ASHP delta T explanation?

[JM97]

I am looking into an ASHP  (air to water) for the first time and have been reading widely around the subject.

 

In various places, I have found lots of information about needing a delta T of about 5C to 7C. So far, though, I have not been able to find a simple explanation as to why this sort of delta T is required, or why a heat pump works best like this. I would be delighted if anyone could explain the basics, or point me in the right direction. Thanks.

[JohnMo]

19 minutes ago, JM97 said:

I am looking into an ASHP  (air to water) for the first time and have been reading widely around the subject.

 

In various places, I have found lots of information about needing a delta T of about 5C to 7C. So far, though, I have not been able to find a simple explanation as to why this sort of delta T is required, or why a heat pump works best like this. I would be delighted if anyone could explain the basics, or point me in the right direction. Thanks.

Simply because the flow temperature are low. You can't have a flow temp of say 30 deg, a room temperature of 20 and return temp of 10. The return temp cannot be lower than the room temperature, to get satisfactory heat transfer to the room the return temperature will also be above the room temperature. The more work the heat pump has to do the less efficient it becomes.

 

A low temperature gas boiler is exactly the same.

[JM97]

36 minutes ago, JohnMo said:

Simply because the flow temperature are low. You can't have a flow temp of say 30 deg, a room temperature of 20 and return temp of 10. The return temp cannot be lower than the room temperature, to get satisfactory heat transfer to the room the return temperature will also be above the room temperature. The more work the heat pump has to do the less efficient it becomes.

 

A low temperature gas boiler is exactly the same.

 

Thank you, that makes perfect sense.

 

Are there also any mechanical considerations? I know with boilers you need minimum flow rates to prevent damage to the heat exchanger. There was a brief reference in something I read to "pressure" problems if the delta T on an ASHP goes above 7C. And that ASHPs will quickly trigger a fault if the delta T strays.

[JohnMo]

There is a min flow switch, if you don't meet min flow it trips the unit. The dT is managed by the heat pump it modulates the circulation pump so it's always at target. As dT reduces the heat pump add more temperature to also manage.

 

Work best in a fully open system, otherwise you need to start adding buffers and possibly reducing efficiency.

[Conor]

Most units now manage output temp to maintain the delta T on the desired range. I just upped mine from 3.5c to 5c.

[HughF]

And modulate pump speed, if you’ve got a unit with a pwm pump.