ASHP defrosting question

[BoulderDash]

I moved into a 4-bed dormer bungalow (176m²) last summer that had just been gutted, extended and completely renovated, including ASHP for the DHW and UFH.  The ASHP is a Nibe F2040-8 (8kW).  The UFH is in screed downstairs and on aluminium trays in the suspended floor upstairs (both with 200mm pipe spacing).  There is no buffer tank, although there is a volumizer unit. Room temps are set to 18C for bedrooms and 20C for the kitchen/lounge etc (no night-time setback).

 

The UFH downstairs can maintain target room temps with a flow temp of 25-35C.  However, the upstairs seems to need a higher flow temp of ~45C to maintain target room temps. The problem with this is that when the outside temp falls below 5C, the ASHP continually defrosts and is unable to deliver a flow temperature >30C (i.e. the opposite of the intended weather effect compensation!).  I have attached some screenshots of plots from my Nibe Uplink – the first is when the outdoor temp was ~10C, which shows the flow and return temps oscillating around the target flow temp (which is what I would expect to see) and the other shows the same plot when the outdoor temp is ~2C.  In the latter plot, the pump is defrosting so frequently that the flow temp never achieves its target, resulting in high electricity consumption (>£200 p/m), a really noisy pump, angry neighbours, and room temps <18C upstairs. The installer is refusing to investigate this problem and Nibe will only interact with installers.

 

Q: is the impact of the defrosting cycles what you would expect for an ASHP at <10C?  If not, any thoughts on the underlying cause – insufficient pump size?  Lack of a buffer tank in the design?  Anything else you can think of that might cause this…?

 

Looking forward to your expert input and happy to provide any additional details that might be helpful.

[ProDave]

What you have discovered is just above 0 is the worst temperature range for an ASHP as that is when most defrosting is likely.  Once well below 0 there is simply not enough moisture in the air to cause frost to form.

 

So you need the higher flow temperature when it's -10.  What about when it is 0 outside?  Will it work okay on the lower temperature?

 

If so you could set your heat curve for 35 at 0 degrees and 45 at -10 degrees and see how that works.

 

P.S. I have aluminium spreader plates used in what UFH we have upstairs and I do not find them as effective as UFH in a slab downstairs.

[BoulderDash]

Thanks ProDave, that’s interesting. Unfortunately, I don’t have the data to answer your question yet, as the outside temp hasn’t really gone below 0 where I live yet this winter. I do have the weather curve set-up pretty much how you suggested though.

 

Is there anything else I should consider here, or do I just have to accept that this is an inherent flaw in the way ASHPs perform?

[troggy02]

You should only really get frosting between approx 2 and -3c depending on humidity. Our system is designed for a 35c flow temp at -3c. At one point we increased this by 2c mainly as an experiment and it caused significantly more defrost cycles. Back to its original settings it might defrost once every 45 minutes worst case (freezing fog appears to be the worst). It's -1c outside as I type this and while I can see frost on the unit, it's been cycling on the thermostat for several hours without needing to defrost.

[SteamyTea]

2 minutes ago, troggy02 said:

You should only really get frosting between approx 2 and -3c depending on humidity

At 4°C, water is at its most dense, but water can also exist, in gaseous form, well below 0°C.

Ice is a pretty good insulator, at -5°C (we should really be using the kelvin scale), it has a conductivity value of 2.25 W.m^-1.K^-1.  But more importantly, it reduces airflow.

The problem is that one ice has formed, the energy to change state is very high, ~330 J.g^-1.  This means that it can take a long time to defrost if just relying on ambient temperature, and why it is often more economical to just pump some preheated water back though the external heat exchanger.  This is because some of the ice will fall off, so does not need defrosting.

One way, and something that @joe90 and myself have talked about in the past, is to stop, or reduce the power of the ASHP when the conditions are right for ice formation.  But as he has, until this winter (the coldest January in a decade apparently), never noticed it frosting, the idea rather got forgotten.  I still think there is some merit in the idea if you know that the temperature will go below freezing later and you can play catch up then.

[TonyT]

You can use some self regulating trace heating tape to prevent the freezing, instead of the ASHP going on reverse or using its own external heater.

 

at 10 watt per metre it can be cheaper to run.

[SteamyTea]

12 minutes ago, TonyT said:

You can use some self regulating trace heating tape to prevent the freezing, instead of the ASHP going on reverse or using its own external heater.

 

at 10 watt per metre it can be cheaper to run.

Where would you place it, can't cover the heat exchanger.

Also it is cheaper to use stored water that has been heated at a CoP of say 2, rather than use an element that has a CoP of 1.  Even if it does hurt to have your hard won hot water pumped around the heat exchanger.

[PeterW]

14 minutes ago, TonyT said:

You can use some self regulating trace heating tape to prevent the freezing, instead of the ASHP going on reverse or using its own external heater.

 

at 10 watt per metre it can be cheaper to run.

But it won’t work as the energy it produces isn’t enough to melt the ice sufficiently and it won’t “know” when ice as formed as you can’t use temperature as a measure. 

[TonyT]

We have tyrapped  it to the external grill on one of our ASHP in a building I manage.

 

But the trace heating worked a treat.

