Help sizing a heat pump

[Eccentric Dyslexic]

Hi peeps,
I am building an under floor heated workshop, 8m square with a 6" thick heated slab. It's very well insulated with no windows or doors except one roller door.
What size air/water HP should I go for? We have 3 phase and will soon have a decent solar PV system so want to use it to heat the floor in the workshop when the house has had what it needs. So, it will not run at night very often to keep costs down.
Cheers
Steve

[Countrypaul]

You question is a little loke how long is my piece of string.

You should carry out a heat requirement analysis first t determine how much heat is actually needed to keep the place upto the temperature you want (and also possibly how much is needed to keep it down in summer).

You will need to calculate the U values for all walls, door, roof, and floor and from there determine the heat requirement in Watts/K. From that you should be be easily able to work out the amount of heat required to keep the temperature at the desired level (being a workshop that is likely to be lower than an office or living room).

Everyones opinion of well insulated seems to vary, and windows and doors are often the weakest point. Is the roller door insulated (can the supplier give you a U value for the whole door in place?) and how high is the workshop?

If it what is considered well insulated, I would expect the heat erquirement for 25m2 to be under 500W - so using a heat pump and UFH just for that might be overkill. An air to air HP might be more than adequate and have the advantage of being cheaper and provide cooling in summer ( will run well off the PV in sumer). remember the PV may well contribute next to nothing in winter.

[GarethC]

I think one of the guys on buildhub circulated a good calculator. Can't remember who I'm afraid! I'm sure someone on here can point to one.

[A.L.]

hello,

I think one of the guys on buildhub circulated a good calculator. Can't remember who I'm afraid! I'm sure someone on here can point to one.

it was Jeremy Harris but you need to be a member to download it. Here is a more direct link to his website:-

http://www.mayfly.eu/

where can download it from the home page.

[Eccentric Dyslexic]

I definitely want the floor heated, I suffer badly with cold fingers so want a comfortable workshop. Also, will,likely be spraying paint in There so want as little air movement as possible.
The doors have a u value of 5.2W/m2K and are 3.5m by 2.5m.
The roof ceiling is apex type, with insulated bords that interlock and then are sealed. U value 0.14m2 I believe.
and the walls 13blocks high and same u value as roof.
Floor will have 100mm kingspan underneath.
Floor will be 100mm concrete with steel cube reinforcement and UFH pipes cable tied to it.
Can anyone put that through a calc and see what it comes out with?
Cheers
Steve

[ecogeorge]

Heating only 5m2 then you want a very small one ............Stoping any from cycling will be difficult -can you add a big buffer tank ??
Gonna be an expensive  capital cost for area .......  surplus pv ?? dump it into water tank and heat floor via mixer valve ??
George

[Eccentric Dyslexic]

8m square, I have edited my post, why I said 5m I don't know! It's 8m by 8m to save confusion.
Steve

[Countrypaul]

I definitely want the floor heated, I suffer badly with cold fingers so want a comfortable workshop. Also, will,likely be spraying paint in There so want as little air movement as possible.
The doors have a u value of 5.2W/m2K and are 3.5m by 2.5m.
The roof ceiling is apex type, with insulated bords that interlock and then are sealed. U value 0.14m2 I believe.
and the walls 13blocks high and same u value as roof.
Floor will have 100mm kingspan underneath.
Floor will be 100mm concrete with steel cube reinforcement and UFH pipes cable tied to it.
Can anyone put that through a calc and see what it comes out with?
Cheers
Steve

What size are the blocks, if 220 x 440 then the height would be 2.86m in block plus say 0.12 for mortar giving near enough 3m, but your door is how high 2.5m?

No indication of roof slope, so I will assume 45 degrees.

Area of walls = 4 x 5m x2.5m + (5m x 2.5m gables) - 2.5m x 3.5m = 50 m2 + 12.5m2 -9m2 = 52.5m2
Area of roof = 25m2 x 1.4 = 35m2
Total area around 90m2.

Area of door = 2.5x3.5 = 9m2

Heat loss = 90 x 0.14 W/K + 9 x 5.2W/k = 12.6 + 46.8 = 59.4 W/K (call it 60 for ease of calculations)

So if it was -3C outside and say 22C inside that would require 25 x 60 = 1500W

That excludes the heat loss from the floor into the earth, but it is obvious that the door is the major problem, without it you would be around 1/4 of the heat loss.

