Use of heating coils in DHW cylinder

Iderown

The system: Oil burner (Riello RDB), Two zone (rads and DHW), Closed/pressurised.

We have a twin coil domestic hot water cylinder. We had thought of installing a solar water heating arrangement and then re-thought. So for 5 or 6 years the "lower" indirect heating coil in the DHW cylinder was unused. The "used" coil is the upper one within the cylinder. There was maybe 2/3 of the cylinder volume heated. Temperature is fine. The thermostat is at about 60. The boiler temperature is at about 70.

Our DHW needs are now increased. I have included the lower coil "in series" with the upper coil so that the cylinder now gets heated to near its bottom. It seems to be working fine.

Question. Are there any possible medium or long term problems that I could have caused?

shane 007

Iderown wrote: »

Question. Are there any possible medium or long term problems that I could have caused?

No, in fact it may well be better. If you have a 26kw boiler, when hot water only is required, the cylinder coil cannot physically get 23kw of energy through it so this leads to short cycling of the boiler.
With both coils connected, you give the cylinder more energy, heating more water quicker, turning off the cylinder stat quicker.
One possible downside is that it could potentially cause a balancing issue with the radiator circuits but as long as the boiler is sized correctly, the pump is correct & the system is well balanced it shouldn't be a problem.
Just ensure the tee off for the second coil is after the motorised valve, i.e. between the MV & the cylinder.

Iderown

Thanks for that, shane 007. The cylinder coils are directly "in series". No additional teeing of feed or return was used. There's no evidence of radiator/DHW balancing problems. Of course, the DHW cylinder heats much more slowly when the radiators are simultaneously demanding heat. But, once the full cylinder is heated, it can last us all day.

For your interest, the control logic is arranged so that, after the DHW cylinder is at temperature, any heated water in the boiler is circulated in the radiators even if they are not demanding heat. OK, it pushes the house temp up slightly, but, better than having that heat go up the boiler flue in the garage.

shane 007

It would be better if they were plumbed in parallel. They would work much more effedtively & could be isolated independently if required. Also better for balancing.

Iderown

Thank you. You have sussed out that I'm all for doing something the easy way (I'm not a plumber or heating engineer) - easy way is not always the best way.
I'll organise that the next time that the family permits. Mind you, it's something else to adjust. Knowing me, I could spend hours doing just that!

shane 007

Just a balancing valve on each coil. Ask for 2no 367's from any plumbing merchants.
Then when fitted, they can be balanced individually, heat equally & be isolated from the circuit if required.