Undersized

[Alwayslearning22]

Hi guys,

I’ve a 4.3kw split hitachi air to water heat pump. My house is around 120sqm with this recent cold weather, I’m beginning to think that my heat pump is undersized. 
It’s constantly frozen even after a defrosting fully it tries to get itself to a 40 flow and  then just freezes up again. 
 

is there anything I can do to stop it freezing up? Maybe lowering flow temp? 
it’s on weather comp at the moment set for 42.
Never reaches that though. 

[ProDave]

42 degrees is not very high.  Where are you and what is your outside temperature?

 

The worst for icing up is a few degrees above 0.  Is this a new system or has it previously worked okay in cold weather?  Is the big air fan on the external unit blowing properly?

[Alwayslearning22]

26 minutes ago, ProDave said:

42 degrees is not very high.  Where are you and what is your outside temperature?

 

The worst for icing up is a few degrees above 0.  Is this a new system or has it previously worked okay in cold weather?  Is the big air fan on the external unit blowing properly?

I’m in the east of Ireland so not the coldest of places. Currently it’s 1 degree but past few nights it was minus 1 or colder and it performed horribly could not maintain building heat loss. 
 

yes, seems to be blowing ok for the 20 mins it’s not frozen. 

id never get it above 42, only time it can manage that is when it’s heating hot water and That’s because the pipe run is short. 
 

System is 2 years old and never worked properly I think 
 

 

Edited December 11, 2022 by Alwayslearning22

[PhilT]

I have a similar size house (self build 1992) with 11kW HP. It's been as low as -5degC the last few nights. Defrost cycle 5min once or twice an hour on average during darkest coldest times, less during late morning/afternoon. My EPC heat loss is 8kW at -2degC. Do you know yours from your EPC cert?

[Alwayslearning22]

12 hours ago, PhilT said:

I have a similar size house (self build 1992) with 11kW HP. It's been as low as -5degC the last few nights. Defrost cycle 5min once or twice an hour on average during darkest coldest times, less during late morning/afternoon. My EPC heat loss is 8kW at -2degC. Do you know yours from your EPC cert?

It’s reassuring to know my defrost cycles seem to be normal then. I thought it was to often as it struggles to get flow back up again. 
 

im not sure of my heat loss. 
my house is a new build timber frame. 
sounds like it’s undersized going by yours? 

[PhilT]

12 hours ago, Alwayslearning22 said:

im not sure of my heat loss. 
my house is a new build timber frame.
sounds like it’s undersized going by yours? 

It's difficult to tell exactly where the problem is right now. Get an EPC assessment, they don't cost much, or very diligently work out your heat loss using one of the many online tools - others on this forum can recommend the best ones. That will tell you whether or not your HP power rating is adequate. Is it in good working order/have you had it checked/serviced? If it's working well and you can't improve insulation/air tightness any further, have you been warm enough for 95+% of the year? If so you may be better off just using some form of supplementary heating eg electric radiators to get you through the extra cold times.

[Radian]

9 minutes ago, PhilT said:

or very diligently work out your heat loss using one of the many online tools - others on this forum can recommend the best ones.

There's a spreadsheet that keeps popping up on the forums but I can never find it,

correction:

 

However it's really quite easy to do from scratch as I posted here:

 

I tend to like doing these things from scratch anyway as I like to understand every step and get to customise the results in whatever way suits me.

[SteamyTea]

3 minutes ago, Radian said:

get to customise the results in whatever way suits me

Isn't that called what Governments, and social scientists do?

 

If I want to scare people, I tell them they have a 5% chance of dying, not a 95% of surviving, and never mention the activity risk of death.

[JohnMo]

28 minutes ago, Radian said:

There's a spreadsheet that keeps popping up on the forums but I can never find it,

correction:

I used this one also, have to say it is very good.  On 10 degree days, it does say you need more heat that you actually do, but doesn't account for people or solar gain.  However the last 24 hrs, with an average temp at about -6 and no solar gain, almost to the kWh over 24 hrs correct.  Can't ask for better than that.

[Alwayslearning22]

8 hours ago, PhilT said:

It's difficult to tell exactly where the problem is right now. Get an EPC assessment, they don't cost much, or very diligently work out your heat loss using one of the many online tools - others on this forum can recommend the best ones. That will tell you whether or not your HP power rating is adequate. Is it in good working order/have you had it checked/serviced? If it's working well and you can't improve insulation/air tightness any further, have you been warm enough for 95+% of the year? If so you may be better off just using some form of supplementary heating eg electric radiators to get you through the extra cold times.

I can’t make any improvements as it’s well enough insulated. Maybe I can improve by fixing  a few holes on the air tightness barrier for pipes and cables that weren’t sealed by the trades. 
 

it keeps the house warm enough, it  just annoys me that it can’t hold the room temperature when it’s this cold out.

Its also extremely noisy during operation and when it goes into its defrost cycles. 

[Alwayslearning22]

8 hours ago, Radian said:

There's a spreadsheet that keeps popping up on the forums but I can never find it,

correction:

 

However it's really quite easy to do from scratch as I posted here:

 

I tend to like doing these things from scratch anyway as I like to understand every step and get to customise the results in whatever way suits me.

Thanks for that information I’ve always wanted to learn to do a heat loss calculation. I believed it was much more difficult.

I’ll be giving that a go tomorrow. 
 

one thing i hate about heat pumps is the frosting up

[Alwayslearning22]

Should I reduce the flow to 37 degrees to try and reduce defrost cycles? 

