Advice needed on electric UFH idea (outbuilding)

[anarres]

Hi,

I have built an office in the garden (32sqm) and I am looking for advice on how best to heat it in the winter. I'm leaning towards electric UFH but maybe this isn't right. If UFH is the right choice I'm not sure on if my proposed construction would work (and I can't easily find info on this online).

There is a concrete slab base with good insulation underneath, and I have 70mm - 75mm to the final floor height.

I was looking at levelling the slab base (it's up to 10mm low in a couple of places but generally pretty flat) putting 50mm XPS insulation down, an electric heating mat on top, with floating engineered or laminate floor to finish.

The first question - is electric underfloor heating so slow to heat up that I'm better off with wall mounted IR heaters or something else? I'll be using the office for 8 hours a day, 5 days a week so super-reactive heating isn't really required, and I don't mind it a bit chilly in winter as long as it's not totally freezing. The office itself is fairly well insulated (see below).

My second question is can I put an electric mat straight on top of 50mm insulation, or do I have to use thinner insulation board with a screed on top? I'd prefer the first option because it's both easier to do (by myself), and more importantly would push more of the heat into the room rather than heating the slab below.

If it helps the office construction is:
8m x 4m external size
7.6m x 3.6m internal size
Built on concrete slab 170mm thick (reinforced), with 200mm XPS insulation underneath.
Wall construction: plinth of 3 courses engineering brick (225mm high), then 100mm timber frame construction with glasswool insulation between and 80mm wood fibre and render externally, and 25mm PIR + plasterboard internally.
Roof construction: 125mm rafters with glasswool between and 25mm PIR + plasterboard internally, tiled externally

 

Final note - there is no hot water or gas out here (40m from the main building) so electric is the only option for heating one way or another.

[TonyT]

Small Air Con Unit known as A2A

will be 4 times cheaper to run, provide heating and cooling

 

You can also get a cheap night time tariff and bring it to temperature in this time frame and make it even cheaper to run

[JohnMo]

I would have a look at a through wall heat pump, looks like a small storage heater, with an inlet and outlet for air behind through the wall. You will get a CoP of circa 3, where a UFH will be a CoP of 1.

[Iceverge]

Well done on the insulation. 

 

You won't need much heating at all. 

 

A cheap second hand storage heater could allow you to shift all the load to a cheaper tarrif. 

 

UFH mats over a slab would be able to achieve this to a limited extent too. I think they'll be fine. 

[Ferdinand]

Make sure you pay attention to ventilation as well as insulation.

[George]

Another vote for an air to air heat pump. 

 

You can get fully internal units as well but they'll be a bit less efficient. 

 

Bbonus of air conditioning in summer as well.

[anarres]

Thanks for the responses everyone, much appreciated. 😀

 

I've been down an A2A heat pump rabbit hole the last day or so and it looks very promising. If it's ok I have some follow-up questions...

 

Near the top of the list of requirements for me is noise followed by reliability and then cost - are there any brands which you would recommend or avoid? Are there websites you would recommend or avoid?

 

I read that you should take cubic volume and multiply by 5 for a BTU rating which gives me about 15000BTU. Looking at an 18000BTU unit the max amps are under 10, so I'm guessing running it on a dedicated 16A line would make sense?

 

I also read you must have a split unit professionally installed - is this correct? Is there an actual danger to doing it yourself if you are a competent DIY-er (like refrigerant gas leaks??)? I guess it's going to add a few hundreds to the price to get it professionally done and I've not needed any assistance so far with building the office (other than having to get electricial sign-off).

 

I have a question about placement - my room is a rectangle (approx 8m x 4m), with a standard gable roof (2.4m room height with another 1.6m height on the roof). Half the space has a ceiling and the other half is open to the roof. The door (and 2 windows) are all on one of the long sides.

 

I can either place this unit in the middle of one of the small walls (@2.4m high) - this wouldn't look great TBH but circulating air would probably move around the space better. The other option is to place it in (near?) the middle of one of the long sides, opposite the door, and just on the corner where the half-ceiling ends (so just under 2.4m).

 

I'm guessing in the middle of the short wall where it can blow in to the open roof space as well as down the long room is better, but would the other position not work or is it possibility an acceptable trade off?

