UFH Mixing Valve advice on Air Source Heat Pump set up

[MorganP]

This is my first time posting and a massive thank you to all those on here who have helped me on this and other topics 🙏

 

We had an Air Source Heat Pump fitted to replace our old oil boiler. Since the installation, the downstairs underfloor heating hasn't worked properly. Upstairs - running on rads - is lovely and toasty.

 

The issue is a lack of heat going into the UFH circuit - especially around the mixing valve in the picture. The temperature of the pipe going in at the top of the value - coming from the ASHP - is lovely and hot. But what’s coming out at the bottom is quite a bit colder. Even fully open, the mixing valve is bleeding cold water into the circuit and lowering the temp way too much.

 

The original installers suggested that it can be removed completely but I'd like some advice please. I have a really sound heating engineer who can probably remove and re-pipe it but he's not an ASHP or UFH professional. Can we remove the mixing valve completely? Do we need to add in something like a TMV? I've search on here but not quite found definite answers. Help would be greatly appreciated!

 

 

[JohnMo]

 

1 hour ago, MorganP said:

lovely and hot

That's the first alarm bell - at this time of year even with radiators, the water should be cool certainly not hot.

 

Are you running fixed flow temp or weather compensation or anything else?

 

What is your design flow temp at -3 the installer should have given you heat loss and design calculations? 

 

Looks a pretty basic mixer, they normally want to see quite a large dT between flow in and UFH return temperature to operate. But you have hot water on the bottom rail cannot read it very well but it looks about 40 degs.

 

You seem to lots of zones looking at you manifold and the number of actuators. 

 

Do you have a buffer vessel as well? (Small vessel sitting between ASHP and heating system normally with 4 pipes).

 

What heat pump are you using?

[dpmiller]

assuming the thermometers are both reading correctly, that should be a toasty hot floor...

[Nickfromwales]

10mm pipes, and suspended / floating wooden floors?

 

Have you any idea how the UFH was installed? In a pug / biscuit mix of sand & cement, or in aluminium spreader plates, or an overlay system (a-la Wunda)?

 

Temp looks high, on the red (flow) side, so assume that the flow pipes are hot?

 

Looks like 10-15 degrees differential, so you’re inputting heat into the floor, so more info please! 

[MorganP]

Thank you @JohnMo / @Nickfromwales - already really helpful and I'm thinking it's a different issue. Just to mention, with the recent warm weather it's not really an issue yet. It was a big headache all last winter though so I'm desperate to get it sorted before the weather sets in (and before my wife emigrates to Italy, leaving me with the kids).

 

Quote

"Are you running fixed flow temp or weather compensation or anything else?"

 

Fixed flow of 50 degrees at the mo. Tried everything last winter and that was the best setting. Weather compensation wasn't effective. The current performance is like this:

 

- around 12 degrees outside and it'll struggle to hit room temp (20 degrees). Most rooms get to about 18 / 19.

- 5-10 degrees outside and we'll be lucky to get 17 degrees room temp

- anywhere near freezing and it's useless - 15 to 16 degrees at best.

 

Quote

"What is your design flow temp at -3 the installer should have given you heat loss and design calculations? "

I'm afraid I don't know. Is that for the UFH or ASHP? The UFH was installed in the 00s long before we moved - based on an oil boiler. We installed the ASHP end of in 2023.

 

Quote

"Looks a pretty basic mixer, they normally want to see quite a large dT between flow in and UFH return temperature to operate. But you have hot water on the bottom rail cannot read it very well but it looks about 40 degs."

 

Seems to be 34 degree flow - photo attached. Return is 30. Should that be enough?

 

Quote

"Do you have a buffer vessel as well?"

Is it one of these in the photo below? 

 

Quote

"What heat pump are you using? "

 

Mitsubishi Ecodan R32 / 11kw

 

Quote

"10mm pipes, and suspended / floating wooden floors? "

 

Pipes: Strange but there are two sizes. My best guess is 12mm and 15mm but see what you think from the photos.

Floors: Two types - mostly wooden but two tiled. The tiled floors perform a lot better

 

Quote

"Have you any idea how the UFH was installed? In a pug / biscuit mix of sand & cement, or in aluminium spreader plates, or an overlay system (a-la Wunda)? "

 

It's an overlay system using an insulation board - installed circa 2004 I think.