Someone else should check my calculations.

Can you do any better on the door as that amount of heat loss will cause significant air movement in itself. You may also find that opening the door will cause a rapid change in temperature inside that causes air movement and takes some time to restore the temperature.

Does the door have a normal size opening for you to get in/out which would reduce the problems?
.
Not sure where you are in the country, but if you assume an average of 10C over the year and the workshop heated to say 20C  7 days a week, then you will be looking at : 10 x 60 x 24 x 365 Whrs pa. so around 6000kWh pa. You can put more accurate figures in to the calculations , but that sounds reasonable to me. Of course there will be other heat added internally, from people working, equipment running (will the compressor be inside?) etc. The average temperature is not a good measure as you will need heating whenever the outside temperature is below say 15C, but not when it is above unless unoccupied for long periods, but when the temp is above 20C it will affect the calculation for average temperature but there is no corresponding cold period for the temperature.

You could try using degree days to get a value, but that is dependant on what you set as a base temp ie. the lowest outside temperature at which you need no heat inside.

HTH

===========================
Using your new figure of 8m x 8m:
Roof about 90m2, walls about 80m2 so around 72W/K, so at 25K difference that would be near 1800W, or 9000KWh pa

[Eccentric Dyslexic]

Thanks Paul, we are near Chalais an hours drive from Bordaux in France. I can't use another door, but I could maybe use one of those drop down curtains you see in supermarkets to double insulate the door? Or get an identical one and put it the other side of the wall?
I planed a pedestrian door, but not sure it's worth it. It will only be me in there, so the door won't be opened often. The floor slab will hold a fair amount of heat so will quickly feel warm once door is shut again. Open to ideas any way!
Roof slope is typically 33degrees over here.
Your wall block size is not far out I suspect.
I am looking at putting as much PV on the roof as possible, 'tis 22m long by 8m wide the garage building in total. The last 8m will be the workshop so the garage door will be inside the garage. So there will be some protection.
Cheers
Steve

[Countrypaul]

The lower roof slope would reduce the roof/wall area a little (about 10m2 in round terms).

Being near Bordeaux & Cognac means that I may have been a little pessimistic with my annual average temperature, looks like it could easily be well above 10C and worrying about frequent bouts of -3C is ighly unlikely.

I think some sort of curtain inside the roller door could well make a significant difference - read some of Biff's posts about his front door and use of a curtain (iirc.) If you were to use something thick like two layers of dust sheet with some insulation like rockwool between (you could go for something much higher spec - but I am thinking workshop rather than country mansion) I am sure that would make a large impact. How airtight is the roller door, as that could make an equally large impact, if it lets enough breeeze through to move the curtain. Try to ensure there is minimal air gaps round any curtain (though I presume you will need some ventillation) as air leakage can easily double your heat losses. Not sure what the drop down curtain is you refer to (can you provide a link?) but jus about anything will make a large impact on U-value of 5.2.

The floor slab should indeed hold a fair amount of heat, but the floor slab is only likely to be a couple of degrees warmer than you target temperature, after all worst case is 1800W over 60 m2 which is only 30W/m2. It may take some time to warm up the space inside if you have had the whole door open for a little while especially if its windy.

If you are putting a large amount of PV on the roof wll that all have to be exported - I know France has different rules to the UK but not sure about any details.

2KW HP is quite small and if it is only to heat the workshop it may end up short cycling a lot unless you have a buffer tank - short cycling can kill a heat pump so I'm led to believe.
I presume you are looking at an ASHP rather than a ground source?
It may be worth checking out the cost of using direct electric heating in the floor rather than a wet system, or direct electric heating of the wet UFH. getting better values for the amount of heat that is likely to be needed is rather key to being able to estimate the heating requirement accurately. If teh ehating requirement turns out to be much lower that the figures I have suggested the extra cost of an ASHP may not be worth while.

One way to deal with this issue is to put in a wet system but use direct electric heating to start with for a whole year. That should then give you an idea of how much electric has been used purely to heat the workshop (make sure you have a dedicated meter) over the year, and if recorded regularly when that heat is needed - it may only be Nov-Mar for example. Knowing howmuch heat is needed when will also allow you to make a more informed decision on the ASHP size.

Remember that you would not need to meet the worst case from the HP, but could supplement it during extreme cold spells with other sources of heat, that way you could ensure you don't oversize the HP and end up wth it short cycling.