 

I've attached a pic of the office with roof removed and the two positions marked.

Edited July 20, 2023 by anarres

[Mr Punter]

Please don't take the gutter downpipe across the flank wall at an angle.  Straight down and connect underground.

 

40m is a long way to go to the loo when it is cold and raining.  Have you costed running a 110mm drain and a 20mm water feed?  It can all go in the same trench as the electric.

[Alan Ambrose]

Electric under floor is good because it is quick to heat up. It's great sitting on a lot of insulation and it's easy to fit smart controls.

 

It may depend on how often it is used. In my workshop which has OK, not great, insulation, but where I'm fairly active and thus like a lower temperature, I just have a wall mounted fan heater on a smart control. Works fine. I use it maybe 30% and don't notice any particularly high bills. The fan heater only gets use in the 3 winter months anyway.

[anarres]

15 hours ago, Mr Punter said:

Please don't take the gutter downpipe across the flank wall at an angle.  Straight down and connect underground.

Hmm what's the issue? I've already constructed this, and to connect underground the waste pipe would have to go through two 90 degree turns and it's possibly not deep enough at the front to accommodate this.

 

15 hours ago, Mr Punter said:

40m is a long way to go to the loo when it is cold and raining.  Have you costed running a 110mm drain and a 20mm water feed?  It can all go in the same trench as the electric.

Yes, but if it had a toilet it would need planning permission as it would be habitable, and this was done under permitted development. I don't believe they would sign off on building essentially a small house in the garden (also it would need to be signed off on regs - I did in fact do it to regs standard but without notification or anything being signed off).

 

Also, I have many trees in the garden and the route the electrics and water has taken is not a straight line, and it goes to the front (middle) of the build - waste would need to come from one of the sides and the route to the main drainage would really not be simple.

 

If planning rules change in future there is the possibility of adding it in but it's not happening right now.

[anarres]

15 hours ago, Alan Ambrose said:

Electric under floor is good because it is quick to heat up. It's great sitting on a lot of insulation and it's easy to fit smart controls.

 

It may depend on how often it is used. In my workshop which has OK, not great, insulation, but where I'm fairly active and thus like a lower temperature, I just have a wall mounted fan heater on a smart control. Works fine. I use it maybe 30% and don't notice any particularly high bills. The fan heater only gets use in the 3 winter months anyway.

I'm not doubting what you're saying but I've seen a lot of posts about how UFH isn't quick to heat up due to the maximum floor temp of 27 degrees, especially if using laminate/wood?

 

In either case I would use smart controls. I've been looking at a 2 part A2A heat pump and the main downside I can see is noise, but reduced running costs compared to electric UFH.

[Mr Punter]

51 minutes ago, anarres said:

Yes, but if it had a toilet it would need planning permission as it would be habitable, and this was done under permitted development. I don't believe they would sign off on building essentially a small house in the garden (also it would need to be signed off on regs - I did in fact do it to regs standard but without notification or anything being signed off).

 

No consent needed for a WC.

[joe90]

42 minutes ago, Mr Punter said:

No consent needed for a WC.

Oh yes, I put a toilet in my last garage, under permitted development. In my previous garage I put a 110 soil pipe in ready fir a toilet but never got around to it again under PD (And that 45’ down pipe looks pants IMO) just saying 🤷‍♂️

Edited July 21, 2023 by joe90

[anarres]

Thanks for the responses. I'm not sure why I thought there was a restriction on toilets - I'd read that they could not be habitable and I assumed this to mean they could not have everything required to be a dwelling. Instead I just re-read the guidance and it clearly says "NO sleeping accommodation" but that's crazy because it just means don't put a permanent bed in right?

 

In any case there are practical issues about waste pipes... my rainwater goes to a soakaway but proper waste would have to go 45m to reach up to the existing drainage and near some large trees through very heavy clay (or take wide detours). These practical issues make it not worthwhile for now at least. Perhaps when I renovate my house in the next year or two, when I have diggers on site and I am relaying the existing soil pipes in the house.

 

Can anyone answer my questions about A2A heat pump positioning though?

[TonyT]

Put the indoor unit so it isn’t blowing directly at you.

high level wall mounted with the exterior unit through the wall to keep the pipe runs nice and short