I am about to replace my north facing iron roof and I trying to work out if I can cheaply capture warm air in winter and use it to heat the house. I would like to take hot air traveling up between the sisalation and the color bond roof (Which i would assume to be quite hot except on windy days. I imagine I can create a manifold between the two layers near the apex of the roof and then duct it out. The ducting fan may work with a small solar panel. The ducting could be directly into a room or perhaps into a thermal mass such as an insulated stack of bricks under the floor(for later use). Does anyone have any comments / can you do it.
Using the gap between iron and sisalation to gather hot air.
(12 posts) (6 voices)-
Chip
MemberThere are a lot of plans and ideas on net about solar air heaters. It might be difficult to make an efficient airtight system using sisalation as the bottom layer. Perhaps you can leave the old roof in place and put battens on top then your intended layer of new iron. Darker colours are much better heat traps. Wind doesn't make a lot of difference to the temperature of a dark surface in good sunshine.
For efficiency sake, a system that returns air from the house to the lower end of the collector will be better but more complex and expensive to build and drive. If not using a return air system (in a fairly well sealed house, you can regulate where the warm air travels within your house by choosing where the displaced air will exit the house. Best if this is down low on southern side where air will be coolest. When your solar heater is working, you will be also getting good solar gain through north windows.
Because your house will be fairly comfortable when sun is giving good performance of your solar air heater, it would be ideal if your thermal mass absorbed a lot of that heat and gave it back to you when you would otherwise be paying for heating or else needing to rug up!
Solar air heating can be the simplest and most cost effective way of getting sun's energy into your home. Good luck!
Posted Monday 18 Nov 2013 @ 9:26:15 pm from IP # -
Johnnojack
MemberChip has explained it well.
I started building a system like you describe but it hasn't been finished off. I did it with the roof in place but it would be so easy with the roof sheets off. I sealed the bottom roof purlin to the iron with expanding foam and battened RFL between the rafters to make a space. Not finished or connected but the air does get hot inside the cavity. I intended to duct air from below through the air heater and back down again as using air from the ceiling space could introduce dust etc. Besides expelling the air from your room to compensate for the warm air coming in is a problem when you don't have low windows or a cold room. I have used the east facing roof as I want warmth from the sun in the morning, before it starts coming in the northern windows.With the roof off you should have no trouble sealing the RFL and installing ducts. You might consider using a corrugated foam insert to seal your bottom purlin as getting in below the roof to do it afterwards is very difficult.
Posted Monday 18 Nov 2013 @ 11:30:22 pm from IP # -
Frances
MemberI know this is an old post but I am wondering if anyone has anything to add. Does anyone know how hot a roof space gets in winter? All the information I have found is about summer and cooling!
I am thinking I can use a fan system designed for venting an underfloor space to vent the space in a double brick wall, drawing down heat from the roof space. As Chip suggests, this would store the heat from the day in the walls so they could radiate it during the evening. But will the roof temperature be high enough to make this worth while?Posted Wednesday 23 Jul 2014 @ 12:35:31 am from IP # -
Frances
MemberI know this is an old post but I am wondering if anyone has anything to add. Does anyone know how hot a roof space gets in winter? All the information I have found is about summer and cooling!
I am thinking I can use a fan system designed for venting an underfloor space to vent the space in a double brick wall, drawing down heat from the roof space. As Chip suggests, this would store the heat from the day in the walls so they could radiate it during the evening. But will the roof temperature be high enough to make this worth while?Posted Wednesday 23 Jul 2014 @ 12:35:31 am from IP # -
Greg
MemberI'm going with a gut feel and would say I doubt you would get enough heat from the air to do any useful heating. If you look at purpose built solar hot water, flat plate or evacuated tube, these things on sunny winter days might give you a tank of hot water. If you circulated that through a radiator or water to air heat exchanger you would do better I feel.
Someone on this forum has built a heat exchanger in their roof space as I recall with some success.
There is also a thread by someone who built the solar heater I mentioned above. Dig around in the threads, there is a wealth of info there.
You might even like to investigate a trombe wall !
good luckPosted Wednesday 23 Jul 2014 @ 2:01:44 am from IP # -
nhand42
MemberFrances said:
I am thinking I can use a fan system designed for venting an underfloor space to vent the space in a double brick wall, drawing down heat from the roof space. As Chip suggests, this would store the heat from the day in the walls so they could radiate it during the evening. But will the roof temperature be high enough to make this worth while?Unlikely. The heat capacity of air is too low to do useful work in winter. You need materials with decent heat capacity like brick or concrete or water.
