Heating, Cooling, and DHW for an Annexe - Air to Air, Gas boiler, Wunda UFH

[ap534]

Hi, 1st post ?

 

We have a new 50m2 Annexe with 1st fix happening now. It is block and render with 75mm cavity insulation, 130mm floor insulation covered by 100mm slab, 130mm insulated warm flat roof. We would like to be able to use it all year round as a living area/home office, but it won't normally be occupied or heated. So rapid warm up is essential. It will have a kitchenette and wetroom with shower and toilet.

 

I have a 20m run of 25mm insulated flow and return pipes (https://www.flexenergy.co.uk/flexalen-600/) coming underground from the house which has a 37kW British Gas condensing gas boiler. Minimum output around 10kW. Stated heat loss through the piping is 15W/m at 80 deg temp. I generally run the house heating at around 50 deg C, then boost to a max of around 80 while heating the DHW.

 

I also have 10kW of PV, some of which I can divert to the cylinder in the Summer months.

 

My plan so far:-

 

DHW

•  150l unvented cylinder with high area coil, heat from the house boiler. High area coil for rapid heat up, and future use with ashp if we replace the  gas boiler.

 

Heating/Cooling

• 2 x Midea wall mounted air to air units with external compressors https://mideauk.co.uk/products/blanc-series/. 5kW for main room, 3kW for office.
• UFH - Wunda 20mm EPS400 overlaid by renovation screed + vinyl plank. Heat load 2.2kW. UFH fed from house boiler. 

 

Questions

1. Air to Air appears to be the lowest cost to heat the building, and the installers say that most people don't bother using UFH once they have air to air. Does this ring true? I like the thought of being able to sit on the warm floor in the winter.
2. I'm being quoted by 2 different places roughly £1500 + £1400 + vat for the 5kW and 3kW  Midea units installed. The units are readily advertised at £680 and £460 +vat. Is over £2k for install the going rate?
3. If I put UFH in then I need it to warm up quickly, which led me to the Wunda system which seems to fit the bill. I don't want to be heating a slab for hours. Am I wasting my time with UFH?
4. If I put the UFH in, then I think I need a buffer tank otherwise the boiler will be short cycling all the time. Then I read threads on here about also being able to switch in the buffer tank to pre-heat the DHW which then let's me reduce the UVC size perhaps to 80l? By the time I've added all that plus the controls, is there a lower cost/size alternative? What about a small external ASHP for the UFH.?

 

Here is a floor plan,

 

 

 

Here's my piping sketch so far, but not sure how to do the DHW pre-heat. I've omitted the various controls and valves at the house.

 

 

Any pointers would be much appreciated please, as I'm finding it difficult to get advice I can trust!

 

 

Edited January 3, 2022 by ap534
updated piping pic

[SteamyTea]

Can you clarify the insulation levels a bit more, does not seem very good spec.

How much wall area is actually windows.

 

A2AHP has a lot going for it, it heats up the parts you want and does not waste time on heating up a large slab of concrete, which is slow to respond for occasional usage.

Having said that, 8 kW_p of heating for 50 m² seems quite excessive.

[tonyshouse]

Forget ufh , for fast warm up insulation on top of slab , for comfort other water round, there are bigger risks to the structure with intermittent heating and likely condensation hopefully only on windows, you want it where you can see it, ashp can dehumidify . 
 

for the tiny office 3kW seems insane! Possible 1kW convector heater on a timer and thermostat, what does intermittent mean in hours per week 

[ap534]

Thank you for replying SteamyTea and Tonyshouse.

 

The building is supposed to be to building regs (as of 2020).

 

Insulation

Insulation in the floor is actually 150mm PIR boards. There are 75mm PIR boards in the wall cavity, and the roof has 130mm PIR boards from memory - I'll need to check. I'm not sure if that gives you any more information or answers your question though!?

