Help sizing a heat pump

[A.L.]

hello,

So do I need a buffer tank or thicker slab?

First let us consider the heat load due to ventilation/infiltration. The heat loss in Watts is 0.3 x m³ x dT°C. Given your description I cannot believe the ventilation is less than 3 air changes per hour or a heatloss of 3.6kW for a temperature difference of 25°C.

If the workshop will be intermittently heated I do not think a slow responding system which a concrete slab would be should be used. I would use an air conditioning air/air heat pump with lightweight surfaces, including the floor if possible to provide rapid response.

I do not think you need a thicker slab. With an 8kW heat pump it is going to take 2hrs to raise the floor slab by 3°C

[Countrypaul]

As I mentioned earlier, and A.L. has put some figures on, the air tighness is a significant factor and may well completely outweigh the insulation levels of the wall and roof. Do you have any idea on how the roof/wall junction will be formed, what insulation will you have on the walls (I would guess about 100mm) of what and will the inside be sealed in any way (even if just plastered)?

The problem with all these calculations is that they are very dependant on the details of construction, so if for an example there are gaps between the roof and wallplate, then the air tightness will be much worse than that of a sealed roof wall junction as you would get with say SIPS.  Having the door to the workshop  being internal to the garage (if I have correctly understood the design), would significantly improve the situation of the roller door (you said there was only the one door into the workshop and no wndows).

Do you have a design that you could post up on here, it might save us having to guess about the details. If the air tightness is as bad as A.L. suggests then a buffer tank or thicker slab are both unlikely to solve the problem, unless you go for a very large buffer tank (for example well unsulated and beneath the floor to save space in the workshop).

If it is as leaky as A.L. indicates, then you are looking at 3 times the amount of heat required that I suggested just based on the insulation U-values, which might require a 30K increase in slab temperature ie. from 20C to 50C which would not be realistic imho, it would also take 20hrs from an 8kW HP assuming no losses, so you might have a very warm workshop at the end of the day due to duming heat into the slab just to have an acceptable temperature the next morning.

[TT]

I have a joinery workshop with 2 joiners in and out all day.
We have the following for heating.
Oil filled heater on all day in the winter on a thermostat keeps chill off and helps prevent rusting tools etc.
A 3kw infra red heated with selectable output that the guys put on when they are cold or glueing stuff up to increase local temperature.
A near infra red flat heater 600x600 meant to sit ion a false ceiling grid screwed to the ceiling above the table saw, on when they are doing a lot of table work in the winter
Roller shutter door open and closes all day, with a wicket door to help heat loss in winter for deliveries.

I have ASHP in another building with an air admittance pass of 9.999.
Heating is never off, due to builders/architects not wanting to achieve a higher figure and it wasn’t written into the contract.

Cheaper to buy and run some portable heaters than fit renewable land if the building isn’t fit for purpose.

[Eccentric Dyslexic]

It will be very well sealed, the only ventilation via gaps round the door. Even those are pretty well sealed. Gaps between blocks will be sealed as will joins between cieling insulation panels. Because this is a purpose built under floor heated workshop for one, high insulation values can be assumed.
I don't want a buffer tank if I can at all avoid it.
The walls hollow clay type 150mm thick glued not cemented, 10cm with insulation then another 150 hollow clay inside. They call it mur max over here. The cieling is 20cm thick insulation panels, glued and sealed.
100mm insulation below floor slab
Here's a YouTube video of the workshop proposal-
Edit, I can't seem to get the link to show, so add https to the beginning of this
://youtu.be/_6bDz_YZONQ

[Countrypaul]

The video doesn't seem to give much detail of the construction, no indication of the roof, nor the garage (infront of the door?) that you have mentioned.

If you intend to run a vehicle in there, for instance when fixing the engine, you may well need to consider some proper ventilation, also applies if you will be doing gas welding (at least that should keep you warm). This might be especially needed if the roller door merely opens into the garage.  You might be able to use a bathroom type MVHR unit which you could switch on/off when needed which would preserve most of the heat but give you fresh air when needed, or not move the air when painting.

