Should I consider UFH for this extension?

[Radian]

 

Here's an extension we're currently building. It's laughingly called a Sunroom but it's longest wall points N.E. The whole thing is floor to ceiling double-glazed, off a beam & block floor with 100mm celotex + screed and capped with a flat warm-roof construction. This is being built as an alternative to the typical bolt-on conservatory that could only be used comfortably for part of the year. Poking the dimensions and materials into online calculators yields a consensus Heat Loss of around 1500W but I'm only only taking that as a very rough guide.

 

The aim is to be able to wander into the room at any time of day, any day of the week. The concern is the warm-up time when we set back the temperature overnight and just how costly this might prove to be if we find it needs to stay on longer. The main reason for choosing UFH over a wall mounted rad in this case is to get heat out to the glazed sides and counter the inevitable cold-pooling of descending air in those regions. One alternative I could think of was to set a perimeter strip of under-tile electric mat in conjunction with a rad (just to offset the cold pooling) but I instinctively shy away from electric heating having already heavily invested in energy saving appliances. Any wisdom to share here?

[Jeremy Harris]

The heat output of UFH near the edges will be pretty small, around 50 W/m² is on the high side of a typical UFH output, so you can guesstimate how effective this will be around the edges.

 

If you want heat around the edges (seems like a good idea) then trench heating may be a better bet.  It will have a fast response time (much the same as radiators) yet not take up wall space.

[ADLIan]

I assume Building Control are involved. Have they picked up on the fact that the extension is over glazed? This has an impact on overall BR compliance.

I would guess it will be difficult to heat in winter and may also overheat in summer time (and the shoulder period?)

[Radian]

2 hours ago, JSHarris said:

The heat output of UFH near the edges will be pretty small, around 50 W/m² is on the high side of a typical UFH output, so you can guesstimate how effective this will be around the edges.

 

If you want heat around the edges (seems like a good idea) then trench heating may be a better bet.  It will have a fast response time (much the same as radiators) yet not take up wall space.

 

I did look into trench heating but the prices were sky-high!

 

24 minutes ago, ADLIan said:

I assume Building Control are involved. Have they picked up on the fact that the extension is over glazed? This has an impact on overall BR compliance.

I would guess it will be difficult to heat in winter and may also overheat in summer time (and the shoulder period?)

 

Yes, that's one of the drawings submitted by the Architects to BC. There was no feedback r.e. over-glazing. The glazing units are 1.3Wm2/K and there's just under 34m2 of it. 1KW for a 22C differential doesn't strike me as outrageous? This room can be closed-off with the original outside door and the heating cct isolated however there are only a handful of days where it drops below zero here on the South coast (who knows what's to come though)

[Jeremy Harris]

Skirting radiators might be another option.

[ProDave]

We are having a similar orangery but on the south side of the house.We are not putting any heating in as it will be a seasonal room which may end up too cold in winter and too hot in summer.

 

If you want this to be an all year extension, why have so much glass? Why not make it a proper extension?  

[Radian]

Perhaps I've got entirely the wrong mindset. I really don't know.

 

The primary requirement is to create a comfortable indoor space with uninterrupted views of the garden (which the main house does not at all). I am under no illusion that heating this space for winter use will not be a luxury that has to be paid for - but my energy calcs indicate a cost of less than a pint at the local for a day's use by the whole family, even when freezing outside. Now I'm not a drinker so I'd be thoroughly content with the deal. As for summer heat, the end-gable wall of the house sits between the garden room and the South. In practice the old patio it's being built on was our best retreat from the 33C summer heat last year so I'm really not expecting a problem there. Only the early morning Sun gets to one side which should come in handy making it a cozy breakfast experience. The glazing consists of sliding units on all three elevations so it won't take much to get a through draft anyway.

 

But I'm open minded about all this, and if I've got something fundamentally wrong and someone can show me why, then I'd view it like ProDave does as a seasonal retreat.

[Jeremy Harris]

Might be an idea to plan for some form of active cooling I think.  I doubt that winter heating will be a significant problem, but suspect that the impact of reasonably good glazing, which will act a bit like a "one way valve" when it comes to radiated heat, will create some overheating in summer, probably more so than with a relatively less well-insulated conservatory. 

