New Member Seeking Advice On UFH

[Paul Ashley]

Good evening

 

New to this site but already found some great information - thank you.

 

I have inherited a UFH system, which when we bought the house, was run by oil-fired boiler. Semi-rural with no gas feed so switched in 2022 to Air Source Heat Pump. This is used for DHW and heating.

 

The property is a large barn (approx 350m sq) which is long (40m) and mainly on 1 floor. There are rooms upstairs where former loft space has been converted to living accommodation. UFH is downstairs, radiators upstairs. Upstairs rooms rarely used especially in the winter so the heating system is not usually on upstairs.

 

Due to size of property, we have a 16Kw ASHP with 2 sets of manifolds. One serves the kitchen, utility, dining room, hall and study. All of these have stone floors. The other set of manifolds serve the bedrooms (3), hall, lounge and 2 bathrooms. Lounge is engineered wood floor, bathrooms are tile floor and bedrooms and hall are carpeted. I believe the tog on the carpet is 2.0 and the underlay is 0.6, which I know is way higher than efficient.

 

This is a problem in sub-zero weather due to a combination of the main bedroom having carpet, being the furthest away from the heat pump (40m away) and SW-facing, so there is a cavity wall but no cavity wall insulation. I believe there is internal insulation but nothing in cavity due to cost of specialist foam fill when built.

 

I am trying to get my head around what is the most cost-efficient way of improving the UFH performance. At one end of the house, it's fine. Performance from the other end is poor. In sub-zero temperatures, it's impossible to get the main bedroom and en-suite above 15C. A bit better each time you move towards the ASHP and manifolds, so 18C for next bedroom and 19-20C for lounge is doable.

 

The ASHP can definitely produce hot enough water for the system at 0C external temp as input/output temps when driving the radiators are easily 55/52 when asked to deliver. Same for DHW. Input/output temps on UFH though can be more like 35/31. This I don't understand. Why such a huge difference? Am I really losing that much heat by sending the water around the UFH system and if so why? Can someone explain what the system does with the water that's being requested at 55C and why the flow temperatures are so low?

 

It is quite clear that in order to hit my target temps in each zone, I need a flow temp (as read from the flow thermostat on manifold) of around 40C and I just can't get that. I'm thinking that the assumptions (eg about insulation levels in floor and walls) made by the person who did the independent survey pre-ASHP installation, were just plain wrong.

 

What can I do and what would be most efficient and cost-effective? Considerations are - specialist insulation in end wall of house (which gets all the wind and rain), take out carpet in bedroom and replace with something better for UFH, add another heat pump just to deal with end rooms to reduce burden on main system or switch end rooms to radiators rather than using UFH at all. 

 

Where do I start? Any help appreciated. I'm not technical at all so need things explained logically and simply

 

Many thanks

 

 

 

 

[jayc89]

How far away are the manifolds to the furthest rooms they serve?

Are the flow/return supply pipes to/from the manifolds insulated?

Have you measured the flow/return temps of the UFH loops supplying heat to the rooms which are struggling to reach temp?

Do you have a heat loss calc for the house?

[JohnMo]

Distance isn't the issue, carpet and underlay is. Our bedrooms are the same. We just open the bedroom doors which fixes the issue.

 

We have a dog bed on the carpet and when you move it the is super hot underneath, but overall the floor temp stays low so less radiated heat.

 

Long term fix is replacing carpet and underlay for better UFH stuff. But will not happen.

 

Alternatives are 

Switch UFH off replace with wall panel heater, or a combination of the two.

 

Flow at a higher temp - CoP hit, rest of house gets too hot.

 

Open doors when you need it warmer, bigger output towel rail electric or dual fuel.

[Nickfromwales]

I'd say the ASHP survey was 'optimistic' eg to get the job, but it simply sounds like 16kW ain't enough heat energy!

 

I assume the oil system kept the house nice and toasty, and you had none of the above issues when sub-zero?

 

If someone is adamant on coming away from oil but the maths don't make an ASHP a fully kosher solution, a good option is a oil+electric hybrid ASHP, which is probably what I would have recommended to keep you safe for the sub-zero weeks/months, with you having retained the oil tank.

 

Do you have room to cascade a second heat pump with the first at the current location?

