Posted by: @trebor12345

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Why do I want to do this?  Oversized HP, too many thermal offs, bungalow not warm on cold days due to defrost cycles, requires constant management to maintain house temperature. My wife would not be able to manage the system if I was not here.

Welcome to the forum.  Some things you are saying dont make sense together

One of the reasons you want to replace it is because its oversized, yet the bungalow isnt warm - both could be true but in that case the reason the bungalow isnt warm is not because its oversized!

However if the figures 2527 KWh/7527 KWh are energy consumed and energy delivered, then your heat pump is indeed oversized, but that doesn't explain why you are cold (it might explain poor COP but you dont see that)

Posted by: @trebor12345

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PS, as an example, had a thermal off this morning, was 3 hours before the system came back on.  Back on for 1 hour, and not back on after 2 hours

What exactly do you mean by 'thermal off'?

I would advise to get to the bottom of why the bungalow isnt warm before replacing it, otherwise there is no guarantee it will be warm after replacement.  Can I ask a couple of questions

• What do you mean by 'thermal off'?
• Have you got any thermostats, if so how are they set?
• Have you set it up to run according to its room temperature sensor, or to ignore its room temperature sensor?
• Is it set to be on 24x7?
• How far out is the temperature during the defrosts?
• what exactly do you mean by 'requires constant management' what are you having to change?

If you do replace then the installer matters much more than the brand of heat pump, we can give some advice but it really would be best first to get to the bottom of the problem, particularly as you are getting a SCOP which is not bad.  

@trebor12345, Just looking at your spec, your house would seem a dream for a heat pump, have you got to the bottom of why you are having thermal offs? If not I would invest in some monitoring to see whats going on.

Posted by: @jamespa

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Posted by: @trebor12345

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Why do I want to do this?  Oversized HP, too many thermal offs, bungalow not warm on cold days due to defrost cycles, requires constant management to maintain house temperature. My wife would not be able to manage the system if I was not here.

Welcome to the forum.  Some things you are saying dont make sense together

One of the reasons you want to replace it is because its oversized, yet the bungalow isnt warm - both could be true but in that case the reason the bungalow isnt warm is not because its oversized!

Posted by: @trebor12345

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PS, as an example, had a thermal off this morning, was 3 hours before the system came back on.  Back on for 1 hour, and not back on after 2 hours

What do you mean by 'thermal off'?

 

I would advise to get to the bottom of why the bungalow isnt warm before replacing it, otherwise there is no guarantee it will be warm after replacement.  Can I ask a couple of questions

• What do you mean by 'thermal off'?
• Have you got any thermostats, if so how are they set?
• Have you set it up to run according to its temperature sensor?
• Is it set to be on 24x7?
• How far out is the temperature during the defrosts?

If you do replace then the installer matters much more than the brand of heat pump, we can give some advice but it really would be best first to get to the bottom of the problem, particularly as you are getting a SCOP which is not bad.

The Thermal Off is where the temperature into the heat exchanger is too high and it shuts down because the water is too warm.  Others would call this short cycling, Thermal Off is manufactures expression.

Fully open 12 port manifold.  12 thermostats in rooms but one one being currently being used, and that I turn off and on.

Unable to run 24/7 as per this morning and the thermal offs.

After a defrost the temperature never goes back to it's set level, it cycles up and down,  I have attached a chart which shows what happens on the 1st defrost and then the subsequent ones. 

I have spent the last year trying to get to grips with this system, unsuccessfully.

My main reason for wanting to change is that it requires too much daily management to maintain a steady temperature.  You always need to be looking ahead at the weather to see what you do.

Posted by: @trebor12345

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I have spent the last year trying to get to grips with this system, unsuccessfully.

My main reason for wanting to change is that it requires too much daily management to maintain a steady temperature.  You always need to be looking ahead at the weather to see what you do.

ok that makes more sense, in which case simplicity of UI is going to be key if you do change!  I can recommend Vaillant for this because I have one, but others will have their own recommendations.  That said you shouldn't need to be looking at the weather whatever heat pump you have, thats what the heat pumps controller is for!

Given your house low loss any heat pump is going to be cycling a lot, a boiler even more so, thus management of and understanding that is key.  

I now see the chart but cant see what the problem is (I can see its varying, but why do you care?)

Do you have a chart of flow temperature or house temperature?

What are your weather compensation settings?

Posted by: @bontwoody

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@trebor12345, Just looking at your spec, your house would seem a dream for a heat pump, have you got to the bottom of why you are having thermal offs? If not I would invest in some monitoring to see whats going on.

The house only requires 14watts/m2.  So at -2C I only require 2.7kwh.  The HP is a 7kw machine. (forgive me if have some of the annotations incorrect).