 

Quote

 

"Temp looks high, on the red (flow) side, so assume that the flow pipes are hot?

Looks like 10-15 degrees differential, so you’re inputting heat into the floor, so more info please!"

 

 

Yeah this has me thinking now - photos attached and notes above. Should 34 degrees be enough to heat the house? And is the 4 degree difference between flow too little? The floor barely gets warm - especially the wooden one. We've insulated more, put up curtains over doors, draft proofed everywhere etc. No more options on that front now 🤷‍♂️

 

The pump is set to max at the mo - 2 instead of 1. But I've tried both settings and it doesn't make much difference.

 

Thanks again

[John Carroll]

If its the top of the flow meter(s) is the flowrate then you only appear to be circulating ~ 1.0LPM/loop, 4C seems a very low dT for that very low flowrate. Every 1.0LPM at a dT of 4C will give a output of, 1.0*60*4/860, 0.28kW, 11 loops equals 11*0.28, 3.1kW.

[JohnMo]

12mm is the best at carrying heat. An overlay system could be leaking heat downwards as much as to the house. I had an overlay system and 34 flow did exactly what you describe for room temperatures. I bypassed in the end not saying you will need to.

 

What are your floor coverings? That makes a huge difference to how UFH performs.

 

How do you actually run the heating? What is a typical daily pattern?

 

Have you tried to measure actual floor temperature?

 

When you operated the oil boiler how was the house temperatures and UFH flow temps?

 

 

 

 

[JohnMo]

1 hour ago, MorganP said:

The pump is set to max at the mo - 2 instead of 1.

Pump speed will not make much difference, if the flow meters are already limiting flow rate. Increase loop flow will make the loop out more heat into the room.

 

I would leave the pump on its high setting and open the loop flow meters to double the current flow rate before I did anything else. Then once settled back the pump speed down until you see a change in the flow meters. Then leave it alone for 24 hrs. Ideally measure floor temp before and after.

[MorganP]

Thanks again @JohnMo - and @John Carroll

 

14 minutes ago, JohnMo said:

What are your floor coverings? That makes a huge difference to how UFH performs.

 

How do you actually run the heating? What is a typical daily pattern?

 

This is very true. The flooring is 12mm oak laminate but we have a tiled utility room which is the warmest room downstairs! 

All zone stats are set at 20º during day / 17º at night. As the rooms rarely hit temp when it's cold, the ASHP runs all day - and often all night (racking up the leccy cost!).

 

29 minutes ago, JohnMo said:

Have you tried to measure actual floor temperature?

 

When you operated the oil boiler how was the house temperatures and UFH flow temps?

 

Not tried to measure temp but will have a go 👍

We had no issues at all when on oil - lovely and toasty with the kids frequently basking around on the floor enjoying the heat - good times! 

 

I never clocked the flow temp when on oil but I suspect they were a good deal higher - the mixing valve seem near fully open so I'm guessing around 40º - 45º. The setting on each flow meter hasn't changed since we installed the ASHP so I'll try increasing the loop flow to see if that helps - that seems the conclusion from @John Carroll post I'm assuming.

 

1 hour ago, John Carroll said:

If its the top of the flow meter(s) is the flowrate then you only appear to be circulating ~ 1.0LPM/loop, 4C seems a very low dT for that very low flowrate. Every 1.0LPM at a dT of 4C will give a output of, 1.0*60*4/860, 0.28kW, 11 loops equals 11*0.28, 3.1kW.

 

 

41 minutes ago, JohnMo said:

12mm is the best at carrying heat. An overlay system could be leaking heat downwards as much as to the house. I had an overlay system and 34 flow did exactly what you describe for room temperatures. I bypassed in the end not saying you will need to.

 

May I ask how you bypassed it? Back to my original question - could I do this by removing the mixing valve?

[JohnMo]

14 minutes ago, MorganP said:

May I ask how you bypassed it? Back to my original question - could I do this by removing the mixing valve?

I installed a fan coil. Disconnected the pipes to the UFH and directed them to fan coil.

 

 

[MorganP]

Thank you all for pitching in. I'll increase the loop flow and post back. Any further ideas in the meantime will be much appreciated.

[Nickfromwales]

22 minutes ago, MorganP said:

Thank you all for pitching in. I'll increase the loop flow and post back. Any further ideas in the meantime will be much appreciated.