 

An air to air heat pump isn't expensive to buy or run, and could deal with both the winter heating and summer cooling requirement.  One that costs around £800 or so would probably do the job and cost less, and be more useful, than fitting wet heating.

[Radian]

3 hours ago, JSHarris said:

Might be an idea to plan for some form of active cooling I think.  I doubt that winter heating will be a significant problem, but suspect that the impact of reasonably good glazing, which will act a bit like a "one way valve" when it comes to radiated heat, will create some overheating in summer, probably more so than with a relatively less well-insulated conservatory. 

 

An air to air heat pump isn't expensive to buy or run, and could deal with both the winter heating and summer cooling requirement.  One that costs around £800 or so would probably do the job and cost less, and be more useful, than fitting wet heating.

 

Interesting. I hadn't considered air-to-air but it does make quite a bit of sense now I've read-up on it. I will confess to being a little sceptical about the running costs. However, seeing a Panasonic 12000 BTU unit costing £850, I must say I'm intrigued. I just can't get a feel for the thermodynamics involved. It claims to be able to extract heat from air at -15C.❄️? I'm currently clinging to the mental model of a fridge transferring heat from its contents to the fins round the back and into the kitchen.

[Jeremy Harris]

Our whole house is entirely heated and cooled by an air source heat pump.  It uses a bit less than 1/3rd of the amount of energy that it delivers, as it just pumps heat from the outside air into the water we use to run the UFH and pre-heat our hot water.  It works fine down to about -20°C, and that restriction is only because of the type of refrigerant used, as there is still loads of heat in air at -20°C (air at 0°C contains about 3.7% less heat than air at 10°C, air at -10°C contains about 7.3% less heat than air at 10°C).

[ProDave]

A heat pump is exactly that, the workings of a fridge. Just as your freezer extracts heat from the freezer compartment at -15 so does a heat pump.

 

Up here in the Highlands a few weeks each winter when it's -10 at night and not above 0 in the day is usual and my air source heat pump has kept the house warm admirably, and heated the hot water.

[Radian]

I started another topic to see if anyone had experience of electriq brand AC because I'm seeing quite a few plus points in going down this route. The cost of one unit I found is on par with the UFH I had originally planned. However, I would still like to see a way of estimating the warm-up time for UFH. It's easy enough (I think) to calculate the power going into the floor, but with 50mm fibrous screed plus tiles on top, how to go about estimating the time taken to raise the air temperature in the room by a given amount?

[Radian]

Hello again, it's been a while, and thanks for all the advice I received back in 2019.

 

I now want to update my topic with the outcome - which is that I did put in a wet UFH loop - and that I'm very glad I did. Initially I decided to hedge my bets and lay in a PEX loop as well as some ducting for air-to-air during the construction of the extension. Neither were costly and meant we could  decide on the heating solution once we had gained experience using the room.

 

This didn't happen until summer 2020 during which we occasionally  had outdoor temperatures in the high 20's peaking at 32. The garden room mostly behaved very well with only the period between mid-morning and mid-day seeing too much solar gain from the S.E. elevation. Jeremy Harris was right to point this out as being potentially the more significant issue. However, by opening both sliding doors a little way to get a crossflow (on the N.W.- S.E. axis b-b on the plan) the room could be kept comfortable. If the house wasn't occupied we would simply close the doors to the rest of the house. So far, so good.

 

Winter 2020-2021 wasn't particularly cold so we employed a 1200W halogen portable heater and continued using the room whenever we wanted to. This was very encouraging and by October this year, having had another comfortable summer, I decided just to hook-up the UFH and leave the air-con ducting buried in the wall. Now we've had a few sub-zero periods since then it seems that we have a good solution. On a sunny but cold day the room is benefiting from a solar boost in the morning so generally it comes on overnight and goes off after between 4 and 12 hours depending on weather. The only issue I have had is that my boiler only modulates down to 10kW so if the UFH is the only load it would short-cycle too much. Rather than install a buffer tank I simply modified my control algorithm to modulate the zone valve serving the central heating radiators which now get a tiny pre-heat in the early-hours. My zone valves are custom servo-actuated home-brews operated by a Raspberry Pi which means I'm also logging all the temperatures and energy usage to keep on top of things.