ASHP for cooling: provisioning for internal and external units

[tanneja]

Hi all,

 

In another thread I got good feedback regarding the value of provisioning for active cooling in our full house refurb. 

 

My hope is that we don't need it.  We plan 35m2 glazing for our total floor area 180m2, 2 story house, 1930 semi detached house, with 4 additional skylights (probably with built in low solar gain finish).  The back garden is approx North facing, so the main sliders shouldn't be problematic for solar gain.  Also likely to get EWI for our solid block walls.  Solar panels are hopefully in our future to generate and also reduce solar gain through the roof.  Still, I like the idea that we have a solution should the house overheat during warm spells, if the planned MVHR isn't able to do very much for cooling.

 

It seems like the ideal would mean provisioning for:

- all 4 bedrooms,

- the snug at the front of the house,

- the main seating at the rear, and

- the garage (will be a play area primarily, within the thermal envelope of the house). 

 

That seems like at least 7 splits.  I have shown my attempt at these locations on the attached house plan, using the orange lines (and circles to help make more obvious).  I guess the pipe run would go centrally down the middle of the house, through the joists between the ground and 1st floor.  That seems like it would service the upstairs and snug really well, with a lateral runs needed to get to the garage.  Would anyone position the indoor units differently, to perhaps require fewer units, and/or be more effective?  FYI the house length (north to south) is approx 17m.

 

Then there is the outdoor unit(s).  Related to how many internal units are needed, will be how many external.  Multi splits typically go up to 5.  And where would I locate these 1 or 2 outdoor units?  We are a semi detached, and a corner plot, where a neighbour front garden is north of our garage.  It feels like the most convenient place for the run would be on the flat roof outside bedroom 2 so to limit the run lengths.  But that will be close to my adjoining neighbour bedroom (attached by our West wall).  Alternatives seem to be:

- the back of the house (i.e. the north of the property) at ground level, but given the vaulted garage and entrance ceilings, it means for long and not simple runs.

- the front of the house, where the bathroom steps back from the snugs principal elevation, but even if hidden by fencing, could draw attention from the street users.

 

I'm wary of the requirement for planning permission for cool-able units.  We don't live in a conservation area, but the threat of needing an expensive acoustic report and likely neighbour objections looms (especially for multiple outdoor units, if needed).  Still I feel I would sleep better through the build knowing the wiring and pipework (or just ducting) is in place to fit should the house suffer from hot weather overheating.

 

FYI purple rooms indicate vaulted ceilings in those rooms.

Semi transparent wide red lines indicate structural steels.

 

My sincerest thanks for any assistance

JT

Edited June 2, 2020 by tanneja
Added image

[tanneja]

I think the only place we wouldn't disturb anyone would be to have any external unit near to the porch area (the link between the garage and the house), but that will draw attention from the street, and could be an eyesore for the driveway area.  If anyone thinks that is a suitable location I would be happy to hear that, or alternatives.

 

Reading a post on single room systems, the Olimpia Unico Smart 12HP 9000 looks like a good all in one unit just requiring two holes in the wall.  Perhaps one of these in the main downstairs room, and then in the vaulted ceiling area of the upstairs landing with bedroom doors open during the day to get access to that cooling unit.  Can a unit such as this have the feed and extract air pipes ran 2.5m over the top of the bathroom, or does it literally have to be on the external wall?  The upstairs corridor doesn't have an external wall, although is vaulted so is right near the roof.

 

Honestly I have read tens of posts from the forum on this, and I know a conversion project rather than a new build doesn't have much excitement, but if anyone can advise me at all, I can make a decision on the system, as my builder wants to continue with the first fix next week after the Covid offsite.

Edited June 3, 2020 by tanneja
Extra info

[tonyshouse]

If you haven’t done it yet then try to design out the need for any cooling, building physics model will inform such a design

 

reasons, expensive to install and run, higher than necessary energy demand, might be outlawed at some future point. 
 

I like MVHR and love EWI, skylights will need shading to prevent losses and gains where external shading is a must.

