Reverse-plumbed heating coil in copper cylinder.

10-10-20

I was troubleshooting a short-cycle problem with my boiler and was trying to maximise the delta-T on the loop, I noticed that my copper cylinder is plumbed with the hot feed in the bottom and the return on the top. It's a ~220l indirect cylinder on a gravity fed system with a gas boiler (Vokera Mynute 28e). It's able to modulate down to 10.5kW.

Other than the theoretical discussion around heat transfer, is there any justification for the cylinder coil to be supplied from the bottom, or is it considered moot in a pumped system?

Furze99

If it's a gravity system - surely hot water rises and the hot supply connection is at the top and the flow goes down the coil and returns back to the boiler?

10-10-20

https://www.boards.ie/discussion/comment/120146648#Comment_120146648

I stated that unclearly: It's a vented cylinder, so gravity fed from the attic tank. The heating coil is pumped by the boiler and the heating system is low-pressure (topped up off the attic tank).

John.G

Bottom to Top means the whole cylinder at the same temperature.

10-10-20

https://www.boards.ie/discussion/comment/120147842#Comment_120147842

Ok, makes sense now as the house was originally fitted with a very large bath. I was concerned that there might be a technical reason for not going top to bottom. Thank you.

I think that the cycling is caused by the low modulation ratio of 1:3 on the Vokera. It's just not able to modulate down to the 3.2 or even 2.9kW rates of the newer systems so is short-cycling horrifically and delta-T is around 12 degrees for much of the time. Thinking of changing it out for a condensing unit.