 

Windows

The building has 2 main rooms + a wetroom. The rooms are 5mx5m and 2.5m x 2.5m. The main room has a 4m double glazed sliding door, and a 2m double glazed sliding door, and a 1.5m wide double glazed window.

 

Here's a video of what it looks like. It has had a tidy up since then!

https://youtu.be/JifnW4bNmRw

 

 

Usage

 

Summer:-

It is a multipurpose building, next to a small outdoor swimming pool. We'll use it as a place to sit and supervise the kids in the pool in the Summer, and somewhere for them to change and have showers. Doors open in this usage, DHW requirement is for multiple consecutive showers.

 

We'd also use it as a home cinema. In this usage, we'd have the doors and windows closed - which is where the Summer cooling requirement comes from.

 

Winter:-

In the winter we'd use it at the weekend to go "on holiday" for the day as a lounge area or to go and watch a film. We'd want to decide with 30 mins or so notice and for it to be able to warm up quickly. In this instance the UFH would be nice, but not if it takes ages to heat up.

 

Other usage year round - Home office (Cooling and heating)/general entertaining/somewhere for the kids to build a Scalextrix track without cluttering the house.

 

In summary, it may not be used for a few weeks at  a time, but when it is used, the demands on it are quite high.

 

 

Heating

The A2AHP seemed ideal, as if the building is not being used for a few weeks, then we could set it to keep the building dehumidified and at a base temperature at minimal cost.

 

The Wundertherm system seemed to deliver rapid warm up by not heating the slab. It has the pipes within insulation boards, then a 5mm renovation screed then the floor covering. Either way I need to bring the floor level between 50 and 75mm.

 

The UFH seems to be adding a lot of complexity and cost, so maybe I should forget it and just stick to the A2AHPs, and a simple UVC for the HW.

 

 

 

 

 

 

 

 

 

 

[SteamyTea]

37 minutes ago, ap534 said:

Insulation

It may just pass.

But

37 minutes ago, ap534 said:

Windows

It is mainly windows.

 

I would do some thermal modelling and then see what size A2AHP is needed.

You can get systems that have one outdoor unit and 2 or 3 indoor units.

 

How do you heat the pool?

[ap534]

18 minutes ago, SteamyTea said:

How do you heat the pool?

 

There's a 18kW ASHP (cheapish generic model) + 15m2 of solar matting. The solar matting adds about 9kW in Summer sunshine. The pool has a telescopic cover over it and a solar cover which helps to retain the heat.

 

The PV provides most of the power for the pool during heating season.

 

 

18 minutes ago, SteamyTea said:

You can get systems that have one outdoor unit and 2 or 3 indoor units.

 

Yes, that was my starting point. The 2 aircon installers I spoke to said that 2 individual units work out around the same cost as a multi-split, and also modulate down better (If I was just using the office, then I'd only need a low level of heating/cooling to maintain the temp).

 

 

18 minutes ago, SteamyTea said:

It may just pass.

 

Is the wall, ceiling, or floor the problem, or all of it?

Edited January 3, 2022 by ap534

[SteamyTea]

2 minutes ago, ap534 said:

2 individual units work out around the same cost as a multi-split, and also modulate down better

Yes, that may well be true, and easier to install.

 

I forgot you have a decent amount of PV.  A2AHP it is then.

Still worth doing a basic heat loss calculation.  To give you an idea, my whole house is a similar size, but my heat load is less than a kW when it hits 0°C.  My place is 34 years old and nothing special.

[ap534]

ok thank you, so forget the UFH then? Even the Wundatherm "rapid warm up system"?

 

What would be a good way of raising the floor level 50 to 75mm given that I also want to reduce the thermal mass and not have the flooring cold to the touch? I need to self level anyway as there is 20mm difference in level in some places, but was wondering whether I can incorporate more insulation in the build up?

[SteamyTea]

4 minutes ago, ap534 said:

thermal mass

Ahhhhh, run for the hills.

