More Renewable Shenanigans at Nowty Towers

[Fintray]

I had exactly the same thing today as well, not as iced up but getting there, outside temperature was about 2C.

[nowty]

Time for an update after some serious usage of the A2A HP's during this cold spell with the GSHP switched off. I was away at the weekend and I left the A2A HP's on 24/7 + storage heater boost in the cheapslot as SWMBO was home alone with pussy.

When I returned home yesterday SWMBO was complaining that the temp in the house had dropped a bit. I noticed the house core temp thermostat had dropped from its usual 23 degrees to 19.5 degrees.

So it was either push the dirty gas button , or brave the cold and put my GSHP back together and get it running.
I managed to get the GSHP back online and its been running on a continuous cycle now for about 18 hours. I also ran the downstairs A2A HP until bedtime.

Core house temp had climbed to 22 degrees by the time we got up this morning and now its back up to our normal 23 degrees.

My not surprising conclusions are that,

• Over 5 degrees the A2A HPs are fantastic and with their modulation, control the house temperature nicely and efficiently. No GSHP needed here.

• Between 0 and 5 degrees the efficiency of the A2A HPs starts to suffer but still ok. Works best for me to use my GSHP heating the house in the early hours until breakfast time, then I switch to the A2A HPs. But they could still do the job without my GSHP.

• Zero or below, A2A HPs use a bit more energy and occasionally do defrost cycles depending on the air moisture. Never thought about this before but having two heatpumps, means the other one is still running when you lose 15 mins of running time whilst in defrost mode in the other. So that's an advantage over a multi split system (single outdoor unit with multiple indoor units). For here definitely better running with the GSHP in these conditions, no defrost cycle and more efficient, more heat and less power.

[marshman]

• Over 5 degrees the A2A HPs are fantastic and with their modulation, control the house temperature nicely and efficiently. No GSHP needed here.

• Between 0 and 5 degrees the efficiency of the A2A HPs starts to suffer but still ok. Works best for me to use my GSHP heating the house in the early hours until breakfast time, then I switch to the A2A HPs. But they could still do the job without my GSHP.

• Zero or below, A2A HPs use a bit more energy and occasionally do defrost cycles. Never thought about this before but having two heatpumps, means the other one is still running when you lose 15 mins of running time whilst in defrost mode in the other. So that's an advantage over a multi split system (single outdoor unit with multiple indoor units). For here definitely better running with the GSHP in these conditions, no defrost cycle and more efficient, more heat and less power.

Interesting, do you really mean "Between 0 and 5 degrees the efficiency of the A2A HPs starts to suffer", or do you just mean they have to work a bit harder because they need to provide more heat but maybe struggle to provide the heat required - i.e the unit is really too small for the job? "People" keep saying that Air Source Heat Pumps are still OK well into negative deg C temps and the COP is still good. Have you found a way of assessing the actual COP of the A2A HP's?

I have always struggled with the idea that any heat pump can maintain the same efficiency if its input heat source temperature drops significantly. It seems counter intuitive to me. In my simple mind if the input temp drops, but you want to maintain the same output temp, then the efficiency should drop as you have more work to do. BUT I must admit I have never studied the "math" behind it all. With a GSHP I can sort of get it as it controls the flowrate of the ground water to maintain around a 3 deg C drop across the heat exchanger (or what ever it is called). I assume an air source one does a similar thing by controlling the fan speed, but there must come a point where the air temperature is so low that it can't achieve its target - just when you need it the most. With a GSHP with correctly sized loops (or heat source) the input temperature will not drop a lot.

[Stinsy]

I noticed mine was blowing out luke-warm air this morning, then about 5-mins later it went into defrost. Worst possible conditions though: cold and foggy. I noticed earlier in the season when it was foggy that water was pouring out of the condensate drain on the external unit. My conclusion is: cold and dry = good, mild and damp = good, cold and damp = bad.

I run mine from 0500-0800 then 1600-2100. My hope is that any ice built up on the morning cycle naturally melts before the evening cycle without me having to expend any energy. Obviously this plan has limits!

[Stinsy]

• Over 5 degrees the A2A HPs are fantastic and with their modulation, control the house temperature nicely and efficiently. No GSHP needed here.

• Between 0 and 5 degrees the efficiency of the A2A HPs starts to suffer but still ok. Works best for me to use my GSHP heating the house in the early hours until breakfast time, then I switch to the A2A HPs. But they could still do the job without my GSHP.

• Zero or below, A2A HPs use a bit more energy and occasionally do defrost cycles. Never thought about this before but having two heatpumps, means the other one is still running when you lose 15 mins of running time whilst in defrost mode in the other. So that's an advantage over a multi split system (single outdoor unit with multiple indoor units). For here definitely better running with the GSHP in these conditions, no defrost cycle and more efficient, more heat and less power.

Interesting, do you really mean "Between 0 and 5 degrees the efficiency of the A2A HPs starts to suffer", or do you just mean they have to work a bit harder because they need to provide more heat but maybe struggle to provide the heat required - i.e the unit is really too small for the job? "People" keep saying that Air Source Heat Pumps are still OK well into negative deg C temps and the COP is still good. Have you found a way of assessing the actual COP of the A2A HP's?

