Old ASHP cycling - lack of modulation?

michaels

IAS a temp fix for our failed gas boiler I have added a second hand ashp into the rad circuit.  Seems to work fine except that it cycles rather than modulating down.  It has a rated output of 13kw and draws about 4kw.  I know from our modulating boiler and smart meter that at current temps we would use about 3-4kw of heat input per hour.

The heat pump (Grant Aerotherm 1) does not seem to modulate.  Do more modern heat pumps have the same issue or do they adjust their output to meet demand?

I know from our gas boiler that demand will typically be between 2 and 8kw depending on the outside temp. (Hot water obviously on top but currently we are handling this via immersion)

shinytop

michaels said:

IAS a temp fix for our failed gas boiler I have added a second hand ashp into the rad circuit.  Seems to work fine except that it cycles rather than modulating down.  It has a rated output of 13kw and draws about 4kw.  I know from our modulating boiler and smart meter that at current temps we would use about 3-4kw of heat input per hour.

The heat pump (Grant Aerotherm 1) does not seem to modulate.  Do more modern heat pumps have the same issue or do they adjust their output to meet demand?

I know from our gas boiler that demand will typically be between 2 and 8kw depending on the outside temp. (Hot water obviously on top but currently we are handling this via immersion)

I don't know any specifics of your ASHP but most can modulate down to 30-40% of their badged output. So that would be a 4-5kW.  Depending on where you live, you may not need that amount of heat so cycling is inevitable at the moment when it's not that cold outside.

However, as well as having a minimum output, ASHPs also need a minimum water volume and flow, which is a lot more than the average gas CH system.   Although you might need, say, 5kW to hear your house, your radiators may not be able to emit 5kW at the flow rate and temp you have.  What will happen then is the return flow temp will get too close to the leaving flow temp and the ASHP will stop for a while, i.e it will cycle.

Using a higher flow temp will allow your rads to emit more heat for a given flow but of course will be less efficient.  You should also open all zones/TRVs.   A bit of cycling - 3-4 times an hour - isn't anything to worry about.  Do you know if it's more frequent?

michaels

shinytop said:

michaels said:

IAS a temp fix for our failed gas boiler I have added a second hand ashp into the rad circuit.  Seems to work fine except that it cycles rather than modulating down.  It has a rated output of 13kw and draws about 4kw.  I know from our modulating boiler and smart meter that at current temps we would use about 3-4kw of heat input per hour.

The heat pump (Grant Aerotherm 1) does not seem to modulate.  Do more modern heat pumps have the same issue or do they adjust their output to meet demand?

I know from our gas boiler that demand will typically be between 2 and 8kw depending on the outside temp. (Hot water obviously on top but currently we are handling this via immersion)

I don't know any specifics of your ASHP but most can modulate down to 30-40% of their badged output. So that would be a 4-5kW.  Depending on where you live, you may not need that amount of heat so cycling is inevitable at the moment when it's not that cold outside.

However, as well as having a minimum output, ASHPs also need a minimum water volume and flow, which is a lot more than the average gas CH system.   Although you might need, say, 5kW to hear your house, your radiators may not be able to emit 5kW at the flow rate and temp you have.  What will happen then is the return flow temp will get too close to the leaving flow temp and the ASHP will stop for a while, i.e it will cycle.

Using a higher flow temp will allow your rads to emit more heat for a given flow but of course will be less efficient.  You should also open all zones/TRVs.   A bit of cycling - 3-4 times an hour - isn't anything to worry about.  Do you know if it's more frequent?

Thanks
 
Not sure to what extent this old model modulates, not much may be the answer as it is drawing the full current on each cycle. I think the emmiters are not too bad but flow rate may be a problem as I have had to tee into some 15mm rad tails rather than the 22 main flow and return .

Energy monitor says between 6 and 10 cycles per hour

Reed_Richards

If you are using the same controller for the heat pump that you used for your gas boiler you may be able to limit the number of cycles per hour.  Telling the controller that is controlling an oil boiler may do that, if you cannot find an explicit limit setting.

michaels

Reed_Richards said:

If you are using the same controller for the heat pump that you used for your gas boiler you may be able to limit the number of cycles per hour.  Telling the controller that is controlling an oil boiler may do that, if you cannot find an explicit limit setting.

Thanks, there are two controls, a 'call for heat' ie a room thermostat, and the heat pump itself which I guess runs until the return temp reaches a cut off level and then stops until the return has cooled below a turn on level and it is this that is causing the cycling.  My emitters are not able to dispense enough energy to keep up with the heat pump - but then I wouldn't want them to as the heat pump is drawing about 5kw so potentially outputting 12-15kw and the house only needs about 3-4kw to maintain temperature.

One thing is the heat pump says it should draw 2.6kw so I need to work out why it is drawing an extra 2kw - it has a built in immersion but this is not running as I have disconnected it but I think it may have a second built in heater for defrosting and I am wondering if this is running for some reason.  The manual is not great....

Reed_Richards

If the room thermostat is programmable  then you might well be able to limit the number of cycles per hour, the water will cool more between cycles making each on-period longer.  If you don't have a programmable room thermostat, why on earth not?    

michaels

Reed_Richards said:

If the room thermostat is programmable  then you might well be able to limit the number of cycles per hour, the water will cool more between cycles making each on-period longer.  If you don't have a programmable room thermostat, why on earth not?    

Fear not. Room stat is a wireless temp/motion sensor that connects to our smart things hub with automations then controlling a smart relay

matt_drummer

Are you using the heat pump's own controller?

It will have a module that goes in your house where you set things like flow temperatures.

I can't think you have just connected the heat pump to a switched power supply?

Netexporter

13kW is quite a large heat pump. How big is the house, what is the gas boiler rating?

michaels

matt_drummer said:

Are you using the heat pump's own controller?

It will have a module that goes in your house where you set things like flow temperatures.

I can't think you have just connected the heat pump to a switched power supply?

Nope, it would not turn on if you simply powered on and off, the 'thermostat' is connected to the heat pump controls, open circuit signals the heat pump to not supply heat, closed circuit signals heat required.

Manual is here, it is the 130 model: Grant Products HPAW85 user manual : Free Download, Borrow, and Streaming : Internet Archive

It has two control units, an indoor one with limited settings and one on the heat pump itself with a few more but basically only flow temp.

michaels

Netexporter said:

13kW is quite a large heat pump. How big is the house, what is the gas boiler rating?

So the 13kw is based on a 2.59kw compressor and a cop of 4.8(!).  Our gas modulating gas boiler is 19kw and we can see that depending on outdoor temperature it runs at between 3 (minimum modulation) and 8kw on sub zero days, only using higher output when heating the hot water which we are not doing with the heat pump.

Problem seems to be that current temps we need about 4kw heat but the heat pump when on produces probably 7+kw so only needs to run about half the time but rather than say running 15 min on, 15 min off, is instead running 3-4 min on, 3-4 min off.