mixing valve opens to draw heat and dhwp is running.

- do the manifolds not fight each other trying to draw from each other? Then do i need a directional flow control

- or heatmeisers are smart enough to turn the pump off when cylinder calls for heat?

- I know you said no to Low loss header, but this was why I thought it would let the pumps on each manifold to circulate at their leisure without fighting

 

Thanks again,

 

[marshian]

24 minutes ago, RedSpottedSev said:

One more thing I cant get my head around...on DHW priority, when cylinder calls for heat, boiler diverts to dhwp, then do the pumps on each manifold keep circulating water around each manifold....but what happens if the mixing valve opens to draw heat and dhwp is running.

- do the manifolds not fight each other trying to draw from each other? Then do i need a directional flow control

- or heatmeisers are smart enough to turn the pump off when cylinder calls for heat?

- I know you said no to Low loss header, but this was why I thought it would let the pumps on each manifold to circulate at their leisure without fighting

 

Thanks again,

 

 

In a DHWP set up the flow to rads or UFH stop - the heat source re-charges the HW cyl and then the system reverts to heating

 

House doesn't or shouldn't lose much temperature in the time it takes to do HW unless it's a massive tank

[JohnMo]

Been thinking about how best to do this system

 

I would do 3 zones - one for each manifold, so no actuators are needed on the manifold, retain you pumps and mixers, but control pump on manifold with the zone thermostat - no wiring centres are needed, but install if you have them (if you want).

 

It may be way easier to install a buffer, but size to about 100L. This will keep boiler happy.  Nibe (they do heat pumps) have good design buffers, there are a couple on eBay at the moment at decent prices. Buffers can be installed in various ways, I would recommend a 2 port setup.

 

So a 2 port is just that, you use only 2 ports. One at top, one at bottom buffer. Each port has an oversized tee, so if using 22mm pipe do a 28mm tee at each port.  This gives a hydraulic separation between heating and boiler.

 

So you operate the two sides independently, the buffer you add a thermostat and the thermostat controls the boiler only.  Set to about 15 degs above max UFH flow temp this allow UFH mixer to have some authority.

 

Boiler has one job keep buffer happy. UFH just draws from buffer, but when boiler is on, will draw water direct from the boiler and flow and return will by-pass the buffer, as zones switch off, excess flow from boiler will pass through buffer.

 

For DHW you have a 3 port diverter between buffer and boiler. Boiler flips to DHW, runs flat out to charge cylinder cylinder, buffer continues to feed the UFH. At end of DHW, boiler moves to charge buffer. You don't need zone valves if you don't want them.

 

The other option is a fully open system, no buffer, no mixers and feed whole lot from a good boiler that does weather compensation.  You may need to add additional pump to the system return. But you need to be prepared to fully balance system, adding zones will miss with a lot of settings and you would run full WC.

 

 

[RedSpottedSev]

Thank you,

Great advice and I think I am getting close to my design, feedback and corrections welcome please.

 

Greenstar 8000 plus with internal integral diverter kit, to allow for DHW priority and setting temp for cylinder and ufh with weather comp. Becomes a 4 pipe system. 

 

2 pipes to a Telford Tempest indirect unvented heat pump cylinder 300L. Hot water feed has to travel 8m to my hotwater manifold that then supplies each outlet around the house. I am deciding if I need an electric heater/circulater as a secondary return or if there is a better set up.

 

Other 2 pipes to Buffer tank/LLH (to be decided as I am still figuring this out)

From this the pipes to my 3 ufh manifolds (2 downstairs, 1 upstairs).

If I only use 2 heatmeiser UHF-RF V2 (upstairs/downstairs), I can have the downstairs heatmeiser controlling the 14 loops, as I understand each 'zone' on the heatmeiser can control 4 actuators.....

Then I can link the 2 heatmeisers as primary and secondary, with the primary calling for heat direct from boiler on a switch live.

So the idea being if any manifold/heatmeiser calls for heat, the boiler kicks in, and heats the LLH/buffer and the manifold pump draws the heat from there.

