Gravity Cold with Mains Hot

[Cinning]

I wanted to hear if there are many systems out there that reverse the normal 'Mains cold + Stored Hot' pattern.

In this house I'm thinking of combining a mains fed 40kw Vaillant 937 storage combi with a gravity cold water system from 200 gallon loft tank.

The Tank will fill quite slowly leaving the boiler most of the mains feed - ensuring the powerful boiler has sufficient mains cold water supply at all times of the day (neighborhood mains pressure varies a bit). If the the tank (ever) gets half-empty a second full-speed rate stopcock activates.

Is this unique?

Given that the reason for this system design is to ensure the combi can deliver on its promise to supply sufficient hot water flow to several showers simultaneously a second question is

...with 5 ensuite showers proposed - should the system use a single thermostatic and pressure balancing shower valve in the loft - and allow this to distribute premixed shower water to simple shower heads in each ensuite via dedicated pipework - rather than buy 5 separate thermostic bar valves or something (which might struggle with the pressure balancing anyway)?

 

[superdupergasman]

Forget connecting both the mains and the tank into the boiler, the main will overcome the tank supply pressure, you would be better supplying the whole lot from the tank and a fit a pump to increase the supply pressure from the tank like an accumulator sort of thing (but I am no expert on these) this will give you balanced water pressure

A combi however, even a 40kw one will never run 5 showers, someone will end up cold.

 

[Cinning]

(Not sure what you mean about using tank to feed boiler (installed already) - my bad description I think. The tank only feeds the taps and baths - the boiler is only fed by mains).

Given that it exists already I'd argue for keeping cold water tank / gravity distribution - for WC cisterns and cold taps. It reduces demand (and noise) from the mains at critical time. WC cisterns dont need to refill in 10 seconds flat ...running mains to cold cold taps would increase water consumption (its a metered house)?

The showers need more thought - I think the centralised 'pressure / temp valve' approach is surely more effective than running mains-cold to each shower to balance out the combi's hot and installing 5 thermostatic showers?

 

[Norcon]

If you use central mixer then individual users lose the ability to control the temperature !.

Legionella is another issue especially if a circulation loop is installed to maintain temperatures in the outlets with long pipe runs.
I would use individual point of use mixers but its your choice.

 

[Cinning]

Yes - I see the legionella point - all that lukewarm water sitting in 5 shower distribution circuits.

However - if say the pressure balancing is somehow taken care of centrally - before the distribution to the showers - then lower cost thermostatic showers can be installed - probably last longer?

 

[powell30]

Why not connect the boiler to a hw cylinder and feed low pressure h/w and tank fed cold to the mixer showers? new cylinder and controls needed but as has been said, a 40kw won't feed 5 showers. No danger of Legionella (ok small) and the boiler isn't over taxed

 

[Cinning]

Thought about it but realised demand for many simultaneously showers wont happen here. Needlessly complex.