 

looks like great plans - hope it is a cool success in summer and a warm one in winter like mine is ?. 

[tanneja]

Thanks @tonyshouse, I feel we are on the way to that with planned:

 

Relatively airtight

EWI and ceiling / roof more insulated

Tinted windows (SKN 176 II most likely St Gobain spec) for east and south elevations

External shutters for skylights

 

I can attempt a model, but I'm not particularly handy like that, just about cope with the 3D planner from DIY Kitchen website!

 

It feels like it shouldn't cost lots and be low mess to provision for some kind of active cooling now with ducts or pipes, whether we actually fit any system in say 2 years from now will be determined by the living experience.  I cautious as it feels so many builders here say the calcs underestimate overheating, or at least to be comfortable at home, they have ended up needing active cooling despite any calcs prior expecting them to be fine.  SO still very interested to hone in on the ideal active cooling solution for budget, effectiveness, and practicality.

[tonyshouse]

Night time cooling is a good option - basically running ventilation system on high through the night and letting it cool the structure then Keeping the sun out during the day 

[tanneja]

Can I ask this again to the forum, as I am undecided as to what would be the best arrangement for either a single 3-way or single 4-way multi split to service the whole house.

 

My current thoughts are to have a 4-way, with 3 units downstairs, one upstairs.

 

The one upstairs would probably be best placed above the shower room door as you emerge at the top of the stair well.  Presumably the indoor unit can oscillate the air left to right to find it's way into the 4 bedrooms from there?

 

Then downstairs, similar to where I marked previously, probably one in the snug at the front (south facing) room, one in the main living area near the kitchen, and for the third, rather than being in the gym / playroom / garage, maybe have in the porch / reception hallway, again hoping it can oscillate the air between the garage and open plan living area.

 

Would anyone think this is a sensible arrangement?  What would you do differently?  Would you provision for a second unit to better cover the home volume (200m2 / 500m3)?  I liked the idea of just one external unit which can provide heating (heat pump), so that I don't need planning permission.

 

We are doing everything else we feel we can to manage solar gain, I like the idea of being able to live in a fridge if we really wanted to.  Don't currently have PV, if energy usage skyrockets with bringing these in, would obviously have to look at that.

[SteamyTea]

You really do need to do some basic thermal modelling.

 

It is not hard, just a case of multiplying the thermal insulation number, the W/m².K, but the wall, windows, doors, ceiling and floor areas, the m², by a temperature difference, the K or °C.

Then add all those numbers together.

 

Add in some air loss figures, and some solar gain figures (these can be estimated from the external wall and window areas.

 

Air is not a good method to shift energy around, I blowing air into a corridor and hoping it will go into rooms is a bit hopeful.

And remember that if the A/C unit is all indoors (like a portable one), switching it on will actually increase the overall temperature.

Edited August 27, 2020 by SteamyTea

[tanneja]

Thanks @SteamyTea, I will attempt those calcs today.  I had hoped based on some anecdotal accounts from users here that any amount of cooling, especially in high places, had a profound impact on the house temperature, more so than expected.  Sounds like a calculations approach is needed.

 

Also, as of earlier this morning I restarted this post in the ASHP forum, I didn't know there was one for ASHP when I posted this originally.  Happy for either to be removed if causing unnecessary clutter.

Edited August 27, 2020 by tanneja

[tanneja]

Sorry to ask, but could you lead my thoughts on the bits I am unsure of?  I presume temperature difference of 8 deg is the most one would want to realistically achieve (i.e. 33C outside, wanting 25C inside)?  Is there a sensible airloss and SG based on the information

 

Does the resulting W value then tell me the cooling requirement I need from the ASHP? 

 

		w/m2.K	m2	dK	W
Renovation	EWI walls	0.28	115	8	258
	renovation ceiling	0.15	36	8	43
Extension	New Walls	0.16	50	8	64
	New ceiling	0.11	80	8	70
	windows	0.68	41	8	223
	floor	0.22	130	8	229
				Total	887
				air loss
				SG		(area averaged G value 0.46)
				Adjusted total

[SteamyTea]

Air loss can be anywhere between 0.3 ACH to 10 ACH.

