combi for 2 showers

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My query, which I'm sure has been asked many times before, but never seem to hit the nail on the head for my scenario is how can I feed two shower heads simultaneously for a good long soak with my 35 kw combi.

Cost is a huge factor here and non vented cylinders are quite frankly out. My idea was to use a standard cylinder with immersion for backup connected solely to this shower. So how i want it to work is that the cylinder can be heated via the combi to x degrees then stop via thermostatic valve. As the shower runs the cylinder is topped up via the combi to keep the hot water coming beyond the capacity of the cylinder alone. If the cylinder runs dry, stopped from truly doing so with an electronic float switch, then the float switch activates a valve to divert the combi hot water from the cylinder directly to the shower to save me/us from freezing whilst being able to rinse. I hope to be able to control the flow between combi, shower and cold feed with valves, in line thermostats, some inventive wiring and a simple retractable switch so at first click the cylinder is heated via the combi and at float switch activation is topped up via cold feed.

I know it's not standard, but please, if you think it can't be done please explain why, after all, I am a mere sparky...
 
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My system is a vented hot water tank supplying hot taps with a second coil at the top which heats mains pressure water for the shower. Although a "custom" built tank it was only a few pounds more than a standard off the shelf tank.

If the shower is a long one and the tank is cooling too much then the boiler fires up to heat the water in the tank.
 
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In theory it should work. In practice it would be downright dangerous.

If you have a combi, I'm assuming you have no cold water storage cistern and, currently, no hot water cylinder. If you install a cylinder you'll need a vent in case of overheat. Any fault in the arrangement of switches you outline could enable overheat. The vent needs to be into a cold water storage cistern, so that the bulk of water in the cistern can cool the overheated water before it hits the overflow pipework. If you tried to vent directly to outside you'd need other devices to cut out in the event of a blockage to the venting system.
 
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I know it's not standard, but please, if you think it can't be done please explain why, after all, I am a mere sparky...
It could be done.

However would probably cost 10x more than a standard unvented cylinder and have multiple points of failure, most of which would create a dangerous situation.
 
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Thanks for all the replies.

Bernardgreen, do you find your shower ever runs cold, or just a noticeable drop it temperature?

Dan_Robinson, the point to your post is helpful how?

oldbuffer, are you suggesting that the system would be dangerous even with the addition of a cistern and overflow? And is the potential danger from overheat alone?
To clarify, the house has never had a wet heating system, i just want a combi for the instant hot water, so open to all possibilities/suggestions.

flameport, can you justify the 10x cost? even with the unvented cylinder costing £1k+ then annual servicing and added into the mix, only from what I have read from other posts, the safety question marks placed upon unvented cylinders from other plumbers?
You also mention the danger in my idea, can you elaborate and with possible remedies?
 
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Money is tight, yet you want multiple luxurious showers?

My post is useful for pointing out the oxymoron nature of your request.


Now kindly move onto youtube and search for "mythbuster water heater blast" and stick to colour coded wires as your plumbing ideas are worrying. :p
 
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My query, which I'm sure has been asked many times before, but never seem to hit the nail on the head for my scenario is how can I feed two shower heads simultaneously for a good long soak with my 35 kw combi.

Why don't you just have a bath instead?
 
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I'd like my car to do 200mph but cost is a problem so I thought I'd buy a couple of motorbikes and gaffer tape one each side of my ford fiesta. If I rig up some complicated linkages and electronic controls I'm sure it could be made to work. Helpful comments only please.
 
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I'd like my car to do 200mph but cost is a problem so I thought I'd buy a couple of motorbikes and gaffer tape one each side of my ford fiesta. If I rig up some complicated linkages and electronic controls I'm sure it could be made to work. Helpful comments only please.

Will you have to pay 3 loads of road tax with that assembly?
 
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