In our house I ran our first full year of occupation on direct electric heating of our thermal store used to drive the UFH and DHW which the allowed us to have some accurate figures on which to size our HP.

[Eccentric Dyslexic]

The solar will be used onsite before exporting any (which will be lost as I won't get any money back) so i plan to have quite a large bank of solar PV over 3 phases with 3 immersuns dumping excess electricity into a well. The well is a feed to our heat pump for the house(which is away from the workshop) the plan will be to only run the HP when the sun is out and it's costing nowt to use.
So I will want to pump heat Into the slab, probably close to the bottom during the day and hope it stays warm all night. I want to avoid a buffer tank if possible. I could make the slab thicker...

[Countrypaul]

Will the heat pump for the house also be supplying the workshop, of will the workshop HP be seperate and if so what type ASHP or GSHP?

Dumping heat into the well to extract it via a heat pump sounds a very inefficient way of using your PV. If you used a HP directly off the PV (I know an Eddi can do this not sure about an Immersun) then you get potentially 3 or 4 times the amount of heat. You could use an ASHP if you would cool the well too much.

To heat the workshop slab during the day in winter when you have less PV and need more heat without using a buffer tank would probably mean having a large number of loops closely spaced together. Not sure if running at a higher temperature might cause problems with the concrete cracking.

100mm of concrete covering 64m2 gives you 6.4m3, iirc that is equivalent to about 13m3 of water. IIRC it takes 4.2 KWh of heat to raise 1m3 of water 1K (C), so about 55KWh for each degree in your slab. 1800W x 24hrs = 45KWh so you should have no problems storing the heat in your slab, but you may ant to control how quickly it can escape by puting something insulating over the top, for example a thick carpet Roll it out in the late morning to stop it getting too warm to work   and un roll it late on to keep it warm overnight ready for morning.

[A.L.]

hello,

100mm of concrete covering 64m2 gives you 6.4m3, iirc that is equivalent to about 13m3 of water. IIRC it takes 4.2 KWh of heat to raise 1m3 of water 1K (C), so about 55KWh for each degree in your slab. 1800W x 24hrs = 45KWh so you should have no problems storing the heat in your slab,

Unfortunately no, 6.4m³ of concrete has the same thermal capacity as about 3m³ of water and it takes about 1.16kWh to raise 1m³ of water by one degree K. So it takes about 5kWh to raise the slab by one degree.

[Countrypaul]

hello,

100mm of concrete covering 64m2 gives you 6.4m3, iirc that is equivalent to about 13m3 of water. IIRC it takes 4.2 KWh of heat to raise 1m3 of water 1K (C), so about 55KWh for each degree in your slab. 1800W x 24hrs = 45KWh so you should have no problems storing the heat in your slab,

Unfortunately no, 6.4m³ of concrete has the same thermal capacity as about 3m³ of water and it takes about 1.16kWh to raise 1m³ of water by one degree K. So it takes about 5kWh to raise the slab by one degree.

Yep, thanks A.L. - totally screwed my figures up - trouble with being interupted by an 8 yr old not willing to do his schoolwork 
Should have been half not double the amount of water, and it was 4.2MJ  I forgot to divide by 3600.  So I was only a factor of about 10 out and  having shown my workings I can't just claim a decimal point in the wrong place 

The result is that the floor slab will need to raise about 10K to cover the amount of heat needed for 24hrs.  This assues that my previous calculations weren't as badly screwed up of course!

[Eccentric Dyslexic]

hello,

100mm of concrete covering 64m2 gives you 6.4m3, iirc that is equivalent to about 13m3 of water. IIRC it takes 4.2 KWh of heat to raise 1m3 of water 1K (C), so about 55KWh for each degree in your slab. 1800W x 24hrs = 45KWh so you should have no problems storing the heat in your slab,

Unfortunately no, 6.4m³ of concrete has the same thermal capacity as about 3m³ of water and it takes about 1.16kWh to raise 1m³ of water by one degree K. So it takes about 5kWh to raise the slab by one degree.

So do I need a buffer tank or thicker slab?

The HP will be just for the workshop and air/water type.
If it got to hot in there, I will leave the workshop doors open and let the heat escape into the 14x8m garage.
The access to the workshop being through the garage will help keep it warm in the winter.