Importance of pressure on Combi's ?

[richard3009]

Noticed my combi mains water pressure had dropped to 0.3 of a bar tonight so re-pressurised to just over a bar my fault haven't topped it up for ages and noticed the bath taps running luke warm.
My question is why do combi's require such pressure and how is it maintained when there is a throughput of water when running taps, does it also pressurise the central heating system and is there an effect on economy.

Thanks Richard

 

[SimonG]

Your hot water and your 'system' pressure are seperate. Depending on make of boiler some will stop operating below 1/2 bar. Always find that when there is an issue with hot water temp it tends to be pressure or expansion vessel related.

 

[Plumb Ugly]

Quite right the mains pressure at your taps is independant of the system pressure shown on the pressure gauge on the front of your boiler this relates to the water filling the boier and the radiators rather than what's coming through your taps. The reason combis are always sealed (pressurised) rather than open vented is to do with the required flow rates for the circualtion through the boiler's heat exchanger

 

[lame plumber]

flow rates thro heat exchanger are to do with pump output not whether sealed or unsealed sytem, the benefits of a sealed sytem are less pipework and extra storge tanks along with less chance of corrosion, as well as being cheaper to install against loss of stored water and additional safety devices called for due to lack of f&e cistern.

Pressure in the sealed sysyem is maintain when a tap is running as it is a seperate system from the dhw, hence no drop in pressure when taps on. Any slight drops in pressure on the sealed sytem are topped up when req as you have already done!

 

[Bunker]

Be careful - if you are losing pressure regularly then you have a leak or the expansion vessel is corrupt or both. If its the former, find it and fix it. If its the latter then there could be no where for the heated water to expand to so will come out of your pressure relief valve, this is what its for BUT they have a nasty habit of getting stuck open with a bit of grit/ rust etc. So, find out WHY its losing pressure although 0.3bar aint all that much (is that when its hot or cold?)
And btw, whats this post doing in the Oil/ Solid section - should be in Central Heating.

 

[lame plumber]

And btw, whats this post doing in the Oil/ Solid section - should be in Central Heating.[/QUOTE]

prehaps its a heatslave 20/25 combi

 

[Bunker]

prehaps its a heatslave 20/25 combi

Yeah but its not about the oil element of it, its about pressure in a combi generally. Sorry, I guess I'm being picky.