[adri123]

Double doors open each end of our PT.  I've now put half a dozen 210 litre barrels painted black under a section of staging as a heatsink to afford some frost protection.

Is there a best time for thermal efficiency as to when to close the doors at the end of the day?

TIA

Adri

[grinling]

I have the doors closed as it is a cold wind. seeds inside are under plastic sheeting.

[mumofstig]

Before it starts cooling down - the idea is to trap the last warmth of the day inside - before it escapes 

[JayG]

Assuming your barrels are full of water I'd suggest you use a thermometer to find the answer to your question - if you haven't got one, I'd guess the water will be at its warmest as the sun starts to go down (earlier if shadows are being cast on your GH.)

[snow white]

At least an hour before sunset to keep heat in.  I only open on sunny days this time of year.

[Steveharford]

I don't even open mine yet. Only to go in and out of course ! :-)

[3759allen]

i've found an hour before you start to feel the temperature dropping off outside, which is hard to judge especially when your busy. i quite often think "oh getting a bit chilly best check the tunnel" and it's dropped a lot of temp more than i'd want.

for me about 2-3 in the afternoon seems best. it's hard trying to judge the temperature of tunnels, they seem to heat up and cool down so quickly. 

[m1ckz]

i open mine about 10am  close around 2 or 3pm//at the moment i look in about 8am  its aroun d 12c then

[Headgardener22]

I would have thought its either when the sun goes off your polytunnel or when the temperature in the polytunnel starts to fall.

As said elsewhere, the reason to close the doors is to trap the warm air in the tunnel.

Do your heat sinks achieve anything? Have you measured the max/min temperatures of the water and the tunnel to see if there's a benefit? I know it inherrently "feels right", (and I do something similar, putting bottles of water by the seedlings) but I haven't been able to proove it does any good. 

[adri123]

The heat sinks definitely work.  The water in them is currently at +-15 deg C.  Given that there's over 1200 litres of water they have a big effect at night in keeping the temperatures up.

I'm going to get another 6 barrels soon so there'll be 2.5k litres.

[Headgardener22]

Will there still be room for things to grow?

Maybe my issue was that I was only using smaller volumes of water and so couldn't see the effect as much.

[JayG]

The heat sinks definitely work.  The water in them is currently at +-15 deg C.  Given that there's over 1200 litres of water they have a big effect at night in keeping the temperatures up.

I'm going to get another 6 barrels soon so there'll be 2.5k litres.

It is a lot of water and I'm not surprised it makes a difference - by my calculations your 1200 litres can release the equivalent of 28 kilowatt-hours of energy when the temperature difference between the water and its surroundings is 20C. How much difference that actually makes in practice is way beyond my knowledge of both physics and maths, but will depend on many variables, including of course the size of your PT and its insulation properties.

[Headgardener22]

JayG: what you say may be true but based upon what ADRI123 said about the temperature, to get that output, the polytunnel would have to be at -10.

My understanding is for tomatoes, ideally one should be trying to keep the polytunnel at 10c and my physics/maths struggles to see what difference 1200 litres of water at 15c will make. However, that isn't to make negative comments and I'm willing to give it a whirl (anything to reduce the heating bill!).

[JayG]

Adri123 may well be growing tomatoes HG22, but he hasn't mentioned it in this thread (only that he wanted to provide some frost protection.)

As my mum used to say "any improvement can only be for the better"  - according to my local weather station on a sunny summer day the sun's energy is equivalent to around a kilowatt per square metre, which is astonishing really - what would be really nice would be to be able to bottle some of it for winter use! 

[Headgardener22]

Adri123 may well be growing tomatoes HG22, but he hasn't mentioned it in this thread (only that he wanted to provide some frost protection.)

True. 

What I suppose I really meant was : would there be enough heat output from the barrels of water which are at 15c to adequately stop the temperature falling to the point where frost was an issue. I've tried it myself (with smaller volumes) and couldn't detect a benefit. However as you say, that shouldn't stop us experimenting.