Will the roof have trusses like a "normal" house roof or will it have a simple ridge beam and panels forming the sloping roof so being completely void?  Would you bt sotorings up there?

For what it is worth I believe making it well insulated and air tight is the right way to go these days.

The calculations I did on U-values assumed a standalone workshop, if there is a garage attched to one end, and especially if the door opens into that, the calculations should take that into account.
I also assumed the 8m was internal not external dimensions was this correct and if not how thick are the walls expected to be (the video implied quite thick)? That could easily reduce the size by 8m2 and the heat requirement by a similar proportion.  What were the two red boxes under the car in the video, are these area where you will have a hoist for example - again this will affect the amount of area that the UFH can cover (if you need to drill into the concrete to fit these then you don't want UFH pipes being hit), similarly if you need to cast something in place.

Did the designer give any values for target air leakage, eg. 3ACH ?

[Eccentric Dyslexic]

I am the designer with my builder friend. The aim to to keep heat in.  There will be no intended ventilation.
I am aware that I will need to open doors when I don't want to breathe fumes.
Yes the red things are scissor lifts bolted to below the heated slab, so they will be cold spots I can't avoid.
The roof will be apex flat panels, very simple, no joists.
Yes, sorry 8m external dimensions, walls are 15+10+15 thick, cieling 20cm thick Insulation.
It will be warmer than most houses lol

[Eccentric Dyslexic]

[Countrypaul]

I am the designer with my builder friend. The aim to to keep heat in.  There will be no intended ventilation.
I am aware that I will need to open doors when I don't want to breathe fumes.

As the fumes are likely to be warm they should rise to the top, hence my suggestion of the bathroom MVHR ventillation near the Apex. Without something to move the air inside it could be difficult to clear just opening the doors especially if they are the internal to a 14m garage.

Yes the red things are scissor lifts bolted to below the heated slab, so they will be cold spots I can't avoid.

Any idea of the amount of space these will occupy - I assume at this point it will not make much diffence to the heat requirement - but may make a difference to the aera you have available for heating.
Will there be any other areas that won't be available for UFH, for example do you want to store anything in there that is better off being kept cool (and can't be in the garage)?

The roof will be apex flat panels, very simple, no joists.

Yes, sorry 8m external dimensions, walls are 15+10+15 thick, cieling 20cm thick Insulation.

My fault, I should have asked about this at the start, for some reason I just assumed it woud be internal - probably because I only consider the internal dimensions when doing calculations at home.

It will be warmer than most houses lol

It certainly appears to be better insulated than many older homes (and probbly a lot of modern ones), how warm it is will depend on the heating though won't it?

[Eccentric Dyslexic]

I think the lift pads are about 50cm by 1.6m each. So, definitely will have an impact.
The fumes issue I can't seem being a issue as I won't be running engine in there, it's mostly restoration. Paint spray will be done In There though, I will wear an air supply mask for that, then shut the doors on them to dry....

[Countrypaul]

Maybe, but 1.6m2 off 52m2 (7.2x 7.2) is only about 3% - if the calculations are that close and I be very surprised!

If we base the heat loss on 7.2m rather than 8m, we will be looking around 1650W at 25K difference.

The door being the main issue, but if that opens into the garage, what is the door from the garage to the outside and is the garage as well insulated as that could have a significant impact on the heat loss.

[Eccentric Dyslexic]

Maybe, but 1.6m2 off 52m2 (7.2x 7.2) is only about 3% - if the calculations are that close and I be very surprised!

If we base the heat loss on 7.2m rather than 8m, we will be looking around 1650W at 25K difference.

The door being the main issue, but if that opens into the garage, what is the door from the garage to the outside and is the garage as well insulated as that could have a significant impact on the heat loss.

Will do our best to draft proof the main garage but it won't be sealed. It will certainly protect the inner door though. No additional windows and doors exterior due to breakins round here,  brits are often targets for theft as we tend to have stuff.

It will be likely an identical door - https     ://www.garagedoorsonline.co.uk/swsseceuroglide