Self build mobile home with high efficiency ASHP questions

[Tricky]

hI

Im a new member and looking for advice on my new build granny annex for my wife and me in our later years and family visitors till then.

The garden house is under construction now by me with help from a self employed builder.

Its 2 bedrooms with overall dimensions of 15m x 6m and constructed with 63 x 38 cls framework and 100 thk Celotex bonded in with foam. 

The heating system I have in mind is a ASHP  Ebay 354341730323. 5Kw with the water temperature set to approx 45C

The hot water tank would be an unvented indirect cylinder approx 200l and would be on whenever the heat pump is on. There are 2 showers each with thermostatic valves set to 38C.

The room heating would be radiators each with thermostat control. A buffer tank using an unvented direct cylinder again of about 200l would be incorporated. The buffer would also be on all the time.

What do you think?

 

Perspective Views 08-09-2022.pdf

[ProDave]

A few random thoughts meant to be constructive:

 

With such thin sticks for the frame, and your proposed 100mm insulation are you planning a twin wall frame like a Larsen truss?

 

Celotex may give a low U value for a given thickness, but is not really the best insulation, Earthwool or blown celulose in a cavity will give a much longer decrement delay, which in simple terms means it takes longer to heat up or cool down, but you will need thicker walls to get the same insulation.

 

Under floor heating will allow the ASHP to run at a lower temperature and therefore a higher COP

 

ASHP's operate heating the building or heating DHW not normally both together, this allows you to set a different flow temperature for each.

 

That may not be the best choice of ASHP.  Unless they tell you otherwise that one is not inverter driven so it will be on at full power or off and won't modulate down to match demand, and will likely have a large power surge at start up.  you would be better spending a bit more for one that has inverter drive.

 

It looks like a good project, look forward to seeing a floorplan and watching the project progress.

[Radian]

9 minutes ago, Tricky said:

100 thk Celotex

If you're in the UK, you probably wouldn't meet building regs Part L (U-Value <=0.18 W/(m²K))

I would suggest you should aim for better than that anyway seeing as you're starting from scratch. 150mm Celotex might get you 0.15 W/(m²K) for instance, but there are other types of insulation to consider.

[JohnMo]

Have you worked out your design heat loss.  Although the heat pump says it 5kW as it get colder that figure drops. 

 

So you need to know your heat loss.

 

Unfortunately that heat pump only has a single flow temp.  You would really need to charge the buffer from the heat pump.  The ch or cylinder would call for heat from the buffer as they required from the buffer. The heat pump would automatically charge the buffer as needed.

 

 

[Tricky]

Thanks for the comments.

The 100mm celotex insulation was installed last week so no option to change now.

Heat load with a internal temp of 22C and external of 0C is less than 1.8 Kw/hr

I have also found this unit which has an inverter drive and a remote controller Ebay 295373873999

Would seem that controller would offer better control of living area temperature as it has a timer function.

Im sizing the radiators on a water temperature of 45C.

 

 

 

[ProDave]

Not what I expected. Under what planning rules are you building this?  To be a "caravan" in UK planning law it needs to be moveable.  That looks built very close to the ground, I don't see how you could ever move that. What sort of foundations is that on?

[Roundtuit]

23 hours ago, Tricky said:

What do you think?

Honestly, if you're planning on living in this for an extended period of time, I'd have a re-think before you go much further.  Has this been professionally designed?

[SteamyTea]

1 hour ago, Tricky said:

I have also found this unit which has an inverter drive and a remote controller Ebay 295373873999

Looks a bit basic.

[Spreadsheetman]

Is that built under permitted development? (Or whatever applies outside England)

[Redbeard]

What will the roof be? Structure, covering and insulation. Ditto the floor. Do those battens suggest that there may be only 50mm of PIR? I worry that you have a long time to be cold.

[Redbeard]

@Tricky wrote: 

"Thanks for the comments.

The 100mm celotex insulation was installed last week so no option to change now."

Maybe not...

Without an outside pic of the insulated shell I cannot be certain, but I guess you have a 37 x 38mm gap on the outside of each stud, and perhaps offcuts filling that gap. If you mark carefully the positions of the studs you could run another layer of PIR on the outside (unless the 6m and 15m dimensions are maxima under Mobile Home law - (later note, just seen a sugg'n that 6.7 x 20m is max so you'd be inside that). Then batten, then clad.

 

I am sure you had checked this already (though I had to check it as I did not know for certain)  T'internet says Mobile homes are not required to comply with the Building Regs, with the exception of certain groundwork and connections to mains drainage.

 

Nevertheless it does not mean that insulation to, or in excess of, Bldg Regs standards isn't advisable from a current and future comfort point of view. Accepting that it is too late to do anything about increasing decrement delay you could still 'engineer' significantly more insulation.

 

 

 

[Jenki]

On 08/12/2022 at 22:42, Radian said:

If you're in the UK, you probably wouldn't meet building regs Part L (U-Value <=0.18 W/(m²K))

I would suggest you should aim for better than that anyway seeing as you're starting from scratch. 150mm Celotex might get you 0.15 W/(m²K) for instance, but there are other types of insulation to consider.

+1.

I've built 2 cabins in Scotland, the walls needed 140mm frame therm and 25mm PIR to comply.

 

[ProDave]

My concern is this does not appear to meet one of the basic requirements of a "caravan" that it must be possible to move it in some way.  And the frame built so close to the ground level so it will be subject to rain splash which does not seem good.

 

I fear we may have frightened @Tricky from replying?  I do hope not.  

[Radian]

1 minute ago, ProDave said:

I fear we may have frightened @Tricky from replying?  I do hope not.  

I expect he's too busy trying to get it all ready to move in by xmas.

[Redbeard]

Pro Dave wrote: "I fear we may have frightened @Tricky from replying?  I do hope not."

 

Yes, me too. Do 'come back' @Tricky. We want to help you get a comfortable home which is safe from the elements.

[Roundtuit]

I think maybe Tricky just wanted comments restricted to the heating system proposal, rather than the holistic approach, and is probably too far down the line to be able easily change some of the fundamentals...