[ProDave]

8 hours ago, JohnMo said:

I used this one also, have to say it is very good.  On 10 degree days, it does say you need more heat that you actually do, but doesn't account for people or solar gain.  However the last 24 hrs, with an average temp at about -6 and no solar gain, almost to the kWh over 24 hrs correct.  Can't ask for better than that.

Yes that spreadsheet, based on simply physics and some sums, is also pretty much bang on for my house as well.  FAR better than the EPC which was fed all the same information that over estimates my heating usage by some margin.

 

Today was one of those "good" winter days here, yes it was cold, but still, and clear blue sky, and plenty of solar gain from the low sun, and as good as it gets PV generation at this time of year.

[ProDave]

2 minutes ago, Alwayslearning22 said:

one thing i hate about heat pumps is the frosting up

It really should not be frosting up much now.  the worst temperature is just above freezing where the evaporator will be below freezing and the moist air will condense and freeze.  Once you get a day or 2 of air temperature below 0 there really is little moisture in the air, and icing is very rare.  -7 here last night and daytime high of -2 and hardly any defrosting.

[ProDave]

3 minutes ago, Alwayslearning22 said:

Should I reduce the flow to 37 degrees to try and reduce defrost cycles? 

Yes run the flow temperature at the lowest that will deliver enough heat into the house.  What is the temperature at the UFH blending valve set to?

[Alwayslearning22]

1 minute ago, ProDave said:

Yes run the flow temperature at the lowest that will deliver enough heat into the house.  What is the temperature at the UFH blending valve set to?

Sorry i should of mentioned, i have only radiators. 

[ProDave]

Try it and see.  At 37, radiators will not "feel warm" but will still deliver heat to the room.  But it may not deliver enough heat so try it and see.

[Alwayslearning22]

5 minutes ago, ProDave said:

It really should not be frosting up much now.  the worst temperature is just above freezing where the evaporator will be below freezing and the moist air will condense and freeze.  Once you get a day or 2 of air temperature below 0 there really is little moisture in the air, and icing is very rare.  -7 here last night and daytime high of -2 and hardly any defrosting.

My heat pump was popping in and out of defrost all day. Although it was heavy fog here. 
 

[Alwayslearning22]

2 minutes ago, ProDave said:

Try it and see.  At 37, radiators will not "feel warm" but will still deliver heat to the room.  But it may not deliver enough heat so try it and see.

I’ve came down nights to see the flow at 34 because then thing is so frosted up it’s not getting air. For some strange reason it could take over an hour of this before it defrosts.

Temp didn’t drop too much inside even at 34.

Edited December 12, 2022 by Alwayslearning22

[jack]

12 hours ago, ProDave said:

It really should not be frosting up much now.  the worst temperature is just above freezing where the evaporator will be below freezing and the moist air will condense and freeze.  Once you get a day or 2 of air temperature below 0 there really is little moisture in the air, and icing is very rare.  -7 here last night and daytime high of -2 and hardly any defrosting.

 

Weather conditions are really unusual down here at the moment. Despite it falling well into the negative single digits for 12+ hours a day for nearly a week, we've also had a lot of mist and freezing fog, so there is some moisture in the air despite the temperature. My ASHP has been defrosting for a few minutes every 45-70 minutes, even when it's -4 to -7 outside and it isn't working that hard.

 

 

[Alwayslearning22]

15 minutes ago, jack said:

Weather conditions are really unusual down here at the moment. Despite it falling well into the negative single digits for 12+ hours a day for nearly a week, we've also had a lot of mist and freezing fog, so there is some moisture in the air despite the temperature. My ASHP has been defrosting for a few minutes every 45-70 minutes, even when it's -4 to -7 outside and it isn't working that hard.

 

My system came out of a hot water cycle last night and the spent the next hour defrosting every 15 mins.
It just wasn’t able to get the room temp back up without really struggling.

Set at a 40 flow and sounded like it was going to explode. 
Took ages to settle down. 
 

 

[PhilT]

2 hours ago, jack said:

My ASHP has been defrosting for a few minutes every 45-70 minutes, even when it's -4 to -7 outside and it isn't working that hard.

It's been highly variable over the last week, from twice an hour to nothing all day. As a new user it's been very interesting to see how the new HP performs, and hear other's experiences, during these prolonged freezing periods, with plenty of useful learnings along the way, this forum being a great help. Thanks to all contributors

[SteamyTea]

2 hours ago, jack said:

so there is some moisture in the air despite the temperature

Something to do with very high rainfall in November maybe. I think we had a rapid loss of air pressure yesterday, which helps down here went from 109 to 103 hPa)

 

Ice still evaporates.

Temperature, is the mean free path speed, or the mean vibrational frequency, depending if it is a gas, liquid or a solid.

Because it is a mean speed, or frequency, some molecules will be moving faster, some slower.  The fastest ones detach from the main material body and escape as a gas.  This lowers the overall temperature of the main material body (it is how adding laser light, to make helium boil, actually lowers the temperature to very close to 0K).

It also accounts, in part, for phase change in materials and the associated change in heat capacity.  By changing the energy levels, the material tries to expand or contract, this can change the arrangement of the molecules (partly why we don't use nobles gasses as refrigerants, they are don't like to be rearranged).  It is this molecular arrangement that changes the properties. A loose jumble in a gas, to neatly arranged, because of molecular polarity, in a liquid, to  a fixed position in a solid.

 

[Alwayslearning22]

Thanks for the replies folks. You’ve all been a great help as always