Green Building Forum - Using passive solar air heating in combination with MVHR

Using passive solar air heating in combination with MVHR

martinhobson — Mon, 27 Sep 2010 09:42:39 +0100

Hi all,
Anyone got any experience, advice or references on using passive solar air heaters (eg made from aluminium cans under glass or perspex) to pre heat the intake air of an MVHR system. I am in the process of designing an MVHR for our (100m2) bungalow retrofit and realised that the intake will be on a south east facing wall. Checking out 'You Tube' there are plenty of people (mainly USA & Canada) making these units (panels?) out of mainly scrap cans or aluminium downpipe or aluminium sheeting and ducting hot air directly into buildings but as yet I can't find anyone combining them with MVHR. I haven't yet contacted any MVHR specialists with the idea.
Martin

Using passive solar air heating in combination with MVHR

evan — Mon, 27 Sep 2010 09:49:30 +0100

I guess that if the solar-heated air is to provide heating, you'd want to use a "summer bypass" option, otherwise that heat is mostly transferred to the exhaust air!

edit: actually forget that, the temperature rise is going to be very small, so the incoming air will still be cooler than room temp, just slightly less cooler. Hook it up and let us know what happens :)

Using passive solar air heating in combination with MVHR

DamonHD — Mon, 27 Sep 2010 09:50:20 +0100

Ask on fieldlines.com or look around http://www.builditsolar.com/; they may not be calling what they're doing MHRV...

Rgds

Damon

Using passive solar air heating in combination with MVHR

SteamyTea — Mon, 27 Sep 2010 10:16:21 +0100

Did my dissertation in that area, may be able to help.
Classmate did his in using Redbull Cans, typical student.

Using passive solar air heating in combination with MVHR

DamonHD — Mon, 27 Sep 2010 10:22:43 +0100

Whole house done that way would surely smell like a bad nightclub: just add some cheap vodka and a faint smell of p*ss!

Rgds

Damon

Using passive solar air heating in combination with MVHR

SteamyTea — Mon, 27 Sep 2010 10:32:17 +0100

My lodgers bedroom smells like a hamsters cage, I never ask what does on in there!

Using passive solar air heating in combination with MVHR

davidfreeborough — Mon, 27 Sep 2010 10:49:45 +0100

Posted By: martinhobsonChecking out 'You Tube' there are plenty of people (mainly USA & Canada) making these units (panels?) out of mainly scrap cans or aluminium downpipe or aluminium sheeting and ducting hot air directly into buildings but as yet I can't find anyone combining them with MVHR.

There's a good reason for that. As evan said, pre-heating the external air input to an MVHR merely increases the exhaust air temperature. In other words, it increases the external air temperature as seen by the MVHR unit & thereby helps protect it from frost, but very little of the additional heat will be transferred into the house. If the MVHR heat exchanger is 90% efficient then around 10% of the additional heat makes it into the house, the other 90% will come out of the exhaust.

David

Using passive solar air heating in combination with MVHR

Viking House — Mon, 27 Sep 2010 11:59:43 +0100

There's a guy in Dublin offering passive air heaters as an add on to his HRV system, I dismissed it as an option because our Passive Houses and Passive Renovations don't need heating on sunny days. The solar gains in a Passive House more than compensates for heat loss through less efficient HRV.
Better to spend your money on insulation, better windows and airtightness, you will get a benefit from these on days when the sun isn't shining and at night, unlike passive air heaters.
A Fine Wire heat exchanger will anyway deliver return air of similar temperatures to air preheated through earth tubes or passive air heaters.

Using passive solar air heating in combination with MVHR

evan — Mon, 27 Sep 2010 12:06:50 +0100

I can see it helping in the case of a cold and frosty day but with good sunshine - then, the outside air temperature might be close to zero so even if your heat exchanger is very efficient, the air brought into the house is going to be noticeably chilly, and your heating system will use more energy to replace it.

If you add such a collector to the intake of the MHRV then it might be working with 10 degree rather than zero degree air, which would give it a much better chance, especially if it prevents it from icing up. The heat exchanger is still working the "right way" in those conditions, so you will get the benefit of the heat.

And if it's an old property without a large area of south facing glazing then you can probably use the help.

I suspect it might be easier to just turn the MHRV off in cold weather though :)

Using passive solar air heating in combination with MVHR

davidfreeborough — Mon, 27 Sep 2010 14:43:05 +0100

Posted By: evanI can see it helping in the case of a cold and frosty day but with good sunshine

Yes, this is a good point & I have considered a number of such schemes myself of varying complexity. The Sunwarm products from Nuaire in particular allow various forms of solar capture. One of these allows the solar air collector to act as a frost protection pre-heater for the MVHR unit & it will bypass the MVHR altogether if warmer air is available from the solar air collector. However, solar heating is best done by passive means &, for an airtight house requiring continuous ventilation, I think you need frost protection which doesn't rely on the sun being there. So I prefer frost protection based on ground-brine-air heat exchangers, electrical resistance pre-heaters or ground-air pre-heater ducts, in that order.