As it is over heating you are interested in, and windows may well be open, work with between 3 to 5 ACH.  Air 'hold' around 0.00278 kWh of energy for every cubic metre, for every degree.  So if you are changing that air 3 times an hour, that is around 0.009 kW, or 9 W, so relatively small.  But at 8°C delta, that is 72W for every cubic metre.  Quite a lot.

 

Is SG single glazing or solar gain.

If single glazing work on 2 W/m².K

 

If solar gain, then it is a bit more tricky as this changes during the day, but with noon probably having a lesser affect that 9AM and 3PM.

This is because at noon, the sun is hitting the roof more, and the hot air in the roof void is trapped (to a certain extent) and is acting as insulation.

When the sun is lower in the sky, it is less powerful, say 800 to 900 W/m², but is hitting the walls and windows more directly.

as a rough estimate, work on 400 to 500 W/m²  as a worse case.  Unless you like working with compound angles an want to model the whole day.

Edited August 27, 2020 by SteamyTea

[tanneja]

Thank you @SteamyTea

 

We would have an MVHR system, it seems most users are happy with something like 0.5 ACH?  My expectation is we will end up with a fairly air tight house given everything is being changed (walls, floors, ceilings, windows).

 

As such with 500m3 house volume, does that translate to 5.6Kw cooling requirement?

 

Then, indeed SG = solar gain, just to say we plan 3G Internorm, with low SG glass on the upstairs east and south elevations (no west elevation window or exposed walls).  I have the m2 glazing averaged G value of the glazing (0.46).  .  If I were to apply your 500W/M2, would that be of the floor area or wall areas?

 

If I have approx 900W from the floor + wall + ceilings as per my previous post, how do i translate that all into sizing the ASHP to get impactful cooling?  For your guide, I was looking at the attached 4way multi split that claims to deliver around 8kW of cooling: https://www.appliancesdirect.co.uk/p/argo-4ms9k9k9k9k/argo-argo4ms9k9k9k9k-air-conditioner

 

I offer my apologies for my inexperience here, and of course offer my sincere thanks for your guidance.

[joth]

44 minutes ago, tanneja said:

We would have an MVHR system, it seems most users are happy with something like 0.5 ACH?  My expectation is we will end up with a fairly air tight house given everything is being changed (walls, floors, ceilings, windows).

 

I think most users are delighted if they achieve 0.5 ACH on a new build. For a refurb, 1 ACH is a typical (e.g. Enerphit) top-tier goal, and more realistically (e.g. AECB) people shoot for 1.5

 

To get any of these, you need to more than simply expect it, it has to be a very intentional goal. If you're doing everything via a main contractor, you need to make it a requirement of the specification you issue them. If you're handling sub contractors yourself, you need to be on top of every single one ensuring they don't ruin it. A single pipe or electrical cable could wreak havoc to achieving the goal.

As an analogy, no one says the expect their roof will be fairly watertight: they damn well require it as an absolute must! And in many ways airtightness is harder than that...

 

 

[SteamyTea]

1 hour ago, tanneja said:

We would have an MVHR system, it seems most users are happy with something like 0.5 ACH?

MVHR will only contribute to cooling when the external temperature is below the internal temperature.  But you still need to change the air.

So this may change the rate at which you change the internal air when you need cooling i.e. a lot at night, none during the day.

1 hour ago, tanneja said:

As such with 500m3 house volume, does that translate to 5.6Kw cooling requirement?

72W x 500m³ = 36 kW (Ah, that was at 3 ACH, a third of that would be 12 kW)

This seems very high to me, so I may have made an error somewhere.

Thing is that this is the worse case, and it only happens very infrequently, and then for a relatively short time.  Consider you would start cooling well before the temperature reaches your desired maximum, the peak power needed would be smaller.

I would have to knock up a spreadsheet to work it out properly.

1 hour ago, tanneja said:

If I were to apply your 500W/M2, would that be of the floor area or wall areas?

