Series linking coils in a twin coil hot water tank

[madjeff]

I have installed a twin coil indirect HWT. The bottom coil designed for solar has never been used. I was considering series linking the return of the top coil into the bottom coil and then back out to the boiler. The objective is to reduce return temperatures in order to improve condensing mode on the gas boiler.

Feedback from anyone with any knowledge or experience of doing this would be appreciated.
regards

 

[josh]

Remember that the two coils are seperate so having the flow go into the top coil and the return coming out the bottom coil wouldn't work. you could link flow to flow and return to return to have more hot stored water than what you have just using the top coil. In terms of doing that though to increase condensing i don't think it would make much differance once the cylinder was upto temp to justify doing the work.

 

[Best]

Would speed up heat recovery on the cylinder & therefore stop the boiler running longer, saving money. To try to have boiler in condensing mode is probably irrelevant. Just tee off the pipes & pipe to other coil is better & quicker to heat. Joining up a standard twin coil, where the coils are level & heat same volume water, then it will be nearly twice as fast a heat up.

 

[ecowarm]

I have installed a twin coil indirect HWT. The bottom coil designed for solar has never been used. I was considering series linking the return of the top coil into the bottom coil and then back out to the boiler. The objective is to reduce return temperatures in order to improve condensing mode on the gas boiler.

Feedback from anyone with any knowledge or experience of doing this would be appreciated.
regards

Yes good idea but may reduce temp too far imo

 

[SimonG]

I installed solar unvented cylinder a couple of years ago, customer wasn't getting panels straight away so I piped the coils in parallel. Speeds up recovery.

 

[buckley plumb]

fitted a few twin coil cylinders in blocks of flats and had each coil to a flow and rtn (its own circuit) with a zone valve and cyl stat to each that worked well and good recovery

 

[Gray0689]

No need for its own circuit just do as above as said more hot water and better recovery times

 

[SimonJohns]

Would speed up heat recovery on the cylinder & therefore stop the boiler running longer, saving money. To try to have boiler in condensing mode is probably irrelevant. Just tee off the pipes & pipe to other coil is better & quicker to heat. Joining up a standard twin coil, where the coils are level & heat same volume water, then it will be nearly twice as fast a heat up.

When a boiler is in condensing mode it is far more efficient

 

[madjeff]

Would speed up heat recovery on the cylinder & therefore stop the boiler running longer, saving money. To try to have boiler in condensing mode is probably irrelevant. Just tee off the pipes & pipe to other coil is better & quicker to heat. Joining up a standard twin coil, where the coils are level & heat same volume water, then it will be nearly twice as fast a heat up.

Thanks to all for your comments; but I'm not sure I agree with them. Quicker recovery time, well yes, more efficient I don't think so. If I had a kettle with one 3KW element, and I added a second element the kettle would boil twice as fast but I'd use twice as much energy. So parallel operation would surely take about the same time use twice the energy and heat up twice the water. (If I have twice as much water as I need, I'm wasting energy not saving it t saving The problem I have with a single coil is the temp drop is quite small unlike radiators where you can choke them to a set temp diff. So when heating returns are up to thre set point and modulating the water return is much higher, over 50 deg assuming set for 65 deg. I felt that putting them in series but only controlling on the top coil would have the effect of the solar return giving up a lot more heat, making it more efficient for condensing mode and also saving on lost heat as the return would be lower so less heat lost on the run back to the boiler.

 

[Best]

. Quicker recovery time, well yes, more efficient I don't think so. If I had a kettle with one 3KW element, and I added a second element the kettle would boil twice as fast but I'd use twice as much energy. So parallel operation would surely take about the same time use twice the energy and heat up twice the water. (If I have twice as much water as I need, I'm wasting energy not saving it t saving I felt that putting them in series but only controlling on the top coil would have the effect of the solar return giving up a lot more heat, making it more efficient for condensing mode and also saving on lost heat as the return would be lower so less heat lost on the run back to the boiler.

Yes, to your point about heating a second coil which is lower than the other coil in parallel, - it will use more energy & heat more water, but only if the boiler is left on long enough! Heat up will be quicker & boiler will cut off sooner.
Also, your idea of going through the bottom coil with the return will surely heat the lower water - so extra water, & will keep the boiler running longer therefore using more fuel? That's why I was thinking condensing mode is not important. Though, on a short running time, it is a good idea.

 

[DirksPlumbing]

Yes, to your point about heating a second coil which is lower than the other coil in parallel, - it will use more energy & heat more water, but only if the boiler is left on long enough! Heat up will be quicker & boiler will cut off sooner.
Also, your idea of going through the bottom coil with the return will surely heat the lower water - so extra water, & will keep the boiler running longer therefore using more fuel? That's why I was thinking condensing mode is not important. Though, on a short running time, it is a good idea.

Depends very much on what is the future plan. And the existing situation. The top coil has only the task to top up the cylinder. It never has been designed to heat the hot water as a sole measurement. But to my understanding you have piped it now anyway.
Yes if there is no additonal recovery time required then you can surely pipe them in series. But what you gain in efficiency at the boiler you will loose twice at extra heated surface of the cylinder. Especially if the insulation is only 50mm or less. Piping parallel will loose you efficiency at the boiler and at the cylinder.
Is it needed? Would a lockshield at the return not give a better meaning of control/balancing?

 

[peteheat]

If the cylinder is of any decent quality and size link the coils at the moment you have a cylinder that is a waste of money example 300 litre single coil means you only heat 150 litres.

Use a gate valve on the connection to the lower coil and at least pre-heat the bottom half.