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726 posts

Ultimate Geek
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Topic # 90270 18-Sep-2011 12:23
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I'm looking to get some ventilation and solar heating in my rather cold house in Karori.  I already have gas central heating but I'd rather avoid using fossil fuels unless really necessary.

I quite like the idea of a box on the roof to capture solar heat and dump it into the house at the same time as providing ventilation.  As far as I can tell there are two competing products .. solarventi from the DVS group, and solamate from the HRV crowd.  

Using the neat calculator from http://mapserve3.nrel.gov/PVWatts_Viewer/index.html it seems the average maximum energy I can capture is 4.4kwh/m2/day.  So, for the largest Solarventi panel which claims to be able to recirculate air for 140 m2, and dimensions of 300 cm x 100 cm, then that's 3 m2, so max I can expect at say 70% efficiency is 9.24 kWh in an average day or less than 4 hours of a 2kW heater running.

I expect that would take the chill off the air, and the drying out of the ambient air would reduce my heating costs.

Anyone have any experience with these units?  And would there be any heat on a day like this which is completely overcast.  Some solar radiation is clearly reaching the ground ...  I asked the DVS guy who came round yesterday and he didn't think they had any installations of the SolarVenti yet in Wgton .. it being a new product.

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  Reply # 522649 18-Sep-2011 14:03
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gchiu: I'm looking to get some ventilation and solar heating in my rather cold house in Karori. ?I already have gas central heating but I'd rather avoid using fossil fuels unless really necessary.

I quite like the idea of a box on the roof to capture solar heat and dump it into the house at the same time as providing ventilation. ?As far as I can tell there are two competing products .. solarventi from the DVS group, and solamate from the HRV crowd. ?

Using the neat calculator from http://mapserve3.nrel.gov/PVWatts_Viewer/index.html?it seems the average maximum energy I can capture is 4.4kwh/m2/day. ?So, for the largest Solarventi panel which claims to be able to recirculate air for 140 m2, and dimensions of 300 cm x 100 cm, then that's 3 m2, so max I can expect at say 70% efficiency is 9.24 kWh in an average day or less than 4 hours of a 2kW heater running.

I expect that would take the chill off the air, and the drying out of the ambient air would reduce my heating costs.

Anyone have any experience with these units? ?And would there be any heat on a day like this which is completely overcast. ?Some solar radiation is clearly reaching the ground ... ?I asked the DVS guy who came round yesterday and he didn't think they had any installations of the SolarVenti yet in Wgton .. it being a new product.


I would do a cost benefit analyisis of capital costs on whether it is cost efffective. Air isn't the best way to heat a house. You are best using mass heating, such as hot water underfloor or radiators. You can get heated water coils installed underfloor boards. Many of these ventillation systems distribute the air at the ceiling, as as heat rises, that heat stays above head level.



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Ultimate Geek
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  Reply # 522661 18-Sep-2011 15:06
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As I understand it, hydronic heating is very expensive to retrofit, and requires a gas or wood chip fired furnace, though possibly a heat pump could be used to heat the water. And it wouldn't have the secondary aim of providing ventilation.

True that roof mounted units do dump the hot air at ceiling level and some of the units can be wall mounted so that the air enters at ground level. That's not an option due to infill housing.

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  Reply # 522673 18-Sep-2011 15:57
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gchiu: As I understand it, hydronic heating is very expensive to retrofit, and requires a gas or wood chip fired furnace, though possibly a heat pump could be used to heat the water. And it wouldn't have the secondary aim of providing ventilation.

True that roof mounted units do dump the hot air at ceiling level and some of the units can be wall mounted so that the air enters at ground level. That's not an option due to infill housing.


It can be done DIY style, as much of the cost is just getting under the house and installing all the piping. I would only consider it with a water heatpump.
But for retrofitting, radiators with a water heatpump, also connected to a woodburner with a wetback, would be a better solution than underfloor heating. I have seen this done and works very well. It does however depend on the construction of the house, and how easy it is to get the pipes in. If you are on pipes it is pretty easy to retrofit. If you are on a concrete pad, it is more difficult, as you have to run all the piping from the roof space.

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  Reply # 522956 19-Sep-2011 11:56
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I would be amazed if a solar system such as that will be of any use in an older house in Karori.


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  Reply # 522982 19-Sep-2011 12:48
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As above, without sounding negative, the problem with these concepts always tends to be, that they work best in bright sunlight, when you don't want any heating, and then don't work too well when it's cold/dark at night time, when you need heating the most....



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  Reply # 522986 19-Sep-2011 12:56
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These units are in use in colder climates than Karori's such as Greenland so I am not so worried.  We know how much energy reaches the ground .. it's just how efficient these things are at capturing it, and at venting it into the house.  Apparently Wgton has more sunshine hours than Auckland so the wind can be useful!

And it's not just a little heating, it's also ventilation which you need at all times, and not just when the sun is out.

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  Reply # 522997 19-Sep-2011 13:09
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How good is your insulation? If it's floorboards does air come up through them? Do you have good thermal drapes? Have you stopped any gaps that air can get in/out through? You need to do all of these before you look at heating, otherwise you're wasting your money with any type of heating.

Does your house have a ceiling cavity? If you do A standard DVS unit can work well. You have the sun hitting the whole roof, warming up the ceiling cavity. I have a little one. On sunny days, even if it's cold, you get significant heat, the house is at least a few degrees warmer than the outside air. On says that aren't sunny it's still a touch warmer than outside, but not much. I have a timer on mine, so it runs from 10am to 4pm in the winter, and in summer i'll run it 8am until 10am and then again maybe 7pm to 9pm (I don't like even the low level of noise the fans make). I run it even on cold days to push the moisture out of the house to make it nicer and also easier to heat. The heat pump comes on in the afternoon after the ventilation unit turns off.

The running cost of a standard DVS unit is pretty low, it's just a big fan.

I'd want figures about the amount of power it would produce in winter. In winter it might not even be able to power a ventilation fan on the dark cold days, I have no idea.

I had a very cold old house in Johnsonville. Between two layers of ceiling insulation, under floor insulation, wall insulation, retrofit double glazing in the living area half of the house (ie not the huge kitchen/dining area), and a heat pump it's now one of the warmest houses I've lived in, and i'm not fully finished with insulation yet. It cost around $15K, not counting the digging to let the guys put in under floor insulation. I'm going to take the reasonably modern fireplace out soon, it smells and there's a hole in the roof to stop it touching things causing fire, but that lets heat out. I'm putting in another heat pump instead, which is also much easier to use.




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  Reply # 523030 19-Sep-2011 14:08
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There was a guy doing this type of thing who was based in Hastings. His web site was floorstore.org but it seems someone may have pinched that site from him. He had a number of successful installs around the place and was taking hot air trapped in the roof between the roof material and building paper from memory. I will keep looking to see if I can find any contact details for him. Sorry I cant be more specific.

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