Wall area, but minus window aperture.  This is affected by the insulation levels as they slow the passage of heat into the dwelling.

Windows can be considered the same as floor area as the light passes though and hits something, and starts to heat that.

1 hour ago, tanneja said:

If I have approx 900W from the floor + wall + ceilings as per my previous post, how do i translate that all into sizing the ASHP to get impactful cooling?  For your guide, I was looking at the attached 4way multi split that claims to deliver around 8kW of cooling

That implies that it is 8.8 times oversized, which sounds odd.

 

I think you are on the right track to sorting it out, just needs the details filling in.

Edited August 27, 2020 by SteamyTea

[tanneja]

@joth thank you for righting my false understanding, as stated previously, I am very green with the science of houses.  I'll carry through 1.5 ACH in that case, does that then equate to 17kW (0.00278 kWh x 1.5ACH x 8deg x 500m3)? 

[tanneja]

Thank you again @SteamyTea

 

Understood on the MVHR.  I thought I had read that those who intend to use AC would likely turn off MVHR during the day and then night purge to recycle the air.  I didn't know if this was ok for health / the house, and a viable strategy since MVHR, as far as I knew, should always be on to retain their balance.

 

On the ACH, it sounds like 1.5 is what most refurbs are lucky to achieve (I don't plan to be lucky, but am going with that standard case), we are not planning to open our sans trickle vent windows.  That seems to need 18kW to cool the all air in the dwelling.  However, if as you say you can do that in a strategic way, where you begin to cool the air as it heats, then perhaps maintaining the differential with outside would be less power heavy.  Would half of that maximum load be reasonable as an approximation (i.e. 9kW)?  Please, if you were considering it, don't go to the trouble of anything more detailed on my behalf, somewhat logical guesses is more than I could ask of anyone.

 

RE SG, walls minus windows is 130m2, how should i interpret 65kW (130m2 x 500W/m2)?

 

The first part where I looked at the house surfaces, their relative conductivity, maintaining 8 deg between their surfaces, seemed to yield 0.9kW

The second part, ACH, seemed to yield 9kW

The last part SG, I'm really not sure, its obviously not 65kW (well, I actually have no idea, I take many things are face value).

 

I'm struggling to have a feel for if these numbers are reasonable.

 

 

Edited August 27, 2020 by tanneja

[SteamyTea]

6 minutes ago, tanneja said:

RE SG, walls minus windows is 130m2, how should i interpret 65kW (130m2 x 500W/m2)?

Only the wall area that is exposed to the sun at any one time.  May not have made that clear, in fact I didn't at all.

[tanneja]

Just now, SteamyTea said:

Only the wall area that is exposed to the sun at any one time.  May not have made that clear, in fact I didn't at all.

Ok, that would still be a third of that (we are a 3 sided house due to being semi detached).  So that is still 22kW...what does this number mean relative to the other two figures calculated?  It can't be the instantaneous power needed to combat solar gain can it?  For clarity we only plan to have 0.28W/m2.k walls as the best we can do with the EWI thickness / soffit overhang we have to play with, is that somehow taken account into the 500W/m2 figure?.

[joth]

Walls will reflect off a lot of the solar energy (especially if you paint them white!), I have no idea really but I can't think it makes sense to model that except on windows (and even there, attenuated by the g-value)

For the walls, I'd start out by modeling it as a the delta-T between worst-case external air temp (40ºC?) (or wall surface temp), and the  indoor temp, passing through the wall U value over the given area.  So e.g.  20 * 0.28 * 120 = 672W 

 

Again  I have no idea really :-) my next step would be reverse engineer how PHPP does this.

 

Edited August 27, 2020 by joth

[SteamyTea]

5 hours ago, joth said:

or wall surface temp

Actually, I think that is a pretty good way to model it.  Certainly a lot easier than working out the forcings as the sun moves around the house during the day.

If the rain stops, and the sun comes out, I may get my IR thermometer out and see what sort of temperatures the walls get to.

Edited August 27, 2020 by SteamyTea