[Paul Ashley]

Thanks for your replies - apologies for slow response, I have been away. 

 

jayc89 - manifolds to furthest rooms around 10m, the flow/return supply pipes are insulated for the 20cm I can see then they disappear into the concrete, yes I have measured the flow/return temps - in very cold weather (sub-zero) they are around 30/25, I don't have a heat loss calc to hand

 

JohnMo - understood but that doesn't explain why the en-suite (tile floor) is colder than carpetted bedrooms next door. I'm thinking of (1) improving insulation (2) add wall heater if needed. Definitely don't want higher flow temp due to the COP hit as you say

 

Nickfromwales - yes oil system did keep it hot. Not surprising - my neighbour is still on oil and has same UFH system. His flow temp is about 75C so no wonder! Oil tank is long gone. I do have room to add another heat pump for far end of house but I think that's a last resort after insulation and timed wall heater in en-suite first and lower tog carpet/underlay.

 

Question on weather comp - Midea heat pump set to 55C at -2C and 37C at 15C on weather comp. Does that mean that my changing the flow temp on the controller overrides this? 

[JohnMo]

On 11/01/2025 at 17:24, Paul Ashley said:

Input/output temps on UFH though can be more like 35/31.

How long are you running the UFH? Is the floor allowed to cool down for many hours or are you adding heat steadily continuously?

 

What happens with an ASHP? the flow temperature is pegged to a combination of return temp and deltaT. So with UFH it can absorb huge amounts of heat, so return temp stays low for quite awhile, so in turn the flow temp is held back.

 

The other thing with UFH output is dictated by insulation and floor build-up.

[Nickfromwales]

1 hour ago, Paul Ashley said:

 

Nickfromwales - yes oil system did keep it hot. Not surprising - my neighbour is still on oil and has same UFH system. His flow temp is about 75C so no wonder! Oil tank is long gone. I do have room to add another heat pump for far end of house but I think that's a last resort after insulation and timed wall heater in en-suite first and lower tog carpet/underlay.

I am suggesting you double-barrel the ASHP's at the same location, with the same plumbing, to keep cost and impact down (referred to as cascading). You simply do not have enough kW of 'oomph' to keep up with the fabric and ventilation heat loss at the new, significantly lower flow temp.

[DamonHD]

I'm likely missing this above but I'm too tired!  Is there some sort of mixer for the UFH that's reducing the UFH flow temperature compared to the rads?  If so, without damaging the floors, can you just not mix it down (so much)?

[Alan Ambrose]

Look up the EPC from the government website and report back so we have an idea of the level of insulation in your barn. EPCs are a hopeless system, but they do give you a very rough idea. Maybe a screenshot of the bit that lists the floor/wall/roof insulation. Similarly, when you had your heat pump installed, you should have been provided with the calcs. Is there any kind of control system for the individual rooms? What was the heat output of the oil boiler you had taken out?

[SteamyTea]

 

On 11/01/2025 at 17:24, Paul Ashley said:

Where do I start?

 

Have you redone a room by room heat loss calculations?

Do you know the pipe spacings in each room?
Do you know the levels of insulation under the UFH in each room/zone?

[ringi]

What size pipes connect the far manifold to the heatpump?

Have the manifolds been balanced?

[sharpener]

On 25/01/2025 at 20:33, JohnMo said:

What happens with an ASHP? the flow temperature is pegged to a combination of return temp and deltaT. So with UFH it can absorb huge amounts of heat, so return temp stays low for quite awhile, so in turn the flow temp is held back.

 

On 25/01/2025 at 22:22, DamonHD said:

Is there some sort of mixer for the UFH that's reducing the UFH flow temperature compared to the rads?  If so, without damaging the floors, can you just not mix it down (so much)?

 

I think these are part of the problem. (Also the low UFH return can starve the rads but it sounds like it is not this.)

 

However 16kW sounds a bit low, we have a 180 sq m barn conversion with uninsulated stone walls and 12kW is absolutely on the limit for about half the floor area.

 

Was it always like this or is the lack of performance new? If you had it put in in 2022 it is less than 3 years old so what do the original installers say? If all else fails you could pay for a Heat Geek qualified engineer to take a look and make recommendations.