 

Adding insulation will add mass, but reduce thermal losses.

 

Can you just add 50 to 60mm of PIR and then board and tile (I assume tiling as it is next to a swimming pool.

You could consider an electric UFH matt for the wetroom.

 

I find wearing shoes and socks cures the cold floor problem.

Edited January 3, 2022 by SteamyTea

[Jilly]

55 minutes ago, SteamyTea said:

Ahhhhh, run for the hills.

 

Adding insulation will add mass, but reduce thermal losses.

 

Can you just add 50 to 60mm of PIR and then board and tile (I assume tiling as it is next to a swimming pool.

You could consider an electric UFH matt for the wetroom.

 

I find wearing shoes and socks cures the cold floor problem.

That's a good look in a swimming pool. It would prevent verrucas too... ? 

[SteamyTea]

12 minutes ago, Jilly said:

That's a good look in a swimming pool. It would prevent verrucas too... ? 

This is my swimming pool. Needs a wetsuit.

Think the GSHP is not working till March.

[ap534]

3 minutes ago, SteamyTea said:

Ahhhhh, run for the hills.

 

I sense I've made a faux pas! Is there anything I should read?

 

3 minutes ago, SteamyTea said:

tiling

 

I don't really want tiles as they are cold and hard. Current plan is to get someone to fit glue down vinyl plank of a type that is not as slippy as the Karndean we have in the house. Or fit the waterproof vinyl click type. I was looking at sheet vinyl, but width is just too wide.

 

Thinking about the floor if I don't put UFH in :-

The slab is out of level by up to 20mm. Is it worth adding self levelling compound 1st to get a proper level to build up from or just put PIR straight down, then board, then self levelling compound on top of that? When we did the extension in the house they just added a few mm of latex compound on top of the slab - That was already level though. I need to fix the level for the floor so that the plasterboard/doors etc. can be fitted.

 

[SteamyTea]

1 minute ago, ap534 said:

sense I've made a faux pas! Is there anything I should read?

Just a thermodynamics book will do. Density, heat capacity and conductivity are not related to each other for any given material.

It is all to do with energy levels in the atomic shells. Gets a bit quantum at the lowest level, but does describe what us happening nicely.

 

4 minutes ago, ap534 said:

The slab is out of level by up to 20mm. Is it worth adding self levelling compound 1st to get a proper level to build up from or just put PIR straight down

Not sure, there are probably better people to ask.

[Marvin]

4 hours ago, SteamyTea said:

a kW when it hits 0°C

A kW per what? 

Is that a Cornish idiom?

Edited January 3, 2022 by Marvin

[joth]

14 hours ago, ap534 said:

•  150l unvented cylinder with high area coil, heat from the house boiler. High area coil for rapid heat up, and future use with ashp if we replace the  gas boiler.

 

This is just for the occasionally used shower, and the occasionally used basin tap? Seems kinda overkill and will likely lose as much in heat losses as actually get useful use.

Why not an instant electric shower heater, and a boiling water tap with hot-water tank (like Quooker combi, but there are cheaper alternatives)? The latter will do for the office tea service too. Easy enough to put it on a timer or motion switch so it's not consuming anything during long winter months.

[SteamyTea]

24 minutes ago, Marvin said:

A kW per what? 

Is that a Cornish idiom?

A kW is power, the rate that energy is delivered at. So in  this context it is the rate that thermal energy is delivered to the house.

If you break down a watt, it is a joule per second, a joule, which is the proper unit of energy, is a kg.m² / s². Which is quite hard to visualise.

[ap534]

3 hours ago, SteamyTea said:

Just a thermodynamics book will do. Density, heat capacity and conductivity are not related to each other for any given material.

It is all to do with energy levels in the atomic shells. Gets a bit quantum at the lowest level, but does describe what us happening nicely.

