Double pass on a cylinder

[Agile]

Its all to do with the diameter of the pipe in the coils!

I think this is a bit to complicated for the OP.

 

[Busby28]

roamer wrote

If we do it in paralell:
-> existing DHW coil will offer the same resistance as it does now.
-> solar coil is slightly smaller in coil length but will only take what circulation in that it's design will allow and not impeed the DHW coil above it?

If we do it in series:
will the flow rate will be reduced after the circulation leaves the DHW coil and enters the solar coil?

My cylinder is number 5 on the chart - 250 litres

It would help if you stated which circuit your're thinking off connecting the extra coil too and who the coil manufacturer is.

Integron coils for example (used by Gledhill) are advised to be used on pumped circuits only.
http://www.britheat.co.uk/products/finned_products/integron_finned_tubes.htm

Also its hardly innovative thinking to have the cylinder heated to the full capacity by the stove and the boiler circuit remain on the top coil only.
A 5 or 6m head dedicated pump from a system link unit should have no problems with the required flowrate through the extra solar coil.
http://www.gasapplianceguide.co.uk/Spec Table.jpg

 

[Onetap]

It's too complicated for me.

If we do it in series:
will the flow rate will be reduced after the circulation leaves the DHW coil and enters the solar coil?

Assuming the flow and return temperature stay the same, you have added the additional hydraulic resistance of the solar coil and the flow rate will be reduced. However, because the solar coil is in contact with colder water, the return temperature will be lower until the cylinder is hot. The differential temperature (dT, Tf-Tr) is proportional to the differential pressure in gravity systems, the dP generates the flow and the larger dP will tend to increase the flow rate.

You can't predict what might happen, hence my suggestion that you just do it and see what happens.

You should monitor the return temperature, you don't want it to drop below 50 (?) or you'll get tar deposits and acid condensation on the stove.

If we do it in paralell:
-> existing DHW coil will offer the same resistance as it does now.
-> solar coil is slightly smaller in coil length but will only take what circulation in that it's design will allow and not impeed the DHW coil above it?

You reduce the circuit resistance, because the water can now pass through either of two coils, so you should get more flow at the same F&R temperatures. The proportion of the flow through any one coil depends on the coil resistance and the height between the F&R connections. The solar coil 'appears' to be very squat, so I suspect that most flow will go through the upper gravity coil and the solar coil will make little contribution. AS before, you can't predict what will happen. I still think you should connect it in series.

You should not connect a pump in the gravity circuit.

 

[Agile]

It's too complicated for me.

You should not connect a pump in the gravity circuit.

Because if you do it ceases to be a gravity circuit?

 

[roamer808]

Thanks guys. I'll ring the cylinder people and check the coil specs.

 

[Onetap]

Because if you do it ceases to be a gravity circuit?

Yes, but it reverts to being a gravity circuit with an obstruction in it when the power fails and the gravity circulation is the only means of dissipating the heat from the stove.

Besides that, he's already got a pumped circuit from the stove to the header thingy.

 

[roamer808]

The alteration is only to the pumped circuit, nothing is being touched on the gravity.

The gravity is working very well and I have already tested it with the power off and the fire high.

 

[Norcon]

OSO recommend the boiler connection to the lower coil (solar) in their twin coil solar unvented cylinder when solar is not being fitted.

You could fit a divertor valve on the flow which allows the top section only to be heated, but energised through a single channel programmer diverts to the lower coil to give the full capacity.

Or position the diversion between the two coils so you have the top one only or when energised both in series again to give the full capacity.

A two or three pipe boiler connection with injector T would be better imo from a control point of view as the low stat is always in the path of the return water.