Interseasonal Store

[knighty]

if you don;t want to buy vacuum tubes etc....


what about coils of black hose ?


it'll be sad to waste the heat exchangers you've already made.... but long length(s) of black hose should be better than heating air then using that to heat the water ?

[SimonHobson]

... and are supposed to  circulate 300 cu metres per hour.

Under what conditions ?
Fans are like centrifugal pumps, and each has a curve showing fluid flow rate vs pressure. Typically, as the pressure goes up, flow rate goes down. It sounds like your long (and relatively small) ducting has too much resistance and so the fans aren't shifting anything like the amount of air you thought they would. Did you do any calculations of required air flow, and pressure drop in the ducting ? If not, then it might be worth sitting down with a duct calculator, then compare the figures you get with the fan performance curves.

EDIT: Ah, looking back I see you've used small inline duct fans. From memory these have p***-poor performance with back pressure and that might explain the problem. Not to mention, the long piece of corrugated pipe the air has to flow through will have a lot of resistance.

[dhaslam]

The ducting is OK  it is polyethylene and has a higher melting point than the PVC pipes in the panels.  The pipes  are perforated drainage pipes so they lose  some pressure  along the way. The outer ones were not fully wrapped because they got covered with clay and then couldn't be reached.   However the centre pipe doesn't heat it's layer much more than the others even though it has much the best return temperature.    I plan on doing a smoke test  to try to measure the rate of air circulation, all the loops are 50 metres long so it would need to circulate the air 750 times in an hour or about every five seconds.  The actual rate could be ten times slower which would explain the slow heating rate completely.    I think that circulating the air a short distance to a sealed box might reach  the required rate, specially because it  would be a closed loop.   

I think that a  big advantage  of using  water to transfer heat is that heat can be used immediately.   Having 30 sq metres  of heating surface will produce a lot of heat on frosty winter days and the store can still gain some heat in the summer to cover cloudy periods.