Alternative Technology Assn Forums

1. 

   Ueee
   Member

   Has anyone ever considered or actually used the heated air in their ceiling cavity to heat there house? I know that several products are available to filter the air and blow it into rooms, but i was thinking about using it to heat the underfloor cavity of the house.

   We live in Canberra and currently have a dark redish tilled roof that is in need of replacing. My thoughts immediately turned to tin with aircell or simmilar underneath, but then i started thinking about the sheer volume of warmed cavity air we have. Our ceiling footprint is probably 250m, and at least 2.5m high at the ridge, so thats a lot of warm air. I put the insulation up in autumn and it still got mighty hot up there.

   Of course, my thoughts then turned to reversing the flow of air in summer to cool the roof cavity down with the cooler underfloor air. I will also be putting in some DIY solar fans to exhaust the cavity in summer.

   The cost of the aircell would be put into more insulation on the ceiling (currently R5.0) to help keep cool in summer.

   In a city like Canberra where we tend to use our heaters a lot more than our aircon, it makes some sense to have the house geared towards being warmer rather than cooler as it would need to be in say Brisbane.

   Any thoughts would be appreciated.

   Posted Tuesday 16 Aug 2011 @ 10:25:35 am from IP #
2. 

   jhamill
   Member

   yes - I'm using HRV system. I have been graphing my temperatures in roof space and house and it seems to be making only a marginal difference to actual internal temperatures according to the graphs..you would expect a bigger increase in internal temperatures compared to outside.

   But saying that, even in winter on a sunny day, the roof space is >30 degrees I can certainly feel warm air coming from the air vents into the house.

   Posted Wednesday 17 Aug 2011 @ 12:45:46 am from IP #
3. 

   ghostgum
   Member

   Before you put in any system that uses roof space heat, I suggest you put a thermometer (e.g. wireless, or logging thermometer) and record how warm it gets during the time of year that you want heating (mid autumn through mid spring).

   It would not work on my house, as I have a light coloured colorbond roof, with a radiant barrier underneath. Yesterday it was sunny for part of the day. With a solar air heater and sun through the windows, the house reached 20.5C. Outside reached 17.5C, and the roof space temperature was 18C. On the other hand, the roof is effective at keeping out lots of summer heat. My ceiling cavity does not get more than 5C above outside temperatures at any time of the year.

   Posted Wednesday 17 Aug 2011 @ 2:10:25 am from IP #
4. 

   tedthebug
   Member

   we are in canberra & have 2 solamate panels & 2 HRV modules (one slaved to the other). I have posted on other areas of this forum but basically we have (expensive) inslab heating which we have now set at about 15-16 degrees in winter. We wear jumpers when we get up, leave the curtains closed when we leave as we aren't home until after dark so would lose anything we gained by leaving the windows open & when we do get home, as long as it has been reasonably sunny, the inside temp is about 20-24 Deg. We cut our electricity bill by about 25% in $ terms & a bit more in usage since the slab is on offpeak.

   we reckon it will take a few years to pay for itself tho I have noticed on some sites that the cost for something similar to ours has dropped below what we paid so pay back would be quicker now. That said, the house has been a lot more comfortable for us so we are happy with the purchase for that reason alone (tho I'd be even happier if I'd paid less).

   Posted Monday 9 Jan 2012 @ 6:06:57 am from IP #
5. 

   photonthief
   Member

   Hi Ueee,

   Yes, I also have a tiled roof in Canberra, have observed how wonderfully warm it is in the ceiling cavity in winter, and have seriously thought about installing ducts and a fan to circulate the cavity air through the main living area, which is about 65 sqm.

   However, we now have a solution which is better IMHO, though not suitable for everyone. Quite independently, we recently installed a large (6kW) PV system. My wife had been asking for airconditioning for years, which I resisted for years because I don't approve of burning fossil fuels to produce electricity used for aircon. However, as we now produce greatly more electricity than we consume, I happily relented and we recently intalled aircon in the main living area. We don't use it much, but it is nice on those small number of stinking hot days, and the power consumption hardly even makes a dent in the power generated by the PV system.

   That is all by way of historical background. We have gas heating, and when I bought the aircon I didn't think too much about the reverse-cycle (R/C) heating capability. However, it slowly dawned on me that my previous plan of circulating warm air from the roof cavity is now completely obsolete, and makes no sense at all for my situation. In reality, the R/C aircon in conjunction with the PV lets us heat the house all day for no cost, and with no CO2 emission. For those that already have or are planning a largish (say >3kW) PV installation, R/C heating is a very efficient and elegant way of using sunlight to heat your house during the daytime. As a bonus, you get cheap and emission free aircon in summer, if you want it. As a further bonus, you make money and save emissions by selling the power generated when you are neither heating nor cooling, which is probably most of the time.

   The numbers work out very well. As you probably know, R/C systems typically operate with a COP of around 3.8, meaning that 3.8kW of heating is produced for every kW of electrical power consumed, which is about as close to a free lunch as you will ever get Our split system Mitsubishi Electric MSZGE71VA is rated at 7.1kW cooling and 8.1kW with COP=3.83, but we would virtually never need 8.1kW of heating. On most winter days, 3.8kW of heating would be (more than) enough, consuming 1.0kW of electrical power, which is easily met with even a small PV system, remembering that the PV system generates electricity ALL of the time, not just on those cold winter days when you want heating. I like playing with numbers, so here are some more FYI. The efficiency of solar panels is around 15.5%, but with a COP of 3.83, the effective efficiency is 15.5x3.83 = 59%, which is remarkably good. To look at it another way, standard solar panels are rated at 190W each, but with a COP=3.83, each panel can (in good sunlight) produce 190x3.83 = 727W of heating in your house, not bad at all for a panel measuring only ~1600x800mm.

   A thorough cost-benefit analysis would be interesting. For me, the environmental benefits of heating in this way are more inportant than the financial cost, in which case PV-RC heating is hard to beat. However, with the cost of solar panels falling dramatically to <$1/watt, PV-RC heating may also make good financial sense. If you invest money in a hydronic heating system, or in fans, ductwork and filters to use the warm air in the ceiling cavity, then that investment is saving energy, emissions and money ONLY on the relatively few cold winter days when it it being used. In contrast, if money is invested in a larger PV system to run R/C heating, then the additional electricity produced is usefully used ALL year, not just when heating is required.

   My apologies for straying slightly off-topic.

   Posted Thursday 12 Jan 2012 @ 6:16:59 am from IP #
6. 

   scott drew
   Member

   Photonthief, Since putting your solar panels on the roof have you measured the internal temperature of the roof space? Wondering if the panels shade the roof and allow the roof space air to stay cooler?

   Posted Sunday 15 Jan 2012 @ 7:36:55 am from IP #
7. 

   photonthief
   Member

   Scott, I have not measured the air tempertaure in the roof cavity directly. However, the panels are all at one end of the house where the bedrooms are. Previously, this end of the house got hotter in summer, whereas now this end of the house stays slightly cooler, which is an added bonus of adding the panels.

   As another indicator, on a sunny summer day the roof tiles literally get so hot they burn you if you touch them, whereas the tiles under the panels are only warm. In other words, the panels are a very effective radiation shield. However, keep in mind that you need a fairly large PV installation if the panel area is to be a significant proportion of total roof area.

   Posted Sunday 15 Jan 2012 @ 9:36:01 am from IP #

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