heat pump for 5400g pond

[DaveB]

hello all
After recently installing a 3.68kw solar panel system last august. I am finding that during very sunny days up until Oct, the panels were producing in access of 3.8kw. The amount of energy I use constantly is between 1 to 1.5 kw ( depending upon what houshold appliances are on ie fridge, freezer etc) I therefore produce in excess of 2 to 2.68 kw for a good few hours each day.
My thoughts have been looking at installing a heat pump of an input power of no more than 2 kw and therefore run it for free.During times such as above. Recently I have been giving my koi a winter period of 7 degs which can be maintained by a gas boiler and is not too expensive. I am however finding that a lot of the cost for heating is now made during spring & autum and even in the summer as I would like temperatures to be higher than the miserable 20degs last summer. can anyone see any problems af running heat pumps in this way.Best Regards DaveB

[Brockp]

Hi Dave;
You will find a discussion on heat pumps (not used in the way you are proposing but it covers some of the pros and cons is some detail) if you just search heat pumps on this site......it was started by another Dave. The thread has a link to some good data on efficiency versus ambient temp.

I have been running a DURA 7 this summer for the first time and with ambient temps even in Hampshire not getting above the mid teens for most of the “summer" this pump kept my pond at 22+ easily and most days using less the 3Kw hours. Even now with ambient at about 10 dropping to 6-8 at night it is keeping my pond at 18 using 6 - 9 Kw hours per day. I planned to drop the pond down to 13 by now but I have left it at 18 as the weather has been so mild.

My only comment is they are big heavy and not very pretty ! and none of them yet have a de icer for the condensation as standard. There is a Heath Robinson kit (£100) you can get and retro fit but if you are using gas heating during the winter it won’t be a problem.

Peter

[DaveB]

Brockp
Cheers for your input, much apreciated. Best Regards DaveB

[Manky Sanke]

My only comment is they are big heavy and not very pretty !

Peter,

Have you been listening to Mrs Manky talking about my old girlfriends?

Is this the thread you mean?
http://www.koiquest.co.uk/forum/viewtop ... 29&t=11873

It wasn't started by a "Dave" but it covered heat pumps in detail plus a way to connect them into a system that has other heaters but without using a control panel. A "Dave" did contribute to it and I think he was the same guy that I ended up discussing heat pumps with in detail on another forum. He sells them. The thread on the other forum was getting a little over enthusiastic about what heat pumps could do, so because I've installed more heat pumps than I can remember, I stepped in, not to rubbish them but to make sure that no one expected more from them than was reasonable.

Heat pumps are brilliant in the summer, you could easily get the equivalent of 6kw of heat for every 1kw of electricity they use.

You won't get that performance in winter, 3kw - 4kw effective output for every 1kw that they use would be reasonable to expect. So instead of reducing your electric heating bill to one sixth as it would in summer, a heat pump in winter might "only" reduce it to one quarter or one third, which is still pretty good.

BUT as the air temperature gets colder, at a certain temperature the reducing efficiency of heat pumps means that they eventually won't be able to produce any useful heat at all and usually they just switch off. That means the pond might go from warm to cold while the heat pump is waiting for the weather to warm up a bit. Worse might be that in a bad winter with the temperature hovering around the critical temperature, it may switch on and heat the pond for a few days, then off for a while, then on, then off. This could cause the pond temperature to yo-yo.

This effect should not be over exaggerated, it would only ever happen in the worst of winters but it is a real possibility and I would always recommend that a back up electric heater be installed to be there in case of such an eventuality. It would hardly ever be needed but I would suggest that you weigh the value of your koi against the cost of having an electric heater "just in case".

The seller (Dave I think) joined the thread and instead of banging on with a load of sales talk, he was actually quite realistic. Obviously he highlighted that the air had to be very cold for this to happen but agreed that in exposed locations it was a possibility and that a back up heater would be a good idea.

So there you have a salesman who wants you to sell you something but he is realistic and isn't prepared to promise what heat pumps can't do. If anyone is thinking of buying a heat pump I would suggest going to him for straight honest answers. Tell him how cold it gets in your neck of the woods and he will tell you honestly what you can and can't expect.
http://www.koiquest.co.uk/forum/viewtop ... 29&t=11873

[Brockp]

Manky et al;

This was the email Dave sent to me when I raised some of the concerns expressed about heat pumps and koi ponds. This Dave is now a member on any Koi forum I know so I have obatined his permission to post his response to me and his contact details. I can honestly say he is a good honest guy to deal with if anybody is interested;

"Dear Peter;

In response to your queries:-

1. Have you seen the COP curve for the Dura+ range? This shows how the output varies with air temp.

see http://www.heatpumps4pools.com/myfiles/ ... 10a%29.pdf

Assuming your water is around 13c (the green line) the COP is around 4 at 0c and 3 at -5c.

eg the Dura+7 draws 1.2kw of power

At 0c it will output 1.2 x 4 = 4.8kw and at -5c 1.2 x 3 = 3.6kw etc.