[SteamyTea]

2 minutes ago, TonyT said:

We have tyrapped  it to the external grill on one of our ASHP in a building I manage.

 

But the trace heating worked a treat.

I am intrigued.

Are you, in effect, preheating the air going into the heat exhanger?

[TonyT]

Don’t think so, just preventing the ice from forming by localised heating.

 

this is a commercial building so we need to maintain the internal  temperature to a certain temp for staff at work previously the unit would freeze over, the inside temp would drop and staff complain, out comes the 3kW fan heaters and the cycle of complaints start, as always they heat the area around the thermostat, fooling it in thinking the room temperature has been achieved and the ASHP shuts off and the room gets colder..

 

educating staff works for 10 seconds, so the trace heating prevents the ice from forming and saves me grief.

[joe90]

As @SteamyTea said above it’s cheaper to use heated water at a decent COP than an element at 1to defrost the unit.  They forecast very cold weather today/tonight so I am turning the room stat up a little, whilst the sun  is shining on the ASHP (to stop any icing ) and get a little heat into the slab which will last many hours.

[TonyT]

Fully get that, but our unit freezes a fair old bit, and stops heating for a while, so the trace heating ticks the box for this particular installation in a commercial building with staff and members of the public.

[SteamyTea]

@TonyT

Be interesting to do an energy analysis on it.

 

As a general note, can't remember much talk about ASHPs actually frosting up in previous winters.

 

@joe90

Be interesting to see if it frosts during the day. Your dewpoint temp is currently 5.2⁰C.

10⁰C here at the moment and quite pleasant.

Was going to post a picture up, but seems the forum has got a problem.

 

https://postimg.cc/Jt2JTzZR

Edited February 6, 2021 by SteamyTea

[ProDave]

44 minutes ago, TonyT said:

Fully get that, but our unit freezes a fair old bit, and stops heating for a while, so the trace heating ticks the box for this particular installation in a commercial building with staff and members of the public.

I would be interested in a photo of your trace heating.

 

How does the ASHP heat the building?  low temperature UFH or high temperature radiators or fan coil units?

[joe90]

31 minutes ago, SteamyTea said:

Be interesting to see if it frosts during the day.

Will keep an eye on it and post results.

[TonyT]

Fan coil unit, ducted to ceiling mounted diffusers

 

[epsilonGreedy]

Is it possible these different ASHP frosting experiences relate to installations with a north or south aspect?

[SteamyTea]

8 minutes ago, epsilonGreedy said:

Is it possible these different ASHP frosting experiences relate to installations with a north or south aspect?

Not really, they draw in lots of air, not sunshine.  So while it may feel, to a person, hot in the sun and cold in the shade, the air is at the same temperature.

Edited February 6, 2021 by SteamyTea

[epsilonGreedy]

29 minutes ago, SteamyTea said:

Not really, they draw in lots of air, not sunshine.  So while it may feel, to a person, hot in the sun and cold in the shade, the air is at the same temperature.

 

 

@Jeremy Harris said much the same a few back when I asked if aspect could affect annual ASHP running costs.

 

At this time of year a southerly brick wall could absorb a useful amount of heat and gently warm an ASPH during an evening but probably a marginal effect.

[troggy02]

3 hours ago, SteamyTea said:

At 4°C, water is at its most dense, but water can also exist, in gaseous form, well below 0°C.

Ice is a pretty good insulator, at -5°C (we should really be using the kelvin scale), it has a conductivity value of 2.25 W.m^-1.K^-1.  But more importantly, it reduces airflow.

The problem is that one ice has formed, the energy to change state is very high, ~330 J.g^-1.  This means that it can take a long time to defrost if just relying on ambient temperature, and why it is often more economical to just pump some preheated water back though the external heat exchanger.  This is because some of the ice will fall off, so does not need defrosting.

One way, and something that @joe90 and myself have talked about in the past, is to stop, or reduce the power of the ASHP when the conditions are right for ice formation.  But as he has, until this winter (the coldest January in a decade apparently), never noticed it frosting, the idea rather got forgotten.  I still think there is some merit in the idea if you know that the temperature will go below freezing later and you can play catch up then.

 

Thank you - very interesting. Our Samsung runs in reverse to defrost. I must say it's pretty quick actually. From talking to the engineer, the unit runs the fans faster and compressor less near zero in an attempt to obtain the same heat extraction without pushing the heat exchanger past zero.

[Spoogster]

Does air circulation have a greater effect? For example if the unit was in a side walkway as opposed to facing into the much larger area of a back garden?

[SteamyTea]

1 hour ago, Spoogster said:

Does air circulation have a greater effect? For example if the unit was in a side walkway as opposed to facing into the much larger area of a back garden?

Unless there is a real constriction of airlflow, not really.

Image putting a fan heater in an alleyway, you would not have to be far away before you cannot feel it.

[joe90]

Been on fir 4 hours and no sign of defrosting ?, house up to 22’ so it’s off now, bring on the beast from the east No2 ?.

[SteamyTea]

16 minutes ago, joe90 said:

no sign of defrosting

Your dewpoint temperature is now at 3°C, your outdoor conditions are 6°C and 81% RH, so a while to go.