Combi boiler: mains or pumped?

[First-time out]

Hi,

I want to switch from an open-vented "traditional boiler" to a combi. The size of the house & usage isn't an issue. I have gotten 3 quotes, and all 3 have given different recommendations for how to provide cold feed/boost pressure.

The 1st is to use the current cold water storage and to pump the outlet to the combi boiler.
The 2nd is to use the mains pressure, which is quite good, to pressurise the system.
the 3rd is to fit a pump (such as a Salamander Homeboost) to the incoming mains under the sink to negate any possible drop in pressure.

I am leaning towards the 1st option as the pump should fit in the space beside the storage tank, so noise shouldn't be too much of a problem. This option would also give a backup in case of water outage.
Although option 2 would be the quietest given the lack of a pump.

Any advice/opinions would be welcome.

 

[bernardgreen]

Options 1 and 3 need not only a pump but equally important they need a control system for the pump,

This control system would have to detect when a hot water tap was turned on and turn on the pump for as long as the tap was open.

 

[flameport]

for how to provide cold feed/boost pressure.

What is the pressure and flow of the mains cold water supply?

 

[First-time out]

Options 1 and 3 need not only a pump but equally important they need a control system for the pump,

This control system would have to detect when a hot water tap was turned on and turn on the pump for as long as the tap was open.

Would the controls not be built into the pump? The plumber recommended a Stuart Turner negative head pump

 

[First-time out]

What is the pressure and flow of the mains cold water supply?

Pressure I don't know, but flow of about 7.5 l/min.

I've been in the house for about 18 months and haven't noticed times of reduced pressure yet, but i'm concerned that demand on water will become more of an issue in the future. That's why i'm leaning towards the pumped storage tank option.

 

[flameport]

Pressure I don't know

Those quoting should have measured it, as without it's impossible to know what system is suitable.

 

[oldbutnotdead]

Any particular reason you want a combi? 7.5 ltrs/minute is very poor - if that's the flow rate at the water suppliers stoptap (the one in the street) it is below minimum service obligation.

 

[Madrab]

To avoid any real issues and added cost, you only want to use the mains for the combi.

If the mains supply pressure is below 12L/Min @ 1.5 bar then as number 3, get a mains booster to augment the mains supply up to that level.

As suggested though I'd get someone from your water transporter out to do a survey on the mains to see what is actually being delivered. If someone is suggesting using a pumped supply from a cistern in the attic for the combi, then unless there is a real issue with your mains then I'd get some more installers in to estimate.