Hello.

Mrs BF and myself are in the middle of a rather large house extension.

About 40% of the build incorporates an annexe for the inlaws.

We currently have a 10 years old boiler and recently replaced the immersion tank with an unvented indirect cylinder.

The boiler is being replaced during the build and the cylinder is being relocated.

I want to run hot water to both kitchens (our and the inlaws) and the utility room from a new combi boiler and all 3 bathrooms hot water from the cylinder

I want to avoid loosing pressure when having a shower if someone is washing up etc.

Can this be done from a combi? or does anyone suggest another way??

Many Thanks

BF

Yes, but explaining it to many plumbers will be hard work as they wont understand that it can be done.

Basically, use the combi hot water as normal – turn on a connected tap, the boiler lights up.

Now the tricky bit, which will flummox many plumbers, you will need 2 zone valves, both running off the central heating output on the boiler.
One valve for the central heating, one for the hot water cylinder.
It will be really easy to wire up/fit, as basically you have an S plan system, plus the combi hot water.
It is not conventional, and I’m sure there will be downsides to it, but it is very easy to implement.
The hardest part will be the separating of the hot water pipes from cylinder fed or combi fed.
Lots of work running new pipes.

I’m not sure this is the best solution, but you must have thought it through.

Just to throw a spanner in the works. I’m also looking at running underfloor heating from the boiler.

My inlaws central heating will be off one zone. My underfloor heating downstairs will be off another and my heating upstairs off the third zone.

I think I’d be looking at 2 boilers.
Your plan can be done, just add new zone valves, but there comes a point when there is just too much for one boiler.
Remember, a combi isnt running central heating when the combi hot water is running, so there may be times when there is no heat coming out – maybe not a big problem if it is only 2 kitchens, but your plan does seem to be running the boiler at its limit.

Nothing tricky in adding more zones, just more 2 port valves.

It becomes an S Plan Plus.

Why do you want to use a combi to provide some of the hot water? I assume because of draw off times?

Regarding losing pressure, best thing would be to fit flow regulators to all outlets, even those off the unvented. Or use a good shower head that regulates flow without any pressure loss. You can really reduce the amount of water that you use with careful choice of fittings.

As above, you might need to think about the rating of the boiler – if getting an additional (or larger) boiler then your gas service pipe up to the meter might need to be upgraded. Ask your supplier for a GT1 check and they will give you the capacity of the pipework, if you are lucky you might be able to get away with a bigger meter (U16). Send me an email for more info if you need it.

If I was fitting a system going by your description I would go with the following.
Fit 1 x Heat only boiler then use a S plan system to allow the independent control of various zone, to make sure the balancing of the system was correct in a ideal world you could would use commission sets on each zone or more crudely a gate valve to distribute flow more evenly.
For the hot water side I would opt for a unvented cylinder (Main pressure) I would install a secondary return for this with short spurs off to the draw off points.
( The secondary return basically involves running the hot pipework in a loop and having a pump fitted to it which circulates the water so that the amount of water drawn off before reaching the correct temperature is minimal, it is key that the secondary return is well insulated)

Personally I think this system is the most feasible. Fitting zone valves and extra controls to a Combi boiler is perfectly do able but I would say you are making a unnecessary compromise in system efficiency to be going down that route.

Most modern taps come with flow restrictors but it wouldn’t harm to fit extra restrictors. If you did find that there was too much pressure drop when multiple users were drawing water then you could retrospectively fit a an accumulator on the incoming water main.

You would need to check manufacturers instructions but most unvented cylinders require a flow rate of at least 20 litres a minute and 3 bar pressure.

Hope thats some use!

what your effectively trying to do is run two homes off one boiler and making the whole thing un necessarily too complicated.. simple things so keep it simple adding loads of valves controls etc involves more opportunities for failure KISS

Just put bigger pipework to and from the cylinder to allow for higher flow rates.
28mm will probably be overkill.
If it’s all running in 15mm then no it won’t work as you would like.

The combi would be running off the same mains water supply and you will have potentially the same issues if more than one area is drawing hot water.

Zoning heating is easy enough.

Note – I do not know how big your house/system/water service is.