Although the air in the loft gets vert hot (temperature) there is not a lot of energy to be gained or saved by this method, yes it will work a bit but building a Trombe wall may not be cost effective.

I'd agree that Trombe walls are likely to be ineffective. The only one I know if the UK (at the Brighton Earthship) was removed because it didn't work. However, EasyBuilder didn't mention one. Trombe walls have, by definition, a high heat capacity and perform indirect heating whereas EasyBuilder seems to be talking about thermally lightweight collectors more-or-less directly heating the air.

https://en.wikipedia.org/wiki/Trombe_wall

“A Trombe wall is a passive solar building design where a wall is built on the winter sun side of a building with a glass external layer and a high heat capacity internal layer separated by a layer of air.”

My plans include solar-warm-air collectors on the east and west gables of the house. I'll see how things work in practice and do some experimentation but mostly I plan to use them to directly heat the house rather than to feed into the MHRV. Feeding into the MHRV (as a preheat) seems attractive in that it effectively increases the apparent out-door temperature. The problem is, though, that you throw away a large percentage of the harvested energy if the air from the collectors is significantly warmer than the indoor air. If your MHRV is 90% efficient then 90% of your collected heat goes to warm the exhaust air. On the other hand, if the airflow rate is high enough that the collector is only raising the temperature of the air by a few degrees you'll still only get 10% of that into the house which won't be a lot.

(PS, limited experiments a while ago: https://edavies.me.uk/2011/12/solar-thermal-end-of-term/)

Posted By: tonyAlthough the air in the loft gets vert hot (temperature) there is not a lot of energy to be gained or saved by this method

Can you spell out why, tony?

My warning wd be that tho the loft gets v hot summer daytime, it v quickly cools as evening approaches and the direct sun goes off it or weakens. That's because of the great cooling effect of a clear sky (not a clouded sky) at any time of day or season - which is only masked by strongish sunshine. Roofs and other upward-facing surfaces get this cooling effect much more than walls - hence
the higher Uw required of roof windows,
dew (condensation on cold grass etc),
ground-frost even when air temp is above zero, and
saturated roof insulation in 'unbreathing' caravan sandwich-construction - litres of it. which can only escape by trickling down thro the walls sandwich (and which everyone thinks is 'leaks').

So despite the summer 'furnace' observation, a loft will be an extra-cold source for a great deal of the time.

You could say the same of the intake's cold outside air that's warmed by recovery from the exhaust. If that cold outside air is pre warmed from the loft (an 'if' which is being questioned here) then every little helps.

Posted By: djhThat's why EasyBuilder is triggering the summer bypass and just using the MVHR to distribute the air.

Posted By: EasyBuilder…when the incoming air is over 22°

But when it's less than 22°, so it's not bypassing, you'll only get a small percentage of the heat harvested by the collector, the remainder will be pushed out of the exhaust by the MHRV. It's a bit of a conundrum how to make effective use of somewhat warmed air in combination with an MHRV.

I haven't done the arithmetic properly but it could well be that the same area of PV as the warm air collector just feeding a post heater in the supply air duct would actually get you more heat. Suppose the warm air collector is 50% efficient, the PV is 15% efficient and the MHRV is 90%. With the warm air collector you'd get 50%×(1-90%) = 5% efficiency whereas with the PV you'd get 15% efficiency. Does the PV cost three times as much?

In the UK, I question if solar air heating is worth it apart from unheated buildings you wish to control damp in.

Windows have a higher heat lose then a wall with a solar air heater on it.

Is it the Genex MVHR system that has a built in heat pump. That seems like a good solution to me.

Posted By: EasyBuilderWell, as an example of performance, today, 23rd September

When reasonable insulation etc you don't need heating at this time of year even without solar are heaters etc...

Posted By: ringiWhen reasonable insulation etc you don't need heating at this time of year even without solar are heaters etc...

Yes, our summer bypass decided to open again today (i.e it's return air temp was over 24°C) and the temperature in the south-facing living room climbed to 26°C (which is fine by us, we quite like it warmer than the 25°C overheat standard). The rest of the house was about 25°C.

There is nothing fundamentally wrong with the idea, it is just that the scale is a problem. A wharehouse in Castle Donington is not the same as your house (I worked in CD when they blew the cooling towers up, it was a foggy morning, we saw nothing).

Posted By: EasyBuilderThe Solcer house http://thecccw.org.uk/wp-content/uploads/2015/07/Solcer-House-brochure.pdf) uses a transpired (perforated metal) solar collector to preheat MVHR input.

Interesting house - be nice when they build the rest of the terrace.

It also has a heat pump to extract heat from the exhaust air; that makes a big difference to how useful prewarming the MVHR inlet air is. By “prewarming” I mean increasing the temperature a bit but not to warmer than the indoor air so you don't want to go into bypass mode.

Posted By: Viking HouseUsing warm air from the south facing loft during the heating season to pre-heat HRV cools the loft, this increases heat-loss between the room below and the loft so you gain nothing.

I'm no building physicist, but I'd have thought that cooling the loft by removing heat, makes it a MORE efficient absorber of solar gain, due to increased delta T ?

gg

Hi gyro, Ok lets throw some rough numbers at it, feel free to correct me! A 1m2 of Solar Thermal collector will collect about 10kWh of hot water in December, a slate or tile will collect at a guess 15% of that because there's no glass over it allowing a slight breeze to cool the slate/tile, also the rough surface tends to be high emissivity, radiating most of the heat gained back out.
So a 100m2 South facing roof on a 200m2 house will collect maybe 150kWh in the month of December, at a guess this will increase by 15%? if the attic is 5 degrees colder, so 22kWh? The heat-loss from a ducted HRV system is about 1,000kWh in a 200m2 Passive House in December.
Any gains made from using the attic air for HRV will increase the heat-loss of the house by a similar amount?
If the Sun is shining on an insulated roof with no attic space it'll reduce heatloss by a similar amount in December so that's already accounted for in heat-loss calculations.
I believe the gains are too small for the effort.

Thanks for the link to your “limited experiments” Ed, there are useful lessons there. I'm hoping that a wall mounted collector under the eaves will not suffer from cooling due to to open sky radiation, snow, or rain (unless driven), and may also capture some heat escaping through the wall. I'm still not clear about the merits of glazed versus transpired collectors for this application. Also I had not realised just how cheap PV is now, so that option deserves serious comparison.