ASHP to Avoid from October 2022

No_No_No_Yes

Reed_Richards said:

No_No_No_Yes said:

Someone mentioned that if the pump is too big, that, will be inefficient too. 

There are two aspects to efficiency.  There is the actual efficiency, which will depend on the difference between the outside air temperature and the output water temperature.  Then there is the "performance efficiency" which is the percentage of the theoretical maximum that is achieved for a given temperature difference.  I read somewhere that heat pumps tend to achieve their best performance efficiency when operating at about half their maximum output.  So maybe you take the mean outside temperature duding the coldest months of the year and size you heat pump such that at that outside temperature it would be running at half power to keep your house warm enough?

I have no idea if this suggestion is right or misguided.  I don't believe there are many people (in the UK) who really know how to right-size a heat pump.  I'm certainly not one of them.       

Good point.
In deed, I need accurate information for a solid installation, there is too much at risk.  The word "theoretical" sends shiver to my spine... 

I heard that the MCS calculator is only a guide and there is no guarantee that a system will perform well... It assumes that the data, like outside temperature and type of insulation is correct.

How would one know the exact level and type of wall insulation, or the floor for that matter.   The surveyor ASSUME it's of a certain type and level.

As you mentioned, it should be better for the pump to over perform at the lowest winter temperatures.

shinytop

No_No_No_Yes said:

The reply I got with regards to efficiency at lower temperatures such as -7C or -10C was, "it will still work but not as efficient".
No recommendations for a larger pump.

Someone mentioned that if the pump is too big, that, will be inefficient too.  It seems that a balance needs to be struck in order to be efficient.
Two thing for sure, I don't want inefficiency, and a want to reduce my heating bills, otherwise I'll stay with my old gas boiler, at least it will keep me warm.

Your heat loss calculation says your house will need 7.6kW at -2.5 degrees.  So a 10kW ASHP would seem to have a bit in reserve. But you do have to be careful that your 10kW ASHP can actually deliver that amount when it's really needed.  The below is for a Midea 10kW monobloc.  It can deliver 10kW but probably not when you want it to.  If this is the same model as you were quoted and the supplier said it could heat your house at -7 or -10 at 50 degrees flow then it looks like they are wrong. 

In fact, with an output of 7.98 kW, this model would be running near flat out at -2 / 50 degrees in your house. 

 
You have to factor in DHW somewhere too. 
  
 

Ceol

No_No_No_Yes said:
As you mentioned, it should be better for the pump to over perform at the lowest winter temperatures.

This isn't necessarily the case. As another user mentioned, the design temperature (-2C for you in this case) is almost always exceeded. Oversizing a heat pump means for basically the whole year your pump will be short cycling (that is to say turning itself on and off again repeatedly) whilst providing heat. This is extremely inefficient and compressor cycles should be as long as possible.

It's therefore best to size correctly, and use backup heat when absolutely necessary. Some heat pumps have integrated backup heaters inside them that do this - this Midea model seems not to - but also you can just buy a single £15 fan heater from a local shop, and that would provide an extra 2kW. With a correctly sized heat pump, you probably wouldn't actually use the backup heat most years, firstly because your house will hold on to heat it had before extreme cold temperatures, and secondly because your house dropping by a degree or two probably doesn't warrant it. However, if you do need it, it's better to whack out the little 2kW fan for one day per year than pay thousands more for both a larger heat pump, and the associated higher bills resulting from running it inefficiently.

One thing I would want to check with your installer though is your altitude as this will affect your design temperature.

shinytop

Ceol said:

No_No_No_Yes said:
As you mentioned, it should be better for the pump to over perform at the lowest winter temperatures.

This isn't necessarily the case. As another user mentioned, the design temperature (-2C for you in this case) is almost always exceeded. Oversizing a heat pump means for basically the whole year your pump will be short cycling (that is to say turning itself on and off again repeatedly) whilst providing heat. This is extremely inefficient and compressor cycles should be as long as possible.

It's therefore best to size correctly, and use backup heat when absolutely necessary. Some heat pumps have integrated backup heaters inside them that do this - this Midea model seems not to - but also you can just buy a single £15 fan heater from a local shop, and that would provide an extra 2kW. With a correctly sized heat pump, you probably wouldn't actually use the backup heat most years, firstly because your house will hold on to heat it had before extreme cold temperatures, and secondly because your house dropping by a degree or two probably doesn't warrant it. However, if you do need it, it's better to whack out the little 2kW fan for one day per year than pay thousands more for both a larger heat pump, and the associated higher bills resulting from running it inefficiently.

