DHW pre-heat question

[JIH]

If I wanted to take heat from my UFH buffer tank to use as pre-heat DHW should I put the pre-heat water through the coil, or should the UFH go through the coil?  Should I refer to my buffer tank as thermal store if it has a coil? 

 

Thanks

[Jeremy Harris]

Is the buffer tank being heated by a heat pump?  If so, then use a buffer tank with a solar coil fitted, with the coil plumbed to the heat pump, as that significantly reduces the amount of expensive antifreeze needed.

 

Our system uses an indirect cylinder like this as the buffer, with the buffer filled with inhibited water.  The top of the buffer feeds a plate heat exchanger for DHW pre-heat, and to reduce the waiting time I fitted the PHE on the first floor, above the buffer, so it is warmed by thermosyphon action before the circulating pump kicks in.  The circulating pump is controlled by a flow switch in the cold supply to the DHW system, so as soon as a hot tap is turned on the pump starts to circulate water from the buffer to the PHE and back to the buffer.  The ASHP is controlled by a thermostat half way up the side of the buffer tank, and the switching is such that a buffer heat call takes priority over the UFH, so that hot water has priority at all times.

 

In practice, our preheat system delivers water at around 30 to 35 deg C to the boost heaters, either the Sunamp PV if it's charged of an instant water heater if the Sunamp has run out of stored heat.  The effective hot water availability of this arrangement exceeds that of a 200 litre unvented cylinder running at around 65 deg C.

 

Edited to add:

Here's a link to our build blog entry that has a diagram of the system: http://www.mayfly.eu/2015/10/part-thirty-nine-getting-into-hot-water-episode-one/

Edited March 5, 2017 by JSHarris

[JIH]

14 minutes ago, JSHarris said:

Is the buffer tank being heated by a heat pump?  If so, then use a buffer tank with a solar coil fitted, with the coil plumbed to the heat pump, as that significantly reduces the amount of expensive antifreeze needed.

 

Our system uses an indirect cylinder like this as the buffer, with the buffer filled with inhibited water.  The top of the buffer feeds a plate heat exchanger for DHW pre-heat, and to reduce the waiting time I fitted the PHE on the first floor, above the buffer, so it is warmed by thermosyphon action before the circulating pump kicks in.  The circulating pump is controlled by a flow switch in the cold supply to the DHW system, so as soon as a hot tap is turned on the pump starts to circulate water from the buffer to the PHE and back to the buffer.  The ASHP is controlled by a thermostat half way up the side of the buffer tank, and the switching is such that a buffer heat call takes priority over the UFH, so that hot water has priority at all times.

 

In practice, our preheat system delivers water at around 30 to 35 deg C to the boost heaters, either the Sunamp PV if it's charged of an instant water heater if the Sunamp has run out of stored heat.  The effective hot water availability of this arrangement exceeds that of a 200 litre unvented cylinder running at around 65 deg C.

 

Edited to add:

Here's a link to our build blog entry that has a diagram of the system: http://www.mayfly.eu/2015/10/part-thirty-nine-getting-into-hot-water-episode-one/

Thanks for reply @JSHarris.  So you have 2 closed circuits, one for UFH and one for DHW pre-heat via plate exchanger.   But is it possible to use the DHW pre-heat directly from the tank, which would appear to be simpler? Or am I missing something? What are the advantages of using the closed circuits and plate exchanger system?

[Jeremy Harris]

1 hour ago, JIH said:

Thanks for reply @JSHarris.  So you have 2 closed circuits, one for UFH and one for DHW pre-heat via plate exchanger.   But is it possible to use the DHW pre-heat directly from the tank, which would appear to be simpler? Or am I missing something? What are the advantages of using the closed circuits and plate exchanger system?

 

It's all to do with pressure.  If the buffer is at mains pressure then it's an unvented cylinder, and that's not really a DIY proposition, as it needs Part G3 sign off for the installation plus annual inspections, which is why I went for a low pressure system that doesn't need any paperwork.

[JIH]

3 minutes ago, JSHarris said:

 

It's all to do with pressure.  If the buffer is at mains pressure then it's an unvented cylinder, and that's not really a DIY proposition, as it needs Part G3 sign off for the installation plus annual inspections, which is why I went for a low pressure system that doesn't need any paperwork.

 

Aaha! Got it, thanks?