The heat pump still produces more output than it’s input power even at low temps.

I normally size the units at 0c allowing some surplus power where possible. In extremely cold weather a backup heat source may be required.

It is however worth remembering that the heat pump can be used all year round providing cost savings. Many koi keepers forget this and just focus on its performance during the depths of winter at say -15c.

2. I appreciate your point on this. I am in talks with various manufacturers to design a heat pump specifically for the koi market. This will have the tray heater fitted as standard. We could of course supply one with all of the current heat pumps to be used on koi ponds, but this would increase the price by c £100. Not all owners experience an ice build up and so it is not always required. Some owners live in milder climate areas and do not suffer from ice build up.

Now that the kit is available, we always offer it with the heat pump when we know that the unit will be used on a koi pond.

Kind regards

David

Tel: 0844-8806749

http://www.HeatPumps4Pools.com

Hope this helps.

Peter

[Manky Sanke]

Peter,

The email you've copied looks like it has been written by the guy I mentioned. While he obviously is quick to underline the benefits of his range of heat pumps, he is being realistic about what you can expect in winter and has sent the performance curves to show what he means.

I've copied this performance curve from the link he put in the email to you because it will be easier for everyone to follow the explanation below if the chart is here rather than having to flick back and forth.
http://www.heatpumps4pools.com/koi-ponds2



The curves show that what you can expect depends mainly on the air temperature but also partly on the pond temperature.

If you were going to use a heat pump in summer and intended to heat your pond to 21 C, as long as the air temperature was above 10 C, you would get a COP between 5 and 6 (and a bit) as shown on the brown curve.

A COP of 6 would reduce the cost of heating your pond to one sixth of what you would have paid if it was heated by a normal electric heater.
A COP of 5 would reduce it to one fifth.

Following the brown curve down, at an air temperature of 0 C and a pond temperature of 21 C, the heat pump would still have a COP of 3.5 which would reduce the bill to less than 30% and at -5 C the bill would be halved.

Below this temperature the heat pump would struggle to produce much useful heat. I don't know at what temperature this range would switch off but all the heat pumps I have dealt with have a cut-off thermostat to prevevent them from getting to the situation where they are producing less effective output than they are using. Ie if it was using, say, 1 kw but producing less than 1 kw of effective output.

If we stopped trying to maintain 21 C in winter and opted for a more realistic temperature between 9 C and 13 C, things are better. Following the green and blue curves down, at an air temperature of 0 C the electric bill would be reduced to one quarter and even at - 5 C it will still be one third but, as you can see, if the air temperature goes much below about - 5 C there soon will come a point where it won't be able to produce any usable heat at all.

If the pond is nicely insulated and covered and you are only trying to maintain around 10 C ish, a heat pump switching off when the air temperature falls below the critical temperature for a few hours or maybe a day here and there over the winter won't really make a great difference in pond temperature. But a sustained cold period where the temperature stays well below freezing for a few days at a time will allow the pond temperature to drop especially for anyone trying to maintain a higher temperature.

This is why a realistic man like Dave recommends that a back up heat source might be needed and this really is a judgment call, balancing the temperature you want to maintain, the degree of insulation and the likelihood of prolonged cold spells in your area.

So as I said earlier, go to Dave and buy your heat pump, he will tell you the truth about what you can and can't expect.
http://www.HeatPumps4Pools.com

(And you might also mention who sent you so he knows where to send my commission. )

[DaveB]

Hi All sorry for not replying earlier and thanks once again for all your input. I have eventually pulled out another 1k for the Dura 7 in the hope of being able to run the unit during the afternoon on a timer and follow up with a thermostatically controlled gas boiler during the cold nights.
I am amazed at the output of my solar panels already producing in excess of 9kwhrs in febuary and mine are almost west facing.The maximum output is already 2.6kw and rising at about 2 oclock in the afternoon. To be able to run the Dura 7 heat pump for free it, the panels will require to produce 2.8kw or more as normally I use a constant 1.5kw for the pond & houshold.
I have read somewhere ( sorry if I can give no links or be more sure, perhaps someone has the exact figures)that due to gas being much cheaper than electricity per kwh the breakeven COP between boilers and heatpumps is around 3 so any thing above this is more efficient. If this is the case I might run the heatpump for longer and the boiler less Your thoughts are appreciated

[DaveB]

Heat pump is now up & running and due to the mild weather it is coping with heat my 5400g pond with running only 4 to 5 hours per day. The newer models are supposed to be fitted with a thermostat which operates within 0.5 deg C instead of 1 deg, so hopefully wiring a digistat into the flow control switch won,t be neccesary. However I was toying with the idea of wiring a timer into the flow switch instead and running it throughout the day when there is a chance( sunny days) of running it for nowt.I might also just use the gas boiler during very cold nights and the heat pump would have low COP figures and might struggle to maintain desired temperatures. Just a few thoughts really