Cylinder losing heat through central heating

A_A_Ron

I have been troubleshooting a hot water loss issue for the past few weeks after noticing that my hot water cylinder isn’t holding heat as well as it should.

i noticed that when the immersion has been on, the pipes which flow into the cylinder from the central heating get hot. The heat even spreads to the bathroom radiator if the immersion has been on for 2+ hours.

If i close the valve to the central heating pipe, the tank WILL hold its heat as expected.

i want to be able to use the central heating and the immersion interchangeably to heat water but dont want to have to close the valve every time i turn off the heat or use the immersion. is this common / What solution could I use here?

System: cylinder has a large cold water tank on top, and slightly smaller hot water tank on bottom. Takes up the whole height of the hotpress. Covered in green foam. Central heating pipes have an input and output to the hot water tank and there is also a little red expansion tank connected to heating pipes. Boiler is gas. Two immersions, one for sink and one for bath. Apartment was built in 2004 i assume these are all originals.

Thanks for any feedback!

Sleeper12

The coil in the hot water cylinder could have a hole in it. If this is the case then you may need a new cylinder

John.G

If there isn't a zone (motorized) valve) on the HW system then same result??, suppose a NR valve could be fitted in the coil just before the coil flow (top) cylinder entry?. as long as no solid fuel system involved.

A_A_Ron

https://www.boards.ie/discussion/comment/119827295#Comment_119827295

I was thinking about this… but the central heating system is holding its pressure of just under 2 bars. I assume this wouldn’t hold pressure if there was a hole in the coil?

https://www.boards.ie/discussion/comment/119827313#Comment_119827313

non return valve seems like just the thing i need! I have no zones on my heating unfortunately, its all or nothing.

The Continental Op

My guess is the coil in the tank that heats the water when you use the central heating is acting as a heat sink and creating a thermosyphon with the nearest rad in the bathroom. Hot water rises and stratifies with colder water at the bottom if the water that will be heated up in the coil can move upwards it will.

If you can trace the pipe run from the coil to the rad in the bathroom I would guess the rad is higher than the HWC and the pipes run fairly level? In that case if you put a sharp downward pipe run followed by a return to the previous position (like a tall U shape) in the pipe that comes out of the top of the coil you should break the thermosyphon. The heated water in the coil now can't rise but the pump when the central heating will move it when the central heating is on.

A_A_Ron

https://www.boards.ie/discussion/comment/119827408#Comment_119827408

This logic sounds right in theory, i would have thought the same thing! But the pipes coming out of the cylinder have a long vertical climb to the ceiling before they come back down to the ground level for the radiators. I tried to illustrate on below picture :)

zoomed out photo:

The Continental Op

Thats fine for a thermosiphon to get started. The longer the vertical run the better for kicking it off. Hot water will rise as it does so it displaces cold water and so the cycle begins. Check were the cold side runs the lower the better for it transfer the colder water back to the bottom of the coil.

Its just a small imbalance in the density of the water in the hot and cold pipes that creates the siphon effect.