[Kristen]

I wonder if one butt, rather than several, connected to some old radiators (to increase area-to-volume collection) painted black to absorb heat, would create a good enough convection thermal current to circulate the water butt's water and heat it more efficiently then just having the sun shining on the surface of the water butt(s) itself?

That said if the hot water returns to the top of the water butt I'm not seeing how, at night, there would be a thermal current to get the heat to circulate from water butt to radiators, and thus radiate to heat the greenhouse. Perhaps it is as simple as the water in the radiators cooling so flowing downhill back to the water butt and thus warm(er) water flowing out of the top of the water butt to the radiators.

Still struggling with what happens on a realy cold night, or during a spell of dull overcast and inclement weather - but I suppose if electric (or whatever) heater available, but the hot water store surficies some/much of the time its an overall saving.

[jrko]

@jrko

So my 8 x 225 litre water filled black barrels in the PT are actually doing harm?  I don't think so.  As far as I'm concerned they provide a buffering of temperature extremes (as much when it's v hot as when cold).

It'll take some persuading for me to believe otherwise.

Maybe re read my post 

The rads might work as the water plus the metal makes for a better thermal mass

[adri123]

But I think that the whole point of thermal mass for a PT or Greenhouse is to smooth out the extremes of either heat or cold.  My Black water barrels (IMHO) served as good a purpose at keeping down the temperature during heat extremes as keeping the temp up when it was cold. 

If you went for the black radiator linked to the barrels you'd soon want a way of disconnecting it when summer came.  I once made a DIY solar hot water heater and it had to be drained several times a day in summer as it was in danger of melt down.  All that heat wouldn't be good for the plants..
 

[jrko]

Just been to the plot.  There's a metal water tank of around 700l.  Right next to it are 7x 150kg reinforced concrete slabs.  They both get the sun for as long as it's out, which is all day today. 

I wasn't able to check actual temps but the concrete was air temp, so 6deg or so.  The water?  Significantly colder, and that's from 700kg of water.

Conclusion? At low temps concrete absorbs more heat than more than 4x the mass of water.  A smaller body of water would heat up faster but would cool down faster too.

[jrko]

Been reading up on water as a thermal mass. It is used, but requires large amounts of space. At this time of year I'd just heat your greenhouse or if you can't use another sealed 'grow house' inside your greenhouse

[Headgardener22]

Just been to the plot.  There's a metal water tank of around 700l.  Right next to it are 7x 150kg reinforced concrete slabs.  They both get the sun for as long as it's out, which is all day today. 

I wasn't able to check actual temps but the concrete was air temp, so 6deg or so.  The water?  Significantly colder, and that's from 700kg of water.

Conclusion? At low temps concrete absorbs more heat than more than 4x the mass of water.  A smaller body of water would heat up faster but would cool down faster too.

Hmmmm. You may (or may not) be right. Checking a temperature by feeling it with the skin is notoriously inaccurate. Water will damp the skin and evaporation will make it feel colder.

It comes back to what we are trying to achieve. Only a heat source will do anything but slow down the rate at which the mass of air in the greenhouse cools. So now, when the evening temperatures are cool and the nighttime temperatures are cold, nothing but a heater will make enough difference to keep the temperature in any part of a greenhouse above 10C. I honestly think that the answer is that any mass (water or concrete) will be better than nothing at all. Sealing and insulating will probably make more difference though.

[Baldy]

It was the song, "Sunday blooming Sunday", by U2 on the song thread was where I first came to truly understand the power of the all seeing Aunty.
I now try to be (at least) better behaved than Cadalot to stay out of trouble... 

Pip pip,
Balders

[cadalot]

Balders where did that come from? 

What the bleeping hell have I done this time! 

It's a ploy I tell you, a ploy to take attention off your antics   

[Baldy]

It doesn't matter what you have (or possibly haven't) done Cadders...

The point is, you've done something... and you are guilty (we all are) - its just a matter of time ' til Aunty works out what you are guilty.

(Its like a slightly more feminine/gardening version of George Orwell's 1984) 

Pip pip,
Balders

[Kristen]

I've got my loggers measuring inside temperature of a 4-shelf blowaway, zipped up in my Greenhouse, another in the greenhouse, and a third outside, to see what the lag / heat-store value of the blowaway is, if any.  Going to be a few more days before I have enough data to publish.

OK, some results ... bit of a cockup, but perhaps more useful data as a result.

I put the logging thermometers:

• On the top shelf of the Blow-away
• On the top of a pot say on the greenhouse border
• Outside the North/Shaded end of the greenhouse, on a brick.

All covered with a pot so that they were not in direct sun.

Result: Blow-away fell to a LOWER temperature than the greenhouse! However, I think this is because its thermometer was up (relatively high) on the shelf, and the one on the pot, stood on the greenhouse border, had the benefit of warmth rising from the soil.

But I think it shows that the temperature in the Blow-away falls just as quickly as the greenhouse itself, and (by itself) provides no additional insulation benefit.


(Click for larger image)

Some additional thoughts:

• The Blow-away was kept zipped-up all day, so it got hotter than would actually be the case. The greenhouse vents were opened on warm days.
• There were no pots / soil / thermal mass in the Blow-away, which might help.
• The Blow-away is a small volume so could be heated cheaply, relative to the whole greenhouse.
• Measurements are shown to 0.1C, but the thermometers are probably not accurate to 0.1C

More detailed article on my blog with a break-out image for one day and tables of falling temperatures / lag to outside etc. etc.

[jrko]

Blog?

Linky please 

[Kristen]

Not allowed to post a link, as per the forum rules.

There's an icon next to my name.

[adri123]

Bought one of these of ebay.  £10.39 inc postage brand new.

http://pcsensor.com/pcsensor-new-double-sensor-thermometer-pc-laptop-thermometers-temper2.html

Will test the PT as soon as it arrives.

[Kristen]

Bought one of these of ebay.  £10.39 inc postage brand new.

I've had a number of gadgets from China, always a bargain   Helps if Mr Customs doesn't get tempted to add VAT on the way in ... he's frequently busy, looking the other way, though

[cadalot]

From the web page Precision:  +-2C  so a 4 degree C accuracy in reading the temperature in the first place! But having the temperature wrong it will at least report variations in temperature accurately