David

Using passive solar air heating in combination with MVHR

martinhobson — Mon, 27 Sep 2010 14:56:00 +0100

Steamy tea, yours is the only positive answer, (except for sunny frosty days ?) so how could you help?

Using passive solar air heating in combination with MVHR

evan — Mon, 27 Sep 2010 14:59:23 +0100

Whilst I respect ST's opinion a great deal, I can't really see how hamsters are going to help in this situ.. Oh, wait, sorry.

Using passive solar air heating in combination with MVHR

Viking House — Mon, 27 Sep 2010 15:10:23 +0100

I don't agree, you're spending loads of money on a gadget that is only required during the heating season, that doesn't work at night, and that doesn't work when the sun isn't shining in winter which is most of the time.
Better to spent this money on insulation and better windows which work for the entire winter, day and night and work when the sun isn't shining!

The same amount of energy can be collected from a south facing window as from a passive solar air heater but a window does much more.

Using passive solar air heating in combination with MVHR

RobinB — Mon, 27 Sep 2010 15:21:40 +0100

surely if you put some wood shavings around the inlet hamsters and other little creature would be attracted to sleep there and warm the incoming air .... any creature that hibernates would be ideal as they'll decamp come spring thus giving you an automatic summer bypass.

Using passive solar air heating in combination with MVHR

martinhobson — Mon, 27 Sep 2010 16:11:08 +0100

I really find this forum frustrating at times; presumptive, obscure and even mocking answers, another thing is that people eg Viking House assume that I am not doing all the other stuff anyway AND that I am about to spend money on a 'gadget'. If I do attempt to use this idea, it'll probably cost about Â£20 at most. I expect to be able to ask questions which may end up being stupid without being patronised. I'm trying to work something out. Perhaps you all do know more than I do, but even if you do, have some respect. Sorry guys but this feels like being bullied at school. I've had some very helpful answers (sometimes) but on other occasions, like this one, the 'GBFcollective' really tests my patience.

Using passive solar air heating in combination with MVHR

evan — Mon, 27 Sep 2010 16:26:19 +0100

Martin, I can't claim to be part of any "collective" but I don't really see any mocking or patronising here, only a little banter (and apologies for my part in that if it offended you).

What you do have is varied opinions on whether it will be useful or worthwhile doing (in our climate), and the reasoning behind that. Perhaps you disagree with the points made?

"Perhaps you all know more than I do" - I doubt it, but why ask a question at all if you aren't looking for input and aren't going to accept any "negative" answers?

Using passive solar air heating in combination with MVHR

Viking House — Mon, 27 Sep 2010 17:33:09 +0100

Posted By: martinhobsonI really find this forum frustrating at times; presumptive, obscure and even mocking answers, another thing is that people eg Viking House assume that I am not doing all the other stuff anyway AND that I am about to spend money on a 'gadget'. If I do attempt to use this idea, it'll probably cost about Â£20 at most. I expect to be able to ask questions which may end up being stupid without being patronised. I'm trying to work something out. Perhaps you all do know more than I do, but even if you do, have some respect. Sorry guys but this feels like being bullied at school. I've had some very helpful answers (sometimes) but on other occasions, like this one, the 'GBFcollective' really tests my patience.

Apologies Martin. I had no idea your solution was so cheap, I hope this information is of use to you, we transferred the air intake pipe from the north wall of a house to inside the garage last winter and it increased the temperature of the air entering the living room by 1.5 degrees. The house we built with the most efficient HRV is the one that first takes the air into the south facing sunroom and then draws the air into the house from there. So it seems to act like a buffer zone!

Using passive solar air heating in combination with MVHR

joe90 — Mon, 27 Sep 2010 17:54:55 +0100

I also want to pre-warm air for the house I plan to build and plan to copy the viking house "solar house" http://www.viking-house.co.uk/solar-house.html. Look at the "cold but sunny day" scenario. My only worry about this is overheating the sun space (and house) on sunny summer days so a form of shading is very important.

Thanks Viking House for all the info given on this forum.

Using passive solar air heating in combination with MVHR

rhamdu — Mon, 27 Sep 2010 18:36:40 +0100

Inclined to agree with davidfreeborough. It is pointless hooking your solar wall directly to your MVHR. It should have its own separate air circulation.

It might need its own small fan. Or you might be able to use passive stack convection - leaving the high-tech MVHR to sort out the fact that the top of the house is now warmer than the bottom.

Perhaps you could store some heat for night-time or when the sun goes in, which in Britain is usually not more than 30 minutes in the future. Could you add some cheap thermal mass to your collector? Maybe part-fill the aluminium tubes with pebbles?

Just ideas. I don't have direct experience here.

