Posted By: John WalshPerhaps counter-intuitively, the further North you are the greater the potential to make use of solar gain in the period from February to June

Say more about that? Why 'further north'?

Fostertom - apologies, but I can't at the moment find the references for this (they're on an old PC in storage). From memory, the thinking is that in the UK for example, the further north the greater the effect of spring time 'seasonal lag' - i.e. average air temps lag behind rising insolation levels. So, on the south coast in mid-April there might be an average daytime temp of, say, 15 deg and nightime of 10 deg, with a maximum sun altitude of around 50 deg. Meanwhile, 'up north' it could be 5 deg cooler or more, while the max sun altitude is similar at around 46 deg. Hence, there might be a greater need for heating at a time when insolation levels are high enough to easily enable harvesting of heat.

I'll dig out the references later.

Ringi said “people designing new homes, are not interested in “free” heat at a time of the year they don’t need it” which is true, but why say it? Maybe you are assuming that anyone with MVHR must live in a new build passive house. I installed MVHR 20 years ago because I live in a bungalow and trickle vents don't work in bungalows unless it's windy. I remember the MVHR salesman saying that he sold lots of systems to people in bungalows, so I'm far from unique.

It's funny when a thread goes so far off topic that even the original poster doesn't address the question originally asked:

Posted By: EasyBuilderWould it be better to take the fresh air from a solar collector on the south wall mounted below the eaves? I've seen them made from twin wall polycarbonate, or from perforated metal, but don't know which would be best, or what the optimum size would be. I'd appreciate tips from folk with experience of these things.

I think the answer is that, yes, it would be better in the winter to use a warm air solar collector mounted on the south wall rather than trying to take heat from the loft which, as the original post says, doesn't work well:

However there's less benefit in winter, not just through less sunshine, but also because the roof loses a lot of heat at night and is slow to warm up.

But, putting warmed air into the MHRV when it's doing HR (ie., not in summer bypass) seems perverse as most of the heat from the collector will just be blown out again in the exhaust air. If the MHRV is 90% efficient then only 10% of the harvested heat will get into the house.

Therefore I think some other form of solar collector to provide space heating would be more cost effective. Options might include:

1) solar warm air warming house air but not associated directly with the MHRV (though you could use one to heat the supply, or even the extract, air);

2) solar hot water (e.g., evacuated tubes) for space heating or fed into a post heater on the warm air supply; or

3) PV panels supplying space heating (e.g., to a MHRV post heater).

A solar warm air collector might be, optimistically, 50% efficient. Preheating a 90% efficient MHRV would result in overall 5% efficiency. Even a PV panel at, say, 15% efficiency naively wired into a simple heater (no attempt to match the load - just pick a resistance suitable for weak winter sunshine) would likely beat that. It might be slightly more costly in raw materials but a lot less hassle to set up (smaller holes in the wall, etc).

(BTW, my house will have elements of all three of the above options. I've given some thought to how to heat MHRV air in various ways and haven't come up with a good solution which doesn't involve heat pumps in some way. There was a time when I thought that HPs were a good idea but now I tend to the buy more PV answer if there's roof (or ground mount) space available.)

OK thanks Ed, back on topic. I agree that warm air input when the system is on HR will not provide much benefit, and as I've found the loft temperature doesn't often get above the bypass trigger temperature in winter. This is exactly why I'm trying to figure out if a wall mounted collector would be better. By better I mean less thermal lag and so more likely to pass the trigger temperature in winter. So far John Walsh has provided some interesting information and I'd like to know more about his system. What I have in mind is to make a twin wall polycarbonate collector of about 3m² on my south wall, which I can do for less than £100, and duct it into my MVHR which will switch to bypass on input over 22°. I was hoping that somebody may have experience with this sort of thing and so give me some clues about what configuration, size and flow rates might be best. Maybe it's time to just do it anyway and post the results for the benefit of others.

Posted By: EasyBuilderWhat I have in mind is to make a twin wall polycarbonate collector of about 3m² on my south wall, which I can do for less than £100, and duct it into my MVHR which will switch to bypass on input over 22°. I was hoping that somebody may have experience with this sort of thing and so give me some clues about what configuration, size and flow rates might be best.

Try Googling "aerogel solar collector" and top of the search results should be a PDF project summary from www.lowenergybuildings.org.uk. It used an aerogel insulated wall mounted solar air collector in the extract leg of the MVHR.

David

... people ...?

Yes, thanks davidfreeborough. That's very useful information. I had not even thought of pre-heating the extract air.

Posted By: Viking HouseDoes anyone know where Aerogel filled Polycarbonate sheets can be purchased?

The only similar product I know of is manufactured by Kalwall.