Using an indirect cylinder with a combi boiler


Postby woz » Fri Feb 19, 2010 5:57 pm

At present I have a 'normal' but 20 year old gas boiler with pumped zoned central heating ( 2 stats 2 motorised valves) and an indirect cylinder for the hot water which is controlled by separate motorised valve and cylinder stat.
I would like to fit a combi boiler and use the instant hot water to supply only one part of the house, the end near the boiler, [u]but to retain the indirect cylinder for the rest of the house[/u].
There are a number of reasons for this: the cylinder is quite a long way from the boiler, our house is long and thin with kitchen at one end and bathroom at the other, so hot water takes a long time to reach the destination from the cylinder. That part of the house would become supplied by direct hot water. The bathroom near the cylinder would be left as is, fed from the existing cylinder. Secondly my wife wants to retain the airing cupboard as she like it as it is, and it would probably cost more to remove it anyway.
My question is Is it possible to run an indirect hot water cylinder from a combi boiler and where can I get wiring diagrams and info.
Thanks
woz
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Postby plumbbob » Sat Feb 20, 2010 4:18 pm

I have never seen it done. I have never even heard of it being done but I can't see why it can't. In fact, I am thinking about fitting a set up similar with the cylinder having a second coil heated by solar.

You already have pretty much all the plumbing and wiring required in your "S" plan system so that part in principle should not be difficult.

How the boiler is connected may be dependant on what make is chosen.
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Postby woz » Sun Feb 21, 2010 3:06 pm

[quote="plumbbob"]I have never seen it done. I have never even heard of it being done but I can't see why it can't. In fact, I am thinking about fitting a set up similar with the cylinder having a second coil heated by solar.

You already have pretty much all the plumbing and wiring required in your "S" plan system so that part in principle should not be difficult.

How the boiler is connected may be dependant on what make is chosen.[/quote]

I am of the same opinion myself. The feed for the hot water is after all just another 'boiler go' signal and a motorised valve controlled by a stat. From the boiler's point of view that is just another circuit that requires the pump and boiler to go. Whether it is another set of radiators and a room stat or a cylinder and cylinder stat seems irrelevant The rest of the control such as boiler overrun etc. should be taken care of by the internals of the boiler. I think however the problem will be having to provide a wiring diagram to the installer as they probably haven't done one before. I had a similar problem when it was installed 20 odd years ago, in the end I drew it out for the installer. I can't understand why more people don't go down that route, it seems logical. I want someone to tell me why it's not a good idea, because there must be a reason!
Just a thought re your solar idea, why would you need a second coil when you could probably feed the existing coil from the solar panel to pre-heat the water. You would of course need some control gear, perhaps a solenoid operated boiler bypass for when the water didn't need to go through the boiler? I haven't looked at solar systems yet, probably quite obvious from my reply!
woz
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Postby plumbbob » Mon Feb 22, 2010 9:28 pm

I think what you have to consider is, if a combi boiler is installed it is chosen because the householder wants to do away with the hot water storage and go the route of instant hot water. If he opts to retain the tank it is just a simple matter of choosing a system boiler instead. People often think modern condensing boilers are only available as combi's. Few people are going to want both options together.

Because a combi is self contained, the only external control required is a link out and back in. The out lead is probably a permanent live anyway so the wiring is likely to be identical to a "S" or "Y" plan. Someone would have to cast an eye over the boiler schematic to make sure.

I am not an expert with solar, but as I understand it the cylinder needs a second coil because firstly, the primary has it's own pump and there is a possibility of the water circulating where it shouldn't and secondly, the solar circuit needs the protection of antifreeze which you don't want mixing with the radiator circuit.

I see now that some combi's can accept pre heated water (from a solar store etc) and will just lift the temperature to the pre-set level. Now that could be useful.
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Postby thegasman » Sat Feb 27, 2010 9:54 pm

right first things first this is really realy easy to do. your new combi boiler will have a live, neutral,earth, live out and switch live return. so all you need to do is get a wire from live out to your wiring centre, do your standard s plan wiring and connect the switch live return to your orange wires from your valves. To make things easier i usually convert to a Y plan but its up to you. Pipework is simple enough to do, i hope this helps.
thegasman
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