I think you’ll need to up the flow temp into the UFH to compensate for your new, low temp heat source.

 

It sounds like whoever sold you the dream didn’t explain things, or survey it properly, to highlight some of the adverse effects that you could expect from switching over to a heat pump.

 

If you up the flow rates, don’t do it to the loops servicing the tiled areas. Try the others at the increased rate and see what happens over 48hrs. 

[MikeSharp01]

11 hours ago, Nickfromwales said:

If you up the flow rates, don’t do it to the loops servicing the tiled areas. Try the others at the increased rate and see what happens over 48hrs. 

You do need to measure the floor temperatures, get a cheap IR gun. (the one below is about £50 from Screwfix but you can get a lot cheaper, 1/3rd the price, versions from amazon). Have you replaced the floor coverings since the Oil boiler? If you have then the flow rate thing will need to be tackled see posts above but also you need to watch out that you do not overheat the floor. You can also work out the TOG value of the laminate, which will give you an idea, along with a bit more working out, just how much to increase the flow rate and / or the delta temperatures across the system. 

 

 

 

[Nickfromwales]

1 hour ago, MikeSharp01 said:

You do need to measure the floor temperatures, get a cheap IR gun. (the one below is about £50 from Screwfix but you can get a lot cheaper, 1/3rd the price, versions from amazon). Have you replaced the floor coverings since the Oil boiler? If you have then the flow rate thing will need to be tackled see posts above but also you need to watch out that you do not overheat the floor. You can also work out the TOG value of the laminate, which will give you an idea, along with a bit more working out, just how much to increase the flow rate and / or the delta temperatures across the system. 

 

 

 

I expect that the area of wooden floor was driven very close to the limit, previously, to get the heat into the room.

 

Good point though. Many heads….

[MorganP]

4 hours ago, MikeSharp01 said:

You do need to measure the floor temperatures, get a cheap IR gun.

 

Thank you for the suggestion @MikeSharp01 - I've just ordered one. The wooden flooring is the same as before so I'm going to wait for the IR gun to arrive and then change flow rates to see what works. 

 

 

15 hours ago, Nickfromwales said:

It sounds like whoever sold you the dream didn’t explain things, or survey it properly, to highlight some of the adverse effects that you could expect from switching over to a heat pump.

 

This is the reason we're no longer using them. They also miscalculated the energy load which meant the electrician on the day disabled our shower to bring it within acceptable levels! Another job on our list is to have the electricity upgraded to get our shower back. 

[JohnMo]

50 minutes ago, MorganP said:

They also miscalculated the energy load which meant the electrician on the day disabled our shower to bring it within acceptable levels

Why isn't the shower(s) just coming from your cylinder? You shouldn't need electric showers, which are pants anyway.

[Nickfromwales]

7 hours ago, MorganP said:

 

Thank you for the suggestion @MikeSharp01 - I've just ordered one. The wooden flooring is the same as before so I'm going to wait for the IR gun to arrive and then change flow rates to see what works. 

 

 

 

This is the reason we're no longer using them. They also miscalculated the energy load which meant the electrician on the day disabled our shower to bring it within acceptable levels! Another job on our list is to have the electricity upgraded to get our shower back. 

I’d firstly survey yourself, and see if the peak demand, with the shower being reinstated, ACTUALLY takes you over the rating of your fuse.
 

Is it an 80a incomer (service fuse?).

 

I think their electrician was being over zealous tbh. MCS box-tickers most prob.

 

Most inverter heat pumps use way less power than their stated output, so I doubt you’re about to go Hiroshima 🔥 any time soon….

 

7 hours ago, JohnMo said:

Why isn't the shower(s) just coming from your cylinder? You shouldn't need electric showers, which are pants anyway.

What he said. 
 

All apart from “pants”. 
 

I’d have said “dogshit”. 
 

Electric showers are dogshit, and a thermostatic mixer off your UVC would be so much better, and cheaper to use.

 

Get the electric one switched to mixer if not too disruptive, particularly if it’s your daily driver.

[MorganP]

23 hours ago, JohnMo said:

Why isn't the shower(s) just coming from your cylinder? You shouldn't need electric showers, which are pants anyway.

 

Good point. The sink next to it already has hot water from the cylinder so this should be an easy option 👍