Most efficient system, tank or combi

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Am fitting out a 5 bed property with 4 bathrooms and two kitchens, and underfloor heating, for multi occupancy.

I am considering a tank and boiler to feed all the hot water requirenments at peak times, mornings and eves, with a separate combi for the heating system.

Out of peak times (ie 16 hours of the day) the combi will supply the hot water instead, not the boiler (this will be switched off).

Am I right to think that keeping 300+ litres of water in a tank hot all day could be expensive - just on the off chance somone will take a shower in the day, and an on demand system more efficient for such off peak demands.

As a side question, a vented tank system would require a pump for the hot water supply to the showers ?, an unvented system does not require any pump - as it works on mains water pressure, but is more onerus in terms of installation cost and maintanence.

Any thoughts appreciated.
 
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A modern well-insulated cylinder will lose almost no heat at all during the day. Two boilers is almost certainly unnecessary for your property. If the incoming mains is up to supplying it, an unvented cylinder connected to a boiler via a hot water priority setup with two flow temperatures would be far and away the best option. You want a reliable, low-maintenance system. An unvented cylinder requires an annual service at the same time as the boiler service, so that shouldn't be too onerous.

I'd suggest an Intergas HRE SB 40 connected to a Joule High Gain cylinder. Set it up for hot water priority for fast heat recovery of the cylinder when required, and a low energy-saving flow temperature for the UFH, and you'll have yourself a very efficient, reliable system
 
First job is to measure the incoming water pressure at various flow rates to decide whether the unvented route is even viable.
Even if it is bear in mind that there's no longer a guarantee that the water companies will provide a decent pressure...certainly not in much of London with such an increasing demand.
So you may still want to consider the stored water/pump options.
 
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As been mentioned check the mains water flow. It may be a good idea to uprate your mains pipe size.

If the mains flow is low, but good pressure, try a cold water accumulator for stored water rather than a loft tank and pump.

I would look at two sequenced boilers on a low loss header. Then you always have backup if one goes down. The two boilers will do heating and hot water. You may want to have two cylinders on timers. One on the other off at low demand times. Only one boiler switched on in summer when only hot water is needed.
 

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