 

 

 

Ah ok, what I was thinking was putting insulation above the slab, moves that slab mass outside the internal thermal envelope, reducing the mass inside the envelope. With only 25 or 50mm extra insulation I guess it doesn't really move it outside, just slows down the heat transfer to the slab which is the point you made if I understand correctly?

[ap534]

3 hours ago, joth said:

This is just for the occasionally used shower, and the occasionally used basin tap? Seems kinda overkill and will likely lose as much in heat losses as actually get useful use.

Why not an instant electric shower heater, and a boiling water tap with hot-water tank (like Quooker combi, but there are cheaper alternatives)? The latter will do for the office tea service too. Easy enough to put it on a timer or motion switch so it's not consuming anything during long winter months.

 

Yes, I started off with that plan, however the electrical load became too high. I have a 16mm2 cable to the annexe - good for 50A which is approx 12kW. So I'd need to fit an 8kW shower max, which are not great for flow rate. I then have 2kw for a Quooker Combi style (we have one in the house) for the kitchen sink, which then leaves the bathroom basin HW to solve for. Then I have the A2AHP load plus any normal appliance load in the building.

 

Alternatively I could fit a 8kW Redring instant heater which could also do the bathroom basin, and use another hot water heater tap for the kitchen unit.

 

I will talk to my electrician again about whether we can go all electrical.

 

 

 

 

 

 

 

 

Edited January 3, 2022 by ap534

[Jilly]

5 hours ago, SteamyTea said:

 

Not sure, there are probably better people to ask.

 

But not about fashion ?

[ap534]

I've reflected on all the comments here - thank you.

 

A2AHP is sorted with a local installer who will do a commission only price and I'll order that in the next day or so.

 

Floor level will be brought up with PIR then T&G chipboard then vinyl click.

 

I'll not bother with UFH.

 

Telford do a 90l indirect UVC with a 21kW coil which looks good to me. Instant Electric shower and water heating isn't going to be practical from a loading point of view.

 

The comments about poor insulation gave me a reality check and made me ponder whether to add anything to the walls now? The plasterboard will be screwed to 2" battens around the perimeter walls which gives me fixing points for shelves and also a a service void. In the large room, OSB will be screwed to the battens 1st and the plasterboard on top of that again to give fixings for cabinets, speakers, TVs etc. I was wondering whether to infill the space in between the battens with PIR and foam the edges, or is it not worth it because the battens themselves will be a cold bridge to the wall?

 

Any other things to worry about while I'm at 1st fix?

 

 

 

[SteamyTea]

9 hours ago, ap534 said:

The comments about poor insulation gave me a reality check and made me ponder whether to add anything to the walls now

If you can add it now, you should.

You can calculate how bad the cold bridging will be.  White Pine has an across the grain conductivity of 0.12 W.m^-1.K^-1.

PIR is around 0.03 12 W.m^-1.K^-1.

Convert to the thickness dependant R-Value, along with the rest of the wall construction ( thickness divided by conductivity), add the all up, then convert to the U-Value (1/R).

Don't for get the roof.  You may find that is easier to do as well.  And you have 50m² of that.

 

  

9 hours ago, ap534 said:

A2AHP is sorted with a local installer who will do a commission only price and I'll order that in the next day or so.

Be nice if he could fit an energy meter at the same time.  Then you can see how much you are using.

https://www.manomano.co.uk/energy-monitors-2271

 

Edited January 6, 2022 by SteamyTea

[JohnMo]

Before you go A2A, have you lived, been in a building where you have A2A wall units? I have and found them noisy, blow drafts at you and are generally annoying. Would not have them in a house again.

[SteamyTea]

42 minutes ago, JohnMo said:

Before you go A2A, have you lived, been in a building where you have A2A wall units? I have and found them noisy, blow drafts at you and are generally annoying. Would not have them in a house again.

This is one of my concerns, especially as my house is tiny.  The two places I have been in with them, I never noticed them at first.  Both were good size rooms, probably 40m².  Which is about the liveable area of my house.