I have always struggled with the idea that any heat pump can maintain the same efficiency if its input heat source temperature drops significantly. It seems counter intuitive to me. In my simple mind if the input temp drops, but you want to maintain the same output temp, then the efficiency should drop as you have more work to do. BUT I must admit I have never studied the "math" behind it all. With a GSHP I can sort of get it as it controls the flowrate of the ground water to maintain around a 3 deg C drop across the heat exchanger (or what ever it is called). I assume an air source one does a similar thing by controlling the fan speed, but there must come a point where the air temperature is so low that it can't achieve its target - just when you need it the most. With a GSHP with correctly sized loops (or heat source) the input temperature will not drop a lot. Mine starts the season around 15/16 deg C and drops to around 7/8 deg C at the end of February. At the moment even though it has been down to -4 deg C over night outside the brine temp has never gone below 12 deg C.

Obviously the larger the delta between the fluid and the outside air, the more efficient the heat exchanger will be. However the UK doesn't typically experience temperatures low enough to trouble a typical ASHP. The same models are used in countries with much lower temperatures than the UK.

AC engineers obsess about the "superheat" and "subcool" as these are the figures that determine if a HP system is working correctly. A typical ASHP will evaporate its fluid at -25℃ so even at -10℃ you still have 15℃ of superheat which is plenty. If it were -20℃ or below you'd have to worry about the heat exchanger not working properly to evaporate the fluid.

However. The problem in the UK is icing up. The UK is a damp country (you may have noticed). When mild/moist air is blown over the heat exchanger, water condenses out and is drained away. In really cold conditions the air cannot contain much water so there is nothing to condense and all is good. However there is a perfect combination of air being just warm enough to hold water and cold enough that the water freezes and stick to the heat exchanger. Eventually the fan can no longer blow air over the fins and the HP is forced to work in reverse (sucking heat out of your house) and/or use a resistive heater to melt the ice.

Last week we had beautiful "alpine" weather. It was -6℃ but bright sunshine and no moisture. The A2A didn't ice up once.

Today is cold and foggy and -4℃. Running at full chat to warm the house up this morning the HP iced up and went into defrost mode. So there was 10-mins when it was no longer providing heat to the house but was consuming electric. Obviously if it does this too frequently efficiency will suffer.

[nowty]

Interesting, do you really mean "Between 0 and 5 degrees the efficiency of the A2A HPs starts to suffer", or do you just mean they have to work a bit harder because they need to provide more heat but maybe struggle to provide the heat required - i.e the unit is really too small for the job? "People" keep saying that Air Source Heat Pumps are still OK well into negative deg C temps and the COP is still good. Have you found a way of assessing the actual COP of the A2A HP's?

It’s a bit of both but remember I am comparing a GSHP wet system with an A2A HP, not a ASHP wet system. So, its maybe not quite like with like. Both are rated at about the same combined heat output but not necessarily at the same operating temperatures (source or output).

I don’t have any empirical data, its just off using them in different conditions and more recently in the cold weather running either only my A2A HP’s or only my GSHP heat pump. Checking how much power they are using and what the house temp is over a long period of time.

Example, in these current cold conditions which are certainly rare events here on the south coast, my GSHP uses under 2kW when rads are under 40 degrees and that’s enough to keep the house at our desired temp of 23 degrees in conjunction with storage heaters.

In similar cold conditions the combined A2A HP’s are using over 2kW and cannot keep the house at our desired temperature.

But if we go back to its about 8 degrees outside, the A2A HP’s can maintain the house temp easily and use less power than my GSHP and are also able to modulate.

[Tinbum]

It’s a bit of both but remember I am comparing a GSHP wet system with an A2A HP, not a ASHP wet system. So, its maybe not quite like with like. Both are rated at about the same combined heat output but not necessarily at the same operating temperatures (source or output).

I don’t have any empirical data, its just off using them in different conditions and more recently in the cold weather running either only my A2A HP’s or only my GSHP heat pump. Checking how much power they are using and what the house temp is over a long period of time.

Example, in these current cold conditions which are certainly rare events here on the south coast, my GSHP uses under 2kW when rads are under 40 degrees and that’s enough to keep the house at our desired temp of 23 degrees in conjunction with storage heaters.

In similar cold conditions the combined A2A HP’s are using over 2kW and cannot keep the house at our desired temperature.

But if we go back to its about 8 degrees outside, the A2A HP’s can maintain the house temp easily and use less power than my GSHP and are also able to modulate.

My new house will be having both, for exactly that reason, and selection will be done by outside temperature.

[nowty]

My new house will be having both, for exactly that reason, and selection will be done by outside temperature.

And so has mine, but by accident rather than by design.

A good report on the GSHP v ASHP report from the Energy Savings Trust is here,
https://www.energysavingtrust.org.uk/si ... web(1).pdf

[marshman]

__

[nowty]

Is the "under 2kW" figure for your GSHP pretty constant no matter what your "stream" temperature is?? - I seem to recall you did some tests a while back but not sure what they were - I have never noticed my GSHP using any more in Feb with incoming brine at 8 deg vs November / early December when it is at 14 deg C, but then again that is not much of a change.

Pretty much, my brine (its only about 3 litres) return to heatpump temp is about 10 degrees in summer and it gets as low as zero in winter, but I cannot tell any difference in power consumption. Its the output temp that is the big factor in the power consumption. I did an experiment with it when I installed it.

Original thread,
https://camelot-forum.co.uk/phpBB3/view ... f=17&t=487