One thing I would want to check with your installer though is your altitude as this will affect your design temperature.

This is often stated but it's not true.  Modern heat pumps can modulate down to 30-40% or so of their nominal output.  So a 10kW ASHP can happily deliver 3-4kW without cycling. If the demand is less, it will cycle but that is normal, as long as it isn't too frequent.  With most current ASHPs, you can't size an ASHP to heat your house when it's cold and not cycle when it's warm.   All other things being  equal, my ASHP is most efficient when it's running about half capacity and least effcient when it's running flat out.  Cycling is a little less efficient but it happens when you're not using much energy. 

Recommending an undersized heat pump that relies on either its own booster or external fan heaters is really poor advice that should be ignored.

Ceol

shinytop said:

This is often stated but it's not true.  Modern heat pumps can modulate down to 30-40% or so of their nominal output.  So a 10kW ASHP can happily deliver 3-4kW without cycling. If the demand is less, it will cycle but that is normal, as long as it isn't too frequent.  With most current ASHPs, you can't size an ASHP to heat your house when it's cold and not cycle when it's warm.   All other things being  equal, my ASHP is most efficient when it's running about half capacity and least effcient when it's running flat out.  Cycling is a little less efficient but it happens when you're not using much energy. 

Recommending an undersized heat pump that relies on either its own booster or external fan heaters is really poor advice that should be ignored.

I am not recommending an undersized heat pump, I am recommending not to get an oversized heat pump, there's a big difference. Getting a 14kW over a 10kW because for a couple of days in 5 years it might be colder than the design temperature is a bad move. It is that that I am saying not to do. If that comes across as advising getting an 7kW system instead, then I do apologise.

As you say, modern inverter systems can only modulate down to about 30-40%, so upsizing does not come without risk of needless wasteful cycling, and that's ignoring the cost, size, noise etc that comes with a unit that is too large to start with.

shinytop

Ceol said:

shinytop said:

This is often stated but it's not true.  Modern heat pumps can modulate down to 30-40% or so of their nominal output.  So a 10kW ASHP can happily deliver 3-4kW without cycling. If the demand is less, it will cycle but that is normal, as long as it isn't too frequent.  With most current ASHPs, you can't size an ASHP to heat your house when it's cold and not cycle when it's warm.   All other things being  equal, my ASHP is most efficient when it's running about half capacity and least effcient when it's running flat out.  Cycling is a little less efficient but it happens when you're not using much energy. 

Recommending an undersized heat pump that relies on either its own booster or external fan heaters is really poor advice that should be ignored.

I am not recommending an undersized heat pump, I am recommending not to get an oversized heat pump, there's a big difference. Getting a 14kW over a 10kW because for a couple of days in 5 years it might be colder than the design temperature is a bad move. It is that that I am saying not to do. If that comes across as advising getting an 7kW system instead, then I do apologise.

As you say, modern inverter systems can only modulate down to about 30-40%, so upsizing does not come without risk of needless wasteful cycling, and that's ignoring the cost, size, noise etc that comes with a unit that is too large to start with.

Fair enough, I do agree that a significantly oversized ASHP isn't a good thing. But I would rather have a ASHP that has 1-2 kW in reserve that can be called on occasionally when it's very cold or lots of people want a bath, rather than using a fan heater.  If the design heat requirement of a house is 10kW at -2 deg then the temperature window where a 12kW ASHP would cycle and a 10kW one wouldn't is quite narrow. 

The MCS guidelines allow for both approaches so we're both right.   

No_No_No_Yes

This is so confusing for the end user!
A qualified Surveyor should make sure that the system is more than adequate, the end user should not be apprehensive about it's efficiency.

A new survey has been carried out and awaiting the findings.
The previous installer wasn't interested in visiting the property before installation ??? He has been replaced.

shinytop

Don't forget to keep us updated  

No_No_No_Yes

Will do. 😊👍

lohr500

New survey with the same firm, or with a different installer? I would be wanting a second opinion from a different business if it was me. Both from a price comparison point of view and specification.