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?