Get your heat pump quick?

michaels

Reed_Richards said:

Baxter100 said:

That's not true, the efficiency of a heat pump is impacted far more when the temperature differential increases.

The efficiency of a heat pump drops around 10% for each degree increase in temperature differential. So a heat pump which runs at 300% efficiency when the output is 50 degrees C and outside air temp 10 degrees C (40 degrees C difference) will only run at 200% efficiency when the outside air temp is 0 degrees C (50 degrees C difference). So the cost of a unit of heat goes up by 50%. That isn't anything like how a gas boiler operates.

You misunderstand (or failed to read) what I wrote.  The important temperature differential that determines the efficiency is the differential between the outside air temperature and the flow water temperature down the heating pipes exactly as you state.  But the influence of room temperature on energy use is the same for all boilers, if you raise the room thermostat setting you will use more energy in the same proportion whatever type of boiler you use.  So if your elderly couple want their thermostat set at 23 C then the fact that they have a heat pump isn't the problem.    

Not sure that is true, my understanding is the hotter the water circulation the less efficient the heat pump will be so it is not just you need x% more kwh at 3% cop but that the cop falls as you try and raise the temperature by heating the circulating water hotter.

Reed_Richards

How many times? You don't need to change the circulating water temperature to change the room temperature! 

If I turn off the weather compensation feature then the flow temperature of my hot water will rise to 50 C and stay there (until the return water gets too hot).  If I want my room temperature to be 21 C lets say I need to run the heat pump for 30 minutes every hour and if I want the room temperature to be 23 C then I need to run the heat pump for 33 minutes every hour (because it's about 1 C outside).

Previously I had an oil boiler which is either off or on and I had different radiators.  Lets say I needed to run the oil boiler for 20 minutes every hour to keep the room temperature at 21 C.  I would need to run it for 22 minutes every hour to keep the room temperature at 23 C.  33/30 = 22/20 = (23-1)/(21-1) = 1.1.  So on a day when it is about 1 C outside it will cost me 10% more to heat the room to 23 C than it would to heat it to 21 C.  And that is the same 10% more whatever heat source I use.     

Meatballs

michaels said:

Reed_Richards said:

Baxter100 said:

That's not true, the efficiency of a heat pump is impacted far more when the temperature differential increases.

The efficiency of a heat pump drops around 10% for each degree increase in temperature differential. So a heat pump which runs at 300% efficiency when the output is 50 degrees C and outside air temp 10 degrees C (40 degrees C difference) will only run at 200% efficiency when the outside air temp is 0 degrees C (50 degrees C difference). So the cost of a unit of heat goes up by 50%. That isn't anything like how a gas boiler operates.

You misunderstand (or failed to read) what I wrote.  The important temperature differential that determines the efficiency is the differential between the outside air temperature and the flow water temperature down the heating pipes exactly as you state.  But the influence of room temperature on energy use is the same for all boilers, if you raise the room thermostat setting you will use more energy in the same proportion whatever type of boiler you use.  So if your elderly couple want their thermostat set at 23 C then the fact that they have a heat pump isn't the problem.    

Not sure that is true, my understanding is the hotter the water circulation the less efficient the heat pump will be so it is not just you need x% more kwh at 3% cop but that the cop falls as you try and raise the temperature by heating the circulating water hotter.

If you are comparing condensing boilers then the hotter the less efficient they are too. However a gas condensing boiler may drop from 93%? to 80% worst case? An ASHP can be dropping from 470% to below 200% so has much further to fall by increasing the water temps?

Meatballs

Reed_Richards said:

How many times? You don't need to change the circulating water temperature to change the room temperature! 

If I turn off the weather compensation feature then the flow temperature of my hot water will rise to 50 C and stay there (until the return water gets too hot).  If I want my room temperature to be 21 C lets say I need to run the heat pump for 30 minutes every hour and if I want the room temperature to be 23 C then I need to run the heat pump for 33 minutes every hour (because it's about 1 C outside).

Previously I had an oil boiler which is either off or on and I had different radiators.  Lets say I needed to run the oil boiler for 20 minutes every hour to keep the room temperature at 21 C.  I would need to run it for 22 minutes every hour to keep the room temperature at 23 C.  33/30 = 22/20 = (23-1)/(21-1) = 1.1.  So on a day when it is about 1 C outside it will cost me 10% more to heat the room to 23 C than it would to heat it to 21 C.  And that is the same 10% more whatever heat source I use.     

I think it's easier to think of it with just constant runtimes than time periods - the magic of modulation and boilers self regulating temps through on off cycles complicates that?

Assuming prefect weather comp  and running the ASHP/oil boiler all of the time by modulating down the flow temperature would mean that the room temperature stays the same so heat in = heat lost.

To warm it up another degree it would therefore need to run hotter for a certain period to deliver the additional temperature change  in a reasonable period of time (but we can probably ignore this) and then slightly hotter than before to maintain the energy as heat loss increases due to a greater temperature differential with outside.

I agree that same energy is required to be delivered for oil/HP however for every degree you increase the heat pump flow temps it is going to be a larger % efficiency loss compared to a gas/oil boiler increasing a degree because the HP is far more efficient to begin with? It has farther to fall and you can see that in the manufacturers tech docs when they show the COP at 35/45/55 (external 7 degrees) etc.

Reed_Richards

Most of the time my ASHP cycles.  Why?  Because it has no idea how big the heat demand is; it can do weather compensation but not load compensation as many modern gas boilers can.  I have no idea why not, it just doesn't seem to be a feature of this generation of heat pumps.  Also it cannot modulate down to as low a fraction of full output as a gas boiler can.  It's actually much more like the oil boiler it replaced; (which was just on/off; oil boilers do not modulate) than a gas boiler.  So I think of time periods rather than constant runtime because that is what actually happens!  

michaels

Interesting, our 20kw gas boiler with weather comp controls modulates down to 4kw and aims to run as much of the time as possible to avoid losses from on/off cycles.  We set a heating curve of flow temp vs outside temp which if it is set right should keep the indoor temp constant (actually we are impacted by solar gain and additonal heat loss when it is more windy but it is pretty good.  Hence my expectation that warmer room temps = higher (continuous) flow temps.  At pretty much any normal UK temperature the flow temp means the boiler will be condensing.

Reed_Richards

Not an Opentherm controller then @michaels?

michaels

Reed_Richards said:

Not an Opentherm controller then @michaels?

Nope - looks interesting but ours has 'weather comp' which has an external temperature sensor and a sensor for the return temperature and tries to maintain a constant return temperature (via modulation) which is higher or lower depending on the external temperature.  The radiator trvs then play a role as they limit rad circulation and thus heat loss as individual rooms get up to the TRV temp resulting in the return flow being warmer and the boiler inputting less energy.

Reed_Richards

With Opentherm you should get both weather and load compensation.  The gas boiler from 1998 in my old house could do both of those and why the latter is not a feature of most heat pumps in 2021 is beyond me.  My heat pump will do weather compensation and reduce the target maximum heating water temperature in warmer weather.  But if it knew when the house was close to the thermostat set temperature and also reduced the heating water temperature then there is the potential for more efficient operation.   

Verdigris

Does anybody here run a Panasonic ASHP? They seem to perform well (on paper at least) and have some useful looking features like linking to solar PV to make the best use of any "free" energy.