View Full Version : Grenhouse effect

12-08-2007, 06:24 PM
If I fitted the outdoor unit of an Air/Air heat pump system in a glass greenhouse will the efficiency be raised in heating mode due to the heat exchanger receiving warmer air than typical outside cooler ambient temperature conditions ??.
Many Thanks.

chillin out
12-08-2007, 07:04 PM
I take it you are just talking about a normal house garden size of green house.

How long do you think it would take before the air got cooled inside by the unit?
I would have thought this would be just like putting the unit in any type of housing.

Fresh air is the best for any out door unit.


12-08-2007, 07:14 PM
Point taken.
So in reality the unit will actually be less efficient.
Many thanks for your reply.

12-08-2007, 07:20 PM
Would it work if the air from the greenhouse was ducted to the outdoor unit hence receiving pre heating as the air travelled through the green house ?.

chillin out
12-08-2007, 07:26 PM
With the cost of ducting and a fan you would be better putting a heater in front of the unit.

All this said, if you try to make the system do more work than it's supposed to do then I think it will break down prematurely.


12-08-2007, 07:31 PM
Surely you only have to rely on the fan in the out door unit ?.
So ducting would be the only expense. If the unit was positioned against the greenhouse exterior then not a lot of ducting would be needed.

Will this raise the efficiency though ?.
I am led to believe that as outside temperatures drop the efficiency of these units decrease.
Hence my idea of pre-heating the outside ambient air via the greenhouse which extracts energy freely from the sun.

The Viking
12-08-2007, 08:55 PM
In principle you are right, the higher the ambient (or on coil) temperature, the better performance you will get from your split heatpump.

Now to the problems with your reasoning;

1. The fan in the outdoor unit doesn't cope well with pressure losses, it doesn't take much to reduce the airflow and with reduced airflow you get reduced heat transfer, thus any gains by having warmer air will be lost.:o

2. Your normal green house does not generate heat, it gets warmed by solar gains, it also lack thermal mass. Therefore, as soon as you start to drag fresh air through it, you will see the temperature inside it dropping. With a 4kW unit circulating about 9 cubic metres per minutes, whilst in heating mode, there will be a lot of air changes.:(

As you can see, my bet is that there won't be any benefits to dragging the air through a green house.
BUT I am all for learning and experimentation so if you decide to try it, please make detailed notes of the running conditions and any savings/losses, then share them with us here on the forum.

13-08-2007, 08:24 AM
What about the poor plants in the green house, you know tomatoes and cucumbers. Wont take long for them to grow into the fan unit and stop the unit all together.

A geen house is for growing food or plants, or are manufacturers now going to sell a green house with all heat pumps, i think not.:D:D:D:D

13-08-2007, 07:27 PM
Good point.
Though I have one which lies idle over the winter period.
Thanks for all the replys.
Much appreciated.:)

chillin out
13-08-2007, 08:25 PM
Solair, you are not really helping with the demise of the stigma surrounding the Irish with coming up with these ideas....LOL

(Only a joke by the way)