 

I do like the alternative of the boiler just concentrating on keeping the buffer tank at a set temperature, and then heatmeisers just activate the manifold pump and not communicate with boiler. But if feels that in hit weather the boiler will essentially have another 100L of water to keep warm alongside the unvented cylinder, is this right?

What do we gain with this instead of a large heatstore that hotwater and ufh all come out from?

 

Which one of these set ups is more straightforward for setting up/service and wiring. Which is going to offer me more tweaking for efficency?

 

Thanks again

Edited November 7 by RedSpottedSev

[RedSpottedSev]

I have just realised a flaw in my plan, as one Heatmeiser can't control 2 manifolds, so it is looking like the buffer tank set up.

[JohnMo]

On 08/11/2025 at 00:19, RedSpottedSev said:

only use 2 heatmeiser UHF-RF V2 (upstairs/downstairs), I can have the downstairs heatmeiser controlling the 14 loops, as I understand each 'zone' on the heatmeiser can control 4 actuators

If you are controlling the two down stairs manifolds as a single zone, you do not need actuators on the manifolds at all. All you need to do is start and stop the pumps. This is done via the pump outlet of the UFH controller - use one controller and take a second power signal to the other manifold. Then you have one thermostat calling for heat for the whole down stairs. But maybe better install several thermostats and allow either to call for heat, any thermostat starts the pumps. Any cold spot is likely to be followed soon after by other ones, so allow whole downstairs to run.

 

With buffer you get hydraulic seperation so you don't even need two port valves in the system.

 

Upstairs. Maybe get a simple stand alone wireless thermostat - something like Computherm Q20RF, very adjustable, come with free standing and wall brackets.

 

LLH are something for industrial systems and need very careful sizing avoid.

 

On 08/11/2025 at 00:19, RedSpottedSev said:

in hot weather the boiler will essentially have another 100L of water to keep warm

Your boiler if you add an outside temperature sensor will manage that for you, you should be able to set at say 15 degs outside temp switch off heating. But I would also add a WC curve, say start at 20 outside flow 20, so the boiler does nothing when it's warm anyway. Then say 45 when -3. You may have to fiddle with the settings depending on how your mixers respond and house temperature. Set the mixers at say 38 as a safety net only.

 

The idea is the flow temp takes an age to heat up house and then you don't allow it cool too much and the heating just ticks away topping up as energy is lost.

[RedSpottedSev]

Thanks again @JohnMo

 

With the buffer tank, as I will likely only heat it with a boiler or stored solar in battery stack... would it be OK for the buffer to be a Direct Unvented one with an emersion heater? Does this part of the system also need expansion vessel and pressure release safety valves too?

 

With it being a 100L buffer, as an alternative design, would a straight up heat only boiler supplying a 300 to 400L heatstore (I've seen the advance Appliances pre plumbed multifuel, 28mm outlet for ufh https://www.advanceappliances.co.uk/product/70-multi-fuel-universal-thermal-store-sfuts/) not be an easier install? 

Is there a significant amount of energy saving with the previously discussed buffer and cylinder system vs one big heatstore?

 

[JohnMo]

2 minutes ago, RedSpottedSev said:

Direct Unvented one with an emersion heater?

That is doable.

3 minutes ago, RedSpottedSev said:

Does this part of the system also need expansion vessel and pressure release safety valves too?

It's all part of the heating system so capacity gets added to system volume and expansion vessel works on system volume.

 

A heat store (thermal store) works with a boiler. But not a fan of pre plumbed as they normally have a load of tat included, you will never use and you have to pay for. Some alternative at a better price point and maybe way better performing, can be specified with a 6.3m² DHW coil on a 300L cylinder.

https://thermal-store.co.uk/thermal-store-price-lmt.php

8 minutes ago, RedSpottedSev said:

stored solar in battery stack.

Use in house, gas is 6-7p to heat things up, electric 27p per kWh and export is 15p, so why heat stuff with expensive electric.

