Portable aircon to cool roofspace + HRV

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MarkEm

3 posts

Wannabe Geek

#311272 1-Jan-2024 13:52

Hi,

Apologies if this has been asked before, i've read through a lot of HRV threads but not seen this talked about. I have an HRV system installed already, which brings down cool air from the roofspace when it's colder up there than in the house (and vice versa). BUT... this would be most helpful to me in the summer when the house is hot, but when the house is hot, the roofspace is usually much hotter so there is no cold air to bring down. At the moment my house is 23 degrees and the roof is 36.

I know the best option to cool my house would be a ducted air conditioning system - my house is a slightly odd shape and to use normal heat pumps we were told we would need three to get coverage. But the cost is more than I can afford right now.

My question is - could I get a portable air conditioning unit (i.e. from mitre 10) and put that up in the roofspace to cool it down, at which point the HRV would kick in and bring all that cool air down into my house? In my head this is a great idea, but i'm wary that i'm missing something obvious since I have googled it and can't seem to find anybody else doing it. I saw some references to heating/cooling a roofspace being inefficient because they aren't well insulated, but given my roofspace seems to hold it's heat very very well during the summer (36 right now!) i'm not sure I have that problem?

I would ideally like the same in reverse in winter i.e. use the aircon unit to heat the roofspace and then the HRV would heat the house.

Any advice would be much appreciated - am I missing something obvious here?

tweake

2391 posts

Uber Geek

#3176700 1-Jan-2024 14:05

simply put, no to both ideas.

first thing is ventilation systems are for ventilation not heating/cooling. the other is simply its going to be way more effective to put the portable aircon in the house. you will use less power for far more gain.

what could help with the ventilation system is a "summer" kit. ie an outdoor air inlet. that way the hrv pulls in cooler outdoor air. even better is automatic summer kit which automatically switch's to outdoor air when the ceiling temp gets to high.

MarkEm

3 posts

Wannabe Geek

#3176704 1-Jan-2024 14:13

Hi, thanks for that. I don't think putting the portable aircon unit inside will be more efficient - I would need three of them due to the shape of my house, otherwise it will only cool one area and the rest will still be hot. The HRV currently brings down cool air when the roof is colder (i.e. at night) so even if that's not it's main purpose, it does seem to function that way. The HRV is ducted into all the rooms in our house, so I think there is efficiency there even if it's not ideal.

I'll look into the summer kit idea, although on stinking hot days would that be any better than just having all the doors/windows open?

Sorry, not trying to argue back to your advice, I just want to understand it better

tweake

2391 posts

Uber Geek

#3176706 1-Jan-2024 14:38

main thing to remember is ventilation system doesn't flow enough air for proper heating/cooling. any heating/cooling by it is a bonus. the main point for the summer kit is so the cooler outside air keeps the ventilation system going longer. if you get a little bit of cooling from that its a bonus.

the whole point of ventilation system is you do not open windows/doors. you don't want all the flies, mosquitos, pollen dirt etc coming into the house.

downsides of portable aircon is they use a duct going outside, so you need to rig a a window etc for it to go through.

i have three heat pumps, i started with small one in the bedroom which makes sleeping a whole lot easier. then got the lounge one in and then the office. bed room ones are often very small and cheap. you might want to avoid the portable and spend the money on a bedroom one.

PJ48

295 posts

Ultimate Geek

#3176707 1-Jan-2024 14:41

We have had the official HRV summer kit installed for the last couple of years. Although it somewhat works, it takes most of the night to cool the house down, as the sheer air volume that needs exchanging is enormous. It does create some air circulation with slightly cooler air, but I am not sure if it is worth the extra expense. If you don't have a mosquito problem, or if you have mosquito screens for your windows/doors, then it is way faster to open all the windows and doors for a couple of hours in the late evening.

MarkEm

3 posts

Wannabe Geek

#3176708 1-Jan-2024 14:56

Thanks - the volume of air flow makes sense. Ah well, seemed too good to be true.

Appreciate the responses!

timmmay

20578 posts

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#3176710 1-Jan-2024 15:12

I modified my old ventilation system that took air from the roof space to take air from the eaves, through a hepa filter (for pollen / car exhaust) then a carbon filter (for smells). The air is much better smelling than when it was taken from the roof space. The air volume is far too low to cool, and the heating I got from the root space previously wasn't huge. I find it better this way.

Heating / cooling I have a ducted system, though if I knew now what I knew then I'd probably go for highwalls in each room. The main disadvantage of a ducted system is the air flows, so for us given how the house / vents are laid out cooling the bedrooms makes the lounge cooler than we want.

tweake

2391 posts

Uber Geek

#3176759 1-Jan-2024 15:22

PJ48:

We have had the official HRV summer kit installed for the last couple of years. Although it somewhat works, it takes most of the night to cool the house down, as the sheer air volume that needs exchanging is enormous. It does create some air circulation with slightly cooler air, but I am not sure if it is worth the extra expense. If you don't have a mosquito problem, or if you have mosquito screens for your windows/doors, then it is way faster to open all the windows and doors for a couple of hours in the late evening.

the last time i looked (quite a number of years ago now) was HRV and others had a very poor "summer kit". basically they ran off room temp rather than ceiling air temp (as they only have fan temp and room temp sensors). a the time only DVS had a system that had a ceiling air temp sensor.

i made my own using a basic mechanical thermostat connect to the motorized damper.

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larknz

1691 posts

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#3176821 1-Jan-2024 20:11

Be aware that in summer your roof space could get up to 40 - 50 deg. Trying to cool it would be a massive cost. In the middle of the day you shouldn't even be considering ventilation from the roof space.

Goosey

2829 posts

Uber Geek

Subscriber

#3176973 2-Jan-2024 06:58

How many sq m’s is your house.

what’s it’s shape? Is it multi level?

mudguard

2114 posts

Uber Geek

#3177092 2-Jan-2024 12:38

larknz:
Be aware that in summer your roof space could get up to 40 - 50 deg. Trying to cool it would be a massive cost. In the middle of the day you shouldn't even be considering ventilation from the roof space.

That's like my brother's house in Auckland. I've never understood how their HRV system is supposed to work. For much of the year the roof space is much hotter than the living areas. Then for a small part of the year the air is too cool to want to bring in.
And they blow annoying dust rings. To be fair I don't think they use it now anyway, they've got aircon installed.

tweake

2391 posts

Uber Geek

#3177095 2-Jan-2024 12:56

mudguard:

That's like my brother's house in Auckland. I've never understood how their HRV system is supposed to work. For much of the year the roof space is much hotter than the living areas. Then for a small part of the year the air is too cool to want to bring in.
And they blow annoying dust rings. To be fair I don't think they use it now anyway, they've got aircon installed.

this is the downside of HRV's (the company) BS advertising. the upside people got ventilation.

they are ventilation systems, not heating/cooling systems. so they should be used regardless of if they have heatpumps etc.

they typically will run at some point. usually the ceiling cools down enough for it to run. a mates place is pretty bad. a sunny day in winter and its 0c in the morning so its off. but his roof space gets to 40c within hours and it turns off. so it runs very little. it often runs later at night as the roof cools, which annoys him as he can hear it. this is why i like the summer kits, switch to outside air so the ventilation can keep going.