You're much better off using the sunlight to directly heat a trombe wall, a concrete floor, or reverse brick veneer. Even solar hot water is borderline in winter.
Posted Wednesday 23 Jul 2014 @ 3:16:47 am from IP # -
Frances
MemberThanks nhand42.I quite agree about heating masonry or concrete directly world be the straight forward option. My problem is that the north face is taken up with windows to allow the sun into the house. The living area is suspended, so no concrete slab.
On the other hand the roof area is about 170 sm and it's a hip roof so I was hoping that air could heat up under the colorbond, rising to the top, from where it could be blown into or sucked down between a double brick wall.
Perhaps I'm better just with more PV panels, but now the feed in tariffs are hopeless this doesn't seem so good. Somehow one wants to conserve the daytime heat for the evening.
Posted Wednesday 23 Jul 2014 @ 3:33:33 am from IP # -
nhand42
MemberFrances said:
On the other hand the roof area is about 170 sm and it's a hip roof so I was hoping that air could heat up under the colorbond, rising to the top, from where it could be blown into or sucked down between a double brick wall.Not enough heat capacity to make it worth your while. Heating water in evacuated tubes and storing the energy in a tank, then releasing it later through radiators, that will work much better, but will be expensive and complicated. I doubt it'll ever pay back.
Frances said:
Perhaps I'm better just with more PV panels, but now the feed in tariffs are hopeless this doesn't seem so good. Somehow one wants to conserve the daytime heat for the evening.PV won't be cost effective either. Not for heating purposes.
You could investigate phase-change materials. I'm skeptical of their effectiveness.
Your best bet is more insulation and draft-stopping. An EER 10 house will stay warm overnight with minimal heating, possibly even none. Active technologies like fans and heat exchangers are most effective if your house is already airtight and well insulated.
Posted Wednesday 23 Jul 2014 @ 4:30:01 am from IP # -
Chip
MemberIf your double brick wall runs north south from a big window then it will pick up a lot of heat if it's painted dark. And since you don't have a slab floor then this wall is not shading other thermal mass so it's your main heat trap. We will use a dedicated home made solar air heater to heat a brick cavity wall. Starting from the bottom: foam insulation board, 3 layers of aluminium flyscreen separated by battens then corrugated clear polycarbonate sheeting. Metal flyscreen is cheap, creates good turbulence and reduces inefficiencies from sub optimal angles. Need to have good filtering on incoming air. Won't use a return air system as we agree it is harder and more expensive. Plus, having some positive pressure inside the house helps defeat inevitable leaks. Testing has shown that on a sunny 12 degree day near Canberra, air arriving in the house will be over 30 degrees for 4 hours.
In a reasonably well sealed house well insulated house with good solar access, it will be already cosy on the days when a solar water or air heater will be working well so this free heat needs to be stored in some way, either in water for later circulation or directly into thermal mass.
I agree that with no slab, trombe walls would be a good option.
Posted Wednesday 23 Jul 2014 @ 7:57:47 am from IP # -
Johnnojack
MemberFrances I posted on this earlier (see above) Not sure why nhand thinks it won't be worth it. You will get a lot more heat per dollar spent than putting in evacuated tubes etc. You are doing the roof anyway and installing the RFL is a worthwhile thing to do on its own. Simply sealing it off and ducting the air into the room below would add very little cost to the whole job. Once installed you will get the benefits immediately the sun comes out, on a cool day. Payback for say $150 extra spent will be far quicker than several thousand spent on solar panels. Not that I'm against solar panels but there are other options which shouldn't be ignored just because they are not common practice. If it is say 16deg inside then you don't need very hot air to add warmth to your room. I have measured 35deg above the foil when it is just 18 outside. This is not rocket science and has been done in many forms already. One method simply ducts air from the top of the whole roof space into the rooms below. It works apparently, despite most roofs having only one or 2 sides facing the sun at once. (the shaded sides would be losing heat). Your north only facing roof section should reach higher temperatures than the above system.
Posted Wednesday 23 Jul 2014 @ 8:10:13 am from IP # -
nhand42
MemberJohnnojack said:
Frances I posted on this earlier (see above) Not sure why nhand thinks it won't be worth it.Because Frances is talking about heating a double brick wall during the day and releasing the heat overnight. You won't move enough heat into the double brick wall using 35C air. Not for the purpose of heating the house overnight.
Direct sunlight onto the double brick wall is effective. That will capture a lot of heat and will be a useful overnight radiator. Passing slightly warm air over a double brick wall? That's not going to be effective. The maths just doesn't add up.
Posted Wednesday 23 Jul 2014 @ 8:56:40 am from IP #