How fast can a 'conventional' boiler heat water?

(a) that is the gas input rating, actual output will depend on the boiler efficiency.

(b) the limiting factor is likely to be the heat exchanger in your hot water cylinder

A Gledhill "high recovery" cylinder heat exchanger is rated 16 - 21 kW depending on cylinder size. "Super recovery" cylinders will operate with boilers up to 40 kW and at 40 degC increase in temperature will reheat 120 litres in 11.9 mins

If you need more than one cylinderful of water for a shower it will use less water and be cheaper to have a bath instead.

Owain

Owain

unless you are a muslim ....

NT

Yes, so that means that the heat output will be around 15kW doesn't it, modern boilers are around 90% efficient aren't they so it should be even a bit more than this.

Isn't the heat exchanger at the boiler end? ... or maybe not as it's actually a Compact HRE 24 OV 'Central Heating' boiler and there is only one water pipe in/out of the boiler which is presumably the pumped CH circuit which also (via the motorised valves) goes through the cylinder heat exchanger too when required.

Yes, I suppose that's true! :-) However it's a new, downstairs, shower room for my M-I-L who is getting on and thus less mobile and doesn't like baths anyway.

Thanks Owain.

A lot of water can be saved if you fit a shower head with a press-to-spray feature, but that might not be acceptable.

Owain

in some circumstances

of course there's a heater exchanger from flame to primary circuit. Direct contact boilers aren't used domestically.

NT

Depends on the type of coil in the cylinder. You need a fast recovery type. That can transfer more heat into the water.

I have one, and it will give a warm shower continuously. And my shower is a high flow low pressure type.

Looking at the installers manual, they claim a max output power of around 23kW for that model. So in theory you could reheat water at that speed however, in practice it will depend on the transfer rate of the heat exchanger coils in the cylinder(s).

The rate of transfer will also not be constant - as it tends to slow down as the cylinder temperature rises - although with good stratification you should get a higher rate for the lions share of the reheat, and the rate then drops off as the cylinder temp gets closer to the flow temperature.

Old style normal cylinders had a max transfer rate of only around 5kW. Modern ones might do 20 - 25 ish. Some very fast recovery ones even more. (My unvented Unistor is rated at about 22kW max IIRC)

That will depend on how quickly the system controls respond to a drop in temperature of the water in the cylinder, and also ho fast you are drawing water for the shower. If you water consumption is say under 8 lpm, and the boiler kicks in soon enough, then yes, you should be able to draw water indefinitely.

John Rumm snipped-for-privacy@nowhere.null wrote: [snip]

Thanks John (and everyone else who has responded) this has been a very enlightening and useful thread. We have decided we will probably go for some sort of gravity fed shower (maybe a venturi) with a thermostatic control.

Then if it's not satisfactory for some reason or another we can change things (like adding 'flanges' or getting a more modern cyclinder) if needed. I suspect though that we will get along fine with a good new shower from the existing hot water supply.

I have an ancient Aqualisa. Gravity fed from a header tank. 22mm pipes. So a high volume low pressure type. Which I far prefer to a high pressure dribble. After all it's the water which gets you clean. But rather obviously costs more to run.

John Rumm submitted this idea :

My near 40 year old 120litre manages to be much of the way in around 20 to 30 minutes from cold. It varies depending on the time of year, CW temperature, but remakes up a bath-full, in less time than it takes me to take a bath.