Boiler size for wet underfloor heating

[Custom45]

Hi,
What size electric boiler should I be looking at to serve a low profile wet underfloor heating system? (Gas & oil not an option).

Looking at Fastwarms 20mm low profile underfloor heating, won't be much in the way of floor insulation as I'll be retro fitting over existing concrete floor. Floor size to be covered is approx 160-180m². Walls and ceiling will be well insulated.

Also, on a single phase supply (100a main fuse) would an electric combi be an option to feed ufh and supply hot water to bathroom?

Although a reasonably large property it only has 2 occupants with 1 bathroom and a kitchen.

Thanks,

 

[trojanhawrs]

You'd be way better going for a heat pump, underfloor heating and heat pumps are a match made in heaven because underfloor requires low temperatures. Minimum 3x cheaper to run than electric boiler, probably more.

 

[Custom45]

Thanks. Air source heat pump? What sort of size would be required? Would a heat pump be able to supply our hot water too though?

 

[trojanhawrs]

Yes, it can be used for hot water storage as well with an immersion as backup for colder days.
You'll need to get a heat loss calculation done and work out what system you're going to use/floor coverings. Assuming 40W/m2 (decent insulation) a 8kW would probably cover you. If you can afford to raise the floor it would likely be worth insulating over the slab and using a low profile screed system on top of that. You'd be looking at 60-100mm of buildup though.

 

[Custom45]

Fastwarm claims typical output is 76w/m2, we'll tiling floor throughout with their 20mm boards and 16mm pipes. They claim you can tile directly on top of their 20mm boards.

Available floor height for buildup in most rooms is only 30-40mm, so not much room for insulation. Was looking at the 6mm insulated tile backer boards to add a bit in though, or maybe 10mm.

With the 8kw output heat pump what is the kw consumption to run it? In our old house we had a 190l hot water cylinder with immersion heater, she complained constantly that it was never enough, hence was looking at a combi for on demand hot water rather than a storage tank that could potentially run out.

Initially I was looking at a 7kw SlimJim electric boiler for the UFH and a 300l Heatrae megaflow tank with solar PV (we have 7 solar panels on roof not doing much) for hot water.

 

[Rodders53]

You really don't want to run a COP of 1 direct immersion heater for UFH...
Not unless you have Loads a Money.

Heat pumps should give a COP of 3-4 on a good day...
8 kW out for 2 kW in (4) or 2.67 kW (3). A bit more on very cold days.

You do need to get pro heat loss calcs done to be syre.

To get hot, hot, water will need use of 1:1 immersion in addition. But heat pumps can do 50C - 55C without and an occasional Legionella cycle. How hot does one need water... showers are 38C-43C nominal and modern bath mixers must be less than 48C to try to prevent scalding of young and elderly...

Some more recent may be able to do HW only with flow hotter at times, then lower flow temps for the room heating. Lower flow temps are more efficient / better cop.

 

[trojanhawrs]

Where did you see the fastwarm figures? I'm sure it could output that much but I suspect a high percentage of that heat is lost to the ground unless it's sitting on a substantial insulation bed.

 

[Custom45]

Was on their data sheet, I would take all their figures with a lunch of salt though, yes, I'm sure they are inflated for marketing reasons.

50-55⁰ for water water would be plenty hot enough, it wasn't water temperature that was the issue before, it's the amount of available hot water.

So, roughly an 8kw heat pump would do the job of running the UFH and providing hot water (via a hw storage tank)? What are good reliable brands of heat pumps?