Maximising the COP of an ASHP

[Jeremy Harris]

28 minutes ago, dpmiller said:

Having watched a number of youtube videos of defrost cycles, I'd say that a heat cycle of 10mins is waay too long and no more than a minute or two of heating should suffice. @JSHarris have you ever watches the defrost to see how much heat is actually needed?

 

I've never been able to quantitatively measure the total heat energy going in to the defrost cycle, only the time it takes and the electrical power drawn.  All I can see is that the fan and compressor both ramp up to maximum electrical power over the first 30 seconds to 1 minute of the defrost cycle, after the heat pump has shut down and the 4 way valve operated, then the compressor shuts off and the fan carries on running at near full speed for a minute or two until the end of the cycle.  It then shuts down, operates the 4 way valve again and ramps back up in heating mode over the next thirty seconds to a minute.

Edited November 25, 2017 by JSHarris

[pdf27]

1 hour ago, JSHarris said:

It was quite a shock the first time I saw a defrost cycle, and saw the 70 litre buffer tank temperature plummeting down to well below room temperature as heat was being drawn from it!

 

44 minutes ago, JSHarris said:

 

I've never been able to quantitatively measure the total heat energy going in to the defrost cycle, only the time it takes and the electrical power drawn. 

Umm... can't you get the thermal power by shutting off the UFH pump and measuring the rate of temperature drop (or even just the total drop) in the buffer tank? 70 litres isn't a lot - you're looking at 0.08 kWh/°C - so you should be able to get a pretty good measurement of heat abstracted, particularly since the pipes will be small by comparison.

[Jeremy Harris]

14 minutes ago, pdf27 said:

 

Umm... can't you get the thermal power by shutting off the UFH pump and measuring the rate of temperature drop (or even just the total drop) in the buffer tank? 70 litres isn't a lot - you're looking at 0.08 kWh/°C - so you should be able to get a pretty good measurement of heat abstracted, particularly since the pipes will be small by comparison.

 

 

Yes, but it would be a lot of faffing about - if you've read the very lengthy discussions on this then things might be clearer.  I'd need to go in and start lifting wires from the wiring box and reconfigure things.  The valve that shuts off the UFH loops from the ASHP would need to be forced to close when the ASHP was operating (you can't just disconnect the UFH circulation pump, as the built in ASHP circulating pump will still cause flow through the UFH system), and I'd need to rewire the valve to the buffer tank to hold that open when the UFH valve was closed (not a normal operating condition).  It would be perfectly possible to do, but I've already spent the best part of a year fine tuning things and I'm reluctant to start taking it apart again just to try and gain a bit of information that would be of no practical value to me.

 

I know, after a couple of years of operation, that just keeping the flow temperature at 40 deg C means the ASHP never defrosts.  Our system works an absolute treat like this, with a high COP (higher than the spec sheet says by at least 10 to 20%), so I'm content to just leave it alone.  We have no electricity bill at all, in effect, and the house is all-electric, so all the initial experiments I did a couple of years ago when trying to both understand how the heat pump actually worked (rather than what the manual said - the two were quite different!) and fine tune the settings needed to get good control over a house with a high thermal time constant and low overall heat losses (which turned out to be relatively easy, but I chose to take an overly complex route to reach that conclusion) have been worthwhile, but may or may not be relevant to a house in a different area, of a different size, shape and orientation, with different thermal characteristics and occupants with a different lifestyle.

[PeterW]

Like @JSHarris I’ve come to the conclusion that ASHP over 45c is pretty pointless as it just goes to defrost too often. 

 

So I will be using the ability of the ASHP to take a variable flow temperature and run the floor buffer at 38c and the hot water tank at 45c. Every night I will boost from 5:30 - 7:30 from 45c to 65c using immersion on E7. That leaves the ASHP to start heating the floor and buffer so everything is warm by 7am. This is a W Plan in reverse too as if the hot water tank needs topping up it will be by the boost immersion for an hour. 

[Stones]

We opted for a packaged ASHP and pre-plumbed cylinder.  I let the ASHP heat all our DHW (to 50C), stored in a 300 litre cylinder.  Over the 9 months that it has been operating, we have returned a CoP of 2.4 for DHW and just under (sometimes over) 4 for heating.  I initially considered preheating DHW using the ASHP and topping up via immersion, but after working it out, concluded there wasn't any financial saving doing so - what you save on ASHP running costs, you spend on direct electric. 

 

Living in a coastal climate, we experience our fair share of damp and wet conditions. Our ASHP does occasionally go into defrost if it's trying to recharge the whole cylinder, but doesn't seem to have to when only recharging a partial draw off (say after a single shower).  Our pattern of use is such that there is generally a fair gap between DHW being drawn off.   I also let our DHW recharge according to a set DHW thermistor temperature drop of 9C rather than a timed early morning / evening schedule.

 

Using an ASHP can work.

 

  

[willbish]

7 hours ago, JSHarris said:

Anything you can do to lower the humidity around the unit would help, too.  I keep meaning to add a small overhanging roof over ours, to both stop it getting wet in the rain (which must raise the local humidity a bit) and also to slightly reduce the radiative heat loss, by shielding the unit and the ground around it from the cold night sky.

 

TGG supply a cover that supposedly allows unimpeded airflow. This would help keep the rain off and could reduce the humidity slightly. It could also allow the heat pump to be orientated back facing out and expose the evaporator to any warmth from the sun without looking it totally odd.

Covers_APN.PDF

[SteamyTea]

15 hours ago, Stones said:

 I also let our DHW recharge according to a set DHW thermistor temperature drop of 9C rather than a timed early morning / evening schedule.

I think that is the way to go as it puts less load on the heating.  If using timing only, it takes no account of of the load needed.

[joe90]

Great reading guys, my ASHP faces west and that elevation gets very windy and carries a lot of rain from the Atlantic so damp a lot of the time. The cover above is just what I was thinking of making so that I will definitely do. I guess it’s down to suck it and see, get it running and monitor how it works for our location and lifestyle.

[Jeremy Harris]

3 minutes ago, joe90 said:

Great reading guys, my ASHP faces west and that elevation gets very windy and carries a lot of rain from the Atlantic so damp a lot of the time. The cover above is just what I was thinking of making so that I will definitely do. I guess it’s down to suck it and see, get it running and monitor how it works for our location and lifestyle.

 

There's some evidence that keeping rain off the unit does help a fair bit, from what I've read.  As long as the air flow isn't restricted, anything that helps to keep the area around the unit dry has to help a bit.  It probably also helps prevent external corrosion a bit, too.  Although designed to be sat outside, there are signs on mine of minor corrosion around some of the fasteners, presumably where the underlying galvanised coating has been scratched.  It it were under cover then spraying a bit of grease or similar around the fasteners might well be more effective at limiting future corrosion.

Edited November 27, 2017 by JSHarris

[SteamyTea]

When I get time, and energy, I shall have a little play with some DHT22s and see if there is really enough of a difference to measure.  It would be nice to think that reducing local RH could make a difference and stop an exterior ASHP being used as a bike rack.

[Jeremy Harris]

Might be worth seeing if there is a difference in humidity with height above ground.  I have a feeling that there will be at certain times, for example when wet ground starts to warm up from the sun early in the morning and the moisture evaporates off.

 

Whether this is significant around the intake to an ASHP as to make a measurable performance different I doubt, personally, but it may do.  I think there are other good reasons for adding a rain shield to one, anyway, so will add it to the list of things I must get around to one day..............

[Andehh]

Sorry to bump an old thread, but curious to see if anyone did look at the wooden cover & whether it made any difference?

[joe90]

Never got round to doing mine ?‍♂️