Oversized Air Source Heat Pump

[Ambaz79]

Hello

 

We are in the process of extending our new house. Extended on front back and side so we only have 1 old external wall that will also be insulated from inside.  We have built it to a high spec with 150mm Cavities with 100mm IKO Enertherm insulation and Triple Glazed windows from Europe that have an average U Value of 1.1.  We also have MVHR being installed soon and the house will be very airtight.  We will have UFH on ground and first floor. 

 

Now the question is we have already purchased our ASHP that 20Kwh however we have recently had a new calculation done (with the actual spec of the house) and it works out to be around 12Kwh.  If we use the 20 Kwh heat pump still would this make the unit less efficient or would it be that the ASHP just works less hard and for this reason uses the same power that a 12Kwh HP would use anyway? 

 

I've tried to find this answer online but not much I can find on this.  Any idea's?

 

[JohnMo]

To make it work you need somewhere to dump heat that is useful. A thermal store would normally be scorned on here.  But install a large one, 500 to 1000 lts.  Use a plate heat exchanger for domestic hot water take off, size the heat exchanger to work with 50 degC water.  Heat TS to 50 degC. Take central heating direct from thermal store.  That should give the ASHP plenty to work against and no short cycling.  Make sure you site the TS where lost heat is useful.

[ReedRichards]

ASHPs don't generally have the modulation range of a gas boiler (but are better than oil boilers which typically don't modulate at all).  So when the demand for heat is low they will tend to cycle.  Whether these cycles are "short" depends on the sophistication of the controller, I have a third party controller which only allows one cycle per 20 minutes.  Anyway, the more over-powered your ASHP the more times during the year it will spend cycling.  Short cycling is bad for conventional boilers and the mentality that is is bad has been carried over to heat pumps.  In reality I think it will depend on what actual time constitutes "short" and how the heat pump performs a cycle, does it start at full output and modulate down or start at a low output and modulate up?  Mine seems to do the latter.

[Kevm]

Most ASHPs can comfortably tick along at 30-40% of their maximum.  Also bear in mind that your 20kW ASHP might not be able to output that much under anything like normal use.  Several well known brands can only deliver up to 75% of their badged output under normal winter use. 

 

Cycling is normal. Heat loss calcs tend to size the heat pump for the 95th centile of coldness so for a lot of the time most ASHPs will be operating below their minumum outputs. But the good thing is, even if it's a little less efficient at very low outputs, it's not using much energy so it's less important.  In the depths of winter when you need 12kW, your 20kW (or whatever) unit will probably be in its sweet spot. 

 

And I'll be careful with my words here but some ASHPs are less efficient when working at or near  their maximum output. 

[Ambaz79]

  On 15/08/2022 at 16:53, JohnMo said:

To make it work you need somewhere to dump heat that is useful. A thermal store would normally be scorned on here.  But install a large one, 500 to 1000 lts.  Use a plate heat exchanger for domestic hot water take off, size the heat exchanger to work with 50 degC water.  Heat TS to 50 degC. Take central heating direct from thermal store.  That should give the ASHP plenty to work against and no short cycling.  Make sure you site the TS where lost heat is useful.

Expand  

I Already have a 320L Water tank as well as a 100L Buffer tank.  Its less then what you suggest but maybe this could be something that I can upgrade.

 

  On 16/08/2022 at 09:19, ReedRichards said:

ASHPs don't generally have the modulation range of a gas boiler (but are better than oil boilers which typically don't modulate at all).  So when the demand for heat is low they will tend to cycle.  Whether these cycles are "short" depends on the sophistication of the controller, I have a third party controller which only allows one cycle per 20 minutes.  Anyway, the more over-powered your ASHP the more times during the year it will spend cycling.  Short cycling is bad for conventional boilers and the mentality that is is bad has been carried over to heat pumps.  In reality I think it will depend on what actual time constitutes "short" and how the heat pump performs a cycle, does it start at full output and modulate down or start at a low output and modulate up?  Mine seems to do the latter.

Expand  

I've asked the manufacturer about this and should have the answer for it soon. 

 

  On 16/08/2022 at 15:53, Kevm said:

Most ASHPs can comfortably tick along at 30-40% of their maximum.  Also bear in mind that your 20kW ASHP might not be able to output that much under anything like normal use.  Several well known brands can only deliver up to 75% of their badged output under normal winter use. 

 

Cycling is normal. Heat loss calcs tend to size the heat pump for the 95th centile of coldness so for a lot of the time most ASHPs will be operating below their minumum outputs. But the good thing is, even if it's a little less efficient at very low outputs, it's not using much energy so it's less important.  In the depths of winter when you need 12kW, your 20kW (or whatever) unit will probably be in its sweet spot. 

 

And I'll be careful with my words here but some ASHPs are less efficient when working at or near  their maximum output. 

Expand  

This is the comment I was really looking for.  I have been told by someone I called today that having a oversized HP should not really mean you use more power so I think your comment does confirm this in a way.  Lets see how it goes for a year and if its too expensive to run and I would save money by downgrading it then ill sell it on Ebay I guess. 

[Nickfromwales]

  On 15/08/2022 at 16:53, JohnMo said:

To make it work you need somewhere to dump heat that is useful. A thermal store would normally be scorned on here.  But install a large one, 500 to 1000 lts.  Use a plate heat exchanger for domestic hot water take off, size the heat exchanger to work with 50 degC water.  Heat TS to 50 degC. Take central heating direct from thermal store.  That should give the ASHP plenty to work against and no short cycling.  Make sure you site the TS where lost heat is useful.

Expand  

Do you really want to have to heat a tank that size to over 60oC all summer just to get DHW? Any cooler would need a monster of a heat exchanger for decent DHW flow rates / temps?

[Marvin]

Hi @Ambaz79

If your going to install PV, I would suggest a Solic immersion heater type device.

 

[Ambaz79]

  On 16/08/2022 at 21:01, Marvin said:

Hi @Ambaz79

If your going to install PV, I would suggest a Solic immersion heater type device.

 

Expand  

Yes we have around 12Kwh of Solar Panels so we will also be having this device. 

[Marvin]

  On 17/08/2022 at 09:43, Ambaz79 said:

Yes we have around 12Kwh of Solar Panels so we will also be having this device. 

Expand  

Well, I think this makes a whole difference to your hot water design. 

[Marvin]

Have you checked your PV expected outputs using the PGIS web site:

 

 re.jrc.ec.europa.eu/pvg_tools/en/tools.html

 

It will give you an understanding on the amount of excess power that you could run to your hot water storage.

 

It changed my plans....

 

 

[HughF]

Care to tell us what ashp you’ve bought? We can check the specs to see what it’s minimum output is.

 

What flow temp has the heating system been designed for ?

[Christiano]

Our 16kW Samsung heat pump ramps up to about 5kW for some minutes to get it going after which it potentially drops down to about 2kW. 

 

It was oversized as we planned to add an extension with UFH, I regret that now and wish we had gone for the right size for the current needs.

[Ambaz79]

 

This is the Specs for the Heat Pump 

[PeterW]

That’s a monster - min heating is 8.83kW and you are going to need somewhere to dump the heat. I have installed a 9kW heat pump and it rarely issues full power, and the spec is not dissimilar to you for build up.

 

I would install twin stats on the hot water tank and only let the ASHP heat it when you’re 2/3rds depleted as otherwise it will be short cycling. Is the buffer installed as a pass through (ie not separated from the UFH) as it will probably not be big enough to stop short cycling unless it can “see” the floor loops direct.

 

Who designed the system overall..?