 

 

[RedSpottedSev]

Thank you,

I have made some progress with the design and decisions. I have put the design in a file attached. I have absolutely zero experience/expertise in this so please do critique/add and modify. If there are critical safety things missing then please also do tell me. I am not fitting this myself but do want to make sure the plan is as tight as it can be.

I would say that I appreciate it isn't the most efficient design as I have had to make do with some kit that I bought previously, and make future compatibility options available.

- With regards to the wiring, the 8000+ has a temp sensor input for LLH. Am I correct I can use that connection for the buffer tank? This way the boiler can modulate the heatoutput to the buffer?

- I would appreciate help on what settings I would then select with this set up in the 'Hydraulics' of the boiler system menu. Attached

On 09/11/2025 at 09:29, JohnMo said:

Your boiler if you add an outside temperature sensor will manage that for you, you should be able to set at say 15 degs outside temp switch off heating. But I would also add a WC curve, say start at 20 outside flow 20, so the boiler does nothing when it's warm anyway. Then say 45 when -3. You may have to fiddle with the settings depending on how your mixers respond and house temperature. Set the mixers at say 38 as a safety net only.

 

 

- I am still not clear/got myself in a muddle with what happens when it is 12 degrees outside vs 2degrees outside. The boiler will modulate to keep buffer at set temp. But the mixing valves on the manifold are manual. So when there is a call for heat from any sensors, the manifold will draw a set temp (mixing valve temp) to the floors. Do we then accept that if it is 2 degrees it may take much longer to heat up if flow temp is set lower, and then perhaps there will be a chance of overshooting/yo yo if the flow temp was set higher? Therefore keeping the system always on helps avoid this? Can you point me to a tutorial/guide to help me get my head around setting and adjusting the flow temp at the manifold?

 -- are you suggesting I remove the blending valves entirely (or set at 38 for safety), and I adjust the buffer tank temp as outside temp changes? If so, are there controls that allow me to remotely adjust buffer tank temp? Or is this where weather comp can adjust the buffer tank temp to try maintain a certain flow temp to the manifolds? I think I need to learn more about WC controls on the 8000+!

 

 

Thanks again

 

LLH sensor.pdf

heating design v1.pdf

Edited 14 hours ago by RedSpottedSev

[JohnMo]

You need to check your attachments, as one is a safety cert

[RedSpottedSev]

sorry, edited

[JohnMo]

First comment is to do buffer in a 2 pipe configuration - not 4 pipe. Use a 28mm tee at a top and bottom connector of the buffer and the then pipe away from there.

You don't need T&P relief on the buffer, but you need it near or part of the expansion vessel (robo kit).

 

You will need a pot water expansion vessel on the DHW cylinder.

 

I would add a filter to cylinder return as well or at least a strainer.

 

Not sure you need a check valve of the hot outlet of the buffer?

 

30kW is a massive boiler are you sure you need one that big?

 

Control of boiler would be to manage buffer, so sensor from buffer goes to boiler.

47 minutes ago, RedSpottedSev said:

not clear/got myself in a muddle with what happens when it is 12 degrees outside vs 2degrees outside. The boiler will modulate to keep buffer at set temp. But the mixing valves on the manifold are manual. So when there is a call for heat from any sensors, the manifold will draw a set temp (mixing valve temp) to the floors. Do we then accept that if it is 2 degrees it may take much longer to heat up if flow temp is set lower, and then perhaps there will be a chance of overshooting/yo yo if the flow temp was set higher? Therefore keeping the system always on helps avoid this

You do this by selecting a flow temp that suits your floor. So set you buffer about 10 degs higher than the UFH flow temp this will give mixers some authority, your different floors may need different flow temps. Set the mixer on the UFH at say 35 circa start. Over shoot too much trim the temp back. Takes days to get up to temp increase temp - couple days you will be just about there. One adjustment per day otherwise you chase your tail.

 

As the floor takes more heat away on a cold day, the mixer allows more hot water in to manage that. So don't over think it.