Another gas boiler/central heating question!

Have mentioned before that we are getting ready to change our Baxi back boiler (25 years old!) in the not too distant future. Would probably have gone for the same system again, but now concerned that future changes may result in spares becoming unavailable etc.
Three adults at home, bath used daily by at least two - no shower and don't want one. So, although the combi boilers seem to be the popular choice, I don't think we could cope with waiting up to 15 minutes for the bath to fill. Would it be feasible (sensible?) to install a combi but keep the existing hot water cylinder (which has an electric heating element) and its associated plumbing purely as a back-up for the bath and separate from everything else?
Tony
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Yes, it is, although you should probably run the kitchen tap from the combi side too. If the boiler is in the kitchen, this will reduce the dead leg. Also, it means that water will occasionally going through the combi section rather than sitting there fermenting.
Christian.
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Generally manufacturers maintain spares stocks for 10-15 years after last production. IIRC they have to to get a Gas Council number for the product.

There are several scenarios that you could do.
- system boiler heating the HW cylinder. Convert the plumbing so that the cylinder is heated on a fully pumped basis rather than the gravity convection method of the back boiler. If you are going to do this, it is worth changing the cylinder for a fast recovery type which will be able to reheat a great deal faster than the one that you have. For backup, you could have an immersion heater in there.
- As above with combi boiler. Use the combi water output for hot water for sinks etc. and the cylinder for the bath. This would allow you to only heat the cylinder an hour or so before you want to use the bath, for example. However since you are not using a shower then you might prefer to have the cylinder heated most of the time anyway.
For your situation, I think I'd be inclined to go for the first because the combi isn't really going to add a lot of value.
.andy
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Andy Hall wrote:

I concur with most of that but found that system boilers were more expensive than the equivalent combi. Presumably because there is much more competition in the combi market. Modern combis will work well on a S plan system, in fact there were wiring diagrams to do this with the two I've fitted. However, as most need volt free contacts to operate it is tricker to use them on a Y plan
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wrote:

Sure. It might really depend on how easy it is to rejig the plumbing to make use of it.

.andy
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On Fri, 30 Jan 2004 12:26:46 +0000, Andy Hall wrote:

That would depend on the make and their marketing/pricing strategy. For Vaillant the Thermocompact system boilers are about 50 cheaper than the Turbomax combis.
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In an earlier contribution to this discussion,

I'm not quite sure what you're suggesting. Are you saying that you would keep the cylinder, but heat it solely with the electric immersion heater? If so, this is a bad idea.
What you *could* do quite feasibly (and it may be what you intend anyway - I'm not sure) is to use the CH side of the combi to heat both the radiators and the water in the cylinder via its indirect coil - in other words to perform both of the functions currently performed by the Baxi back boiler.
If you currently have a fully pumped system with a 3-port diverter valve (Y-plan) or two 2-port zone valves (S-plan), this will be very easy to do. If you currently have gravity HW and pumped CH it can still be done - but there is a bit more work involved.
As others have suggested, use the HW side of the combi to feed the kitcken sink, but the stored hot water for baths.
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Thanks for all the advice. Plenty of food for thought and at least I'm beginning to understand what is available :-)
Thanks again, Tony
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