Using passive solar air heating in combination with MVHR

davidfreeborough — Mon, 27 Sep 2010 19:39:16 +0100

Posted By: martinhobsonI really find this forum frustrating at times; presumptive, obscure and even mocking answers

I hope you didn't find my answers presumptive, obscure or mocking; that certainly wasn't the intention. I was simply trying to say that for a 100% efficient MVHR heat exchanger:

exhaust air temperature = inlet air temperature;
air supply temperature = extract air temperature.

If you increase the inlet air temperature then the exhaust air temperature will go up, but the air supply & extract air temperatures will remain unchanged. In other words, the MVHR unit decouples the temperatures in the house from those outside. So, assuming that the heat exchanger is not frosting up, changing the outside air temperature that it sees will not affect the temperature of the house air supply at all.

A similar thing applies to a less than 100% efficient heat exchanger, just the maths gets a bit more complicated.

David

Using passive solar air heating in combination with MVHR

evan — Mon, 27 Sep 2010 20:19:22 +0100

Hang on a minute, doesn't it split the difference in temperature, even if it's 100% efficient?

Say the room is at 20 degrees and the outside at 10 degrees. Then the heat exchanger will reduce the temperature of the exhaust air by 5 degrees whilst raising the temperature of the incoming air by the same amount: 15 degrees air goes into the building. Right? Or have I gone mad?

Using passive solar air heating in combination with MVHR

Viking House — Mon, 27 Sep 2010 20:34:05 +0100

I always thought you were mad anyway
Last winter we were getting 18 degrees supply air when it was 20 degrees inside and 0 degrees outside with FiWi HRV so I think David's version is more accurate but the colder it is outside the colder the supply air is because you are starting from a lower point. So we'd expect to get 19 degrees when it is 10 degrees outside and 20 degrees inside and 17 degrees if it was minus 10 degrees outside.

Using passive solar air heating in combination with MVHR

rhamdu — Mon, 27 Sep 2010 20:42:57 +0100

Not only are countercurrent heat exchangers allowed by the building regs, they are also permitted by the second law of thermodynamics.

For explanation, Google something like 'why penguins don't get cold feet'.

Using passive solar air heating in combination with MVHR

davidfreeborough — Tue, 28 Sep 2010 08:51:33 +0100

Posted By: evanHang on a minute, doesn't it split the difference in temperature, even if it's 100% efficient?

Another way to think about this is that a heat exchanger is just a metal or plastic plate separating two airflows going in opposite directions. The heat is transferred from one air flow to the other by conduction from one face of the plate to the other. One end of the plate is at the inlet air temperature the other end is at the extract air temperature.

INLET AIR==>==>==>==>==>==>==>SUPPLY AIR
-----------------------------------------Metal/plastic plate
EXHAUST AIR<==<==<==<==<==<==EXTRACT AIR

OK, in a real heat exchanger the air flow is separated into lots of fine ducts to increase the effective contact area with the duct wall/plate, but the same principle applies. As long as the heat exchanger is 100% efficient & the air flows are balanced, the inlet & exhaust temperatures will be identical because those airflows are in contact with the same part of the plate.

If you increase the temperature of the inlet air, that end of the plate will increase in temperature by the same amount & therefore so will the exhaust air temperature. The temperature of the other end of the plate remains unchanged because it is defined by the extract air temperature.

David

Using passive solar air heating in combination with MVHR

davidfreeborough — Tue, 28 Sep 2010 08:59:23 +0100

Posted By: Viking HouseLast winter we were getting 18 degrees supply air when it was 20 degrees inside and 0 degrees outside

I guess this is because the heat exchanger is 90% efficient.

David

Using passive solar air heating in combination with MVHR

RobinB — Tue, 28 Sep 2010 09:41:00 +0100

Sorry - absolutely didn't mean to come across as mocking, just wrote what sprung to mind. Might start a new thread on using animals to help heat a home - e.g. old Swiss houses where the cows live on the ground floor in winter providing a modicum of heat to the upstairs family rooms.

Hopefully on a more practical note I do think a lean-to greenhouse with ventilation around the bottom perimiter would help especially when cold but sunny. Also worth looking up "trombe wall" .
RobinB

Using passive solar air heating in combination with MVHR

evan — Tue, 28 Sep 2010 09:44:13 +0100

Posted By: rhamducountercurrent heat exchangers

That's the one. Of course, silly me :)

Using passive solar air heating in combination with MVHR

Gavin_A — Tue, 28 Sep 2010 16:51:24 +0100

my two penneth

Id worry that what you could be creating is a perfect luck warm breeding ground for germ type nasties in winter, and in summer you'd end up with the MVHR system heating the house rather than ventilating it... or if the panel was bypassed, it'd run the risk of catching fire / melting something unless it was well built enough to cope with high summer stagnation temperatures.

it'd also only do anything for maybe a quarter of the day in winter when it's needed most, and even then it's likely to have minimal effect.

If we were talking preheating the air to an ASHP then that'd potentially be a different kettle of fish entirely, although I reckon you'd need a pretty big surface area to make much different to the volume of air needed.