Posted by: @trebor12345

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The house only requires 14watts/m2.  So at -2C I only require 2.7kwh.  The HP is a 7kw machine. (forgive me if have some of the annotations incorrect).

Yes, but the lowest power heat pump you will get is ~5kW, so its still going to be cycling.  A boiler will be much worse.  Thus getting to grips with why cycling is causing you a problem is the key here!  With your low loss and UFH combined I wouldnt be surprised if the problem is the use of a thermostat to control things, its almost bound to cause the internal temperature to swing wildly.

Do you have a chart of flow temperature or house temperature?

What are your weather compensation settings?

@trebor12345 I see, so a very small heat pump recommendation is required 🙂

Posted by: @jamespa

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Posted by: @trebor12345

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I have spent the last year trying to get to grips with this system, unsuccessfully.

My main reason for wanting to change is that it requires too much daily management to maintain a steady temperature.  You always need to be looking ahead at the weather to see what you do.

ok that makes more sense, in which case simplicity of UI is going to be key if you do change!  I can recommend Vaillant for this because I have one, but others will have their own recommendations.  That said you shouldn't need to be looking at the weather whatever heat pump you have, thats what the heat pumps controller is for!

Given your house low loss any heat pump is going to be cycling a lot, a boiler even more so, thus management of and understanding that is key.  

I now see the chart but cant see what the problem is (I can see its varying, but why do you care?)

Do you have a chart of flow temperature or house temperature?

What are your weather compensation settings?

Re the chart.  I should of added more info.  

The first part of the chart shows the performance during the day with one defrost occurring at 3:40pm. (I have a temperature probe attached to the refrigerant gas pipe coming into the indoor unit so it gives me an idea as to what the heat pump is doing).  You will see that the temperature falls and then recovers to a steady temperature.

The second part of the chart shows the subsequent defrosts occurring approx. every hour, as denoted by the *.  But when the defrost is over the temperature of the refrigerant gas does not achieve a level state, if fluctuates up and down till the next defrost.  During this time the house cools, as the heat pump is not producing as much heat.  While this is occurring the heat pump is constantly running at 52Hz with a consumption of 5 Amps.

Not running weather compensation at the moment (that's another issue), just a fix temperature of 31C.  

Posted by: @bontwoody

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@trebor12345 I see, so a very small heat pump recommendation is required

I am far from convinced this will fix the problem because there isnt much out there < about 4kW, so not that much difference.

I think there is a good chance its a control issue not a source issue (and a boiler could be far worse).

I wouldnt replace this unit until that was understood.

Posted by: @trebor12345

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The second part of the chart shows the subsequent defrosts occurring approx. every hour, as denoted by the *.  But when the defrost is over the temperature of the refrigerant gas does not achieve a level state, if fluctuates up and down till the next defrost.  During this time the house cools, as the heat pump is not producing as much heat.  While this is occurring the heat pump is constantly running at 52Hz with a consumption of 5 Amps.

Not running weather compensation at the moment (that's another issue), just a fix temperature of 31C.  

Whatever you are experiencing in terms of house temperature is not, IMHO, going away 'simply' by getting another heat pump, and is likely to get even worse if you were to go to a boiler because it will cycle even more (because it will be even more oversized).  That's not saying that changing heat pump is not eventually sensible, but it wont magically fix the 'problem'  

You have a very low loss house with a (relatively) high power heat source and are controlling it on the thermostat. 

Thats like trying to drive at an average of 20 mph by alternating between 60MPH and 0mph, but only deciding to when to change between the two by looking in the rear view mirror to decide how far you have gone before reacting and then slamming on the breaks when you realise you have gone too far.  Its going to be a bumpy ride. 

If you change to lower power heat pump 60MPH may become 40MPH, but its still going to be bumpy.  If you change to a boiler 60MPH is likely to become 80mph!

• Have you got any plots of flow temperature and house temperature, not worried about refrigerant temperature - thats for the heat pump to sort out
• If its not set to operate 24*7 what is it set to do?
• when you say the house cools, by how much.  You shouldn't notice if the pump is off for an hour, assuming the slab has warmed up (perhaps that's the underlying problem) because of the vast thermal reservoir the slab represents
• have you tried to set up weather compensation and failed/abandoned, or have you never set it up.  This is mechanism built into your heat pump to look ahead to see what the weather is going to do to your house thus saving you from having to do so!  With low loss and UFH this becomes vital, because control on room temperature alone is almost guaranteed to fail!

Just to add.  As the system is not that common I have been unable to find anyone local who knows anything about the system or what I can do to fix it.  The manufacturer has been involved, but whilst it's improved, there are still issues.

I appreciate that it's going to be difficult to find a Heat Pump that modulates down to less that 2kw.  The same applies to a gas boiler.