Advice on a combi boiler please.

[doitmyself]

large hot water demands that will not deliver what you want because the mains water supply will not do it anyway.

Just fit a conventional system with an unvented hot water store

If the incoming mains supply is not sufficient for a combi boiler, then it also unsuitable for a mains-fed unvented cylinder. Both rely on the mains pressure and flow rate to deliver water to the taps. The only difference is that the combi has to heat the water as it is used.

 

[acecharly]

If you have a store of hot water then the idea here is that your water pressure does not matter because while the hot water is not being used the cylinder is filling to a point where your likely to reach your peak demand. Having an engineer calculate your hot water demand would mean a cylinder of appropriate size will be installed to ensure that you dont run out. Efficiency wise unvented cylinders are good many will heat up in 20 minutes and will stay warm for up to 2 days.

 

[doitmyself]

If you have a store of hot water then the idea here is that your water pressure does not matter because while the hot water is not being used the cylinder is filling to a point where your likely to reach your peak demand. Having an engineer calculate your hot water demand would mean a cylinder of appropriate size will be installed to ensure that you don't run out.

A conventional unvented HW cylinder relies on the pressure provided by the cold water tank in the loft to provide the flow of hot water from the taps. An unvented cylinder relies on the mains pressure to do the same job. So if the mains pressure is not good enough, you will not get an adequate flow from the tap.

All the unvented cylinder does is to allow you to store a quantity of water at a high temperature, which means you do not have to heat the water instantly so the boiler can have a smaller output compared to a combi.

Yes, the cylinder has to be sized correctly to ensure that there is an adequate supply of hot water.

 

[Centralheatking]

So after all this we are back to the traditional conventional boiler set up - hey

2 tanks in roof space - f& e & cold water storage and a cylinder which will be sized
to serve the property heated by a suitable sized boiler. ok some fancy controls
regarding zones etc to comply

So its really back to the early 80's for larger buildings -with new boilers etc

centralheatking

 

[doitmyself]

So after all this we are back to the traditional conventional boiler set up - hey

Only if it's not possible to get sufficient flow and pressure from the mains, e.g by upgrading the incoming pipe.

Vaillant, for example say:

To obtain optimum cylinder output, there must be a suitable cold mains inlet, i.e. the measured static line pressure must be at least 2.0 bar. A corresponding flow rate of at least 20 to 25 l/min must be available. ... The cylinder still works satisfactorily with a pressure below 2 bar but the flow rate is reduced. ... If the pressure is below 1 bar, you should not install an unvented cylinder.