how does a combi boiler hot water work?

Helen28

When I turn on the hot water tap the water flows and eventually it gets hot, obviously.
Say the combi boiler uses 28kWh of gas when on full does the amount of gas being used vary depending on the flow rate of the water or does it remain constant?
Put another way, if I turn the flow rate of my thermostatically controlled shower down, even though the temperature of the mixed water remains the same and I use less water, am I still using the same amount of gas even though less water is being used? 

Undervalued

Helen28 said:

When I turn on the hot water tap the water flows and eventually it gets hot, obviously.
Say the combi boiler uses 28kWh of gas when on full does the amount of gas being used vary depending on the flow rate of the water or does it remain constant?
Put another way, if I turn the flow rate of my thermostatically controlled shower down, even though the temperature of the mixed water remains the same and I use less water, am I still using the same amount of gas even though less water is being used? 

No
A combi boiler, when providing hot water, shuts off the flow to the radiators and just heats a small heat exchanger which in turn heats the mains water flowing through it to the tap / shower. When the water is flowing more slowly, so needs less energy to heat it, the size of the gas flames reduce in proportion.

The figure you quote is its maximum gas consumption but with a reduced flow it will consume far less.

Incidentally, when you say it "eventually" gets hot is that long enough to be a problem? Most boilers have a setting that will keep the hot water heat exchanger pre-heated so that you get hot water quicker when you turn on the tap. Obviously that uses a bit of gas so may not be the most economical.

daveyjp

It depends how old the combi is.

Ectophile

Older combi boilers were either on or off.  If you turned on the hot water, it would run at full pelt.  If you turned the hot water flow down a bit, the water got hotter.  Turn it off too far, and the boiler would switch off.

Modern ones (paritcularly condensing boilers) have variable gas burners.  The burner will most likely run at a high power to start with, but will then reduce the power to maintain the temperature you set on the front panel.

Helen28

The boiler is 5 years old.
So reducing the flow of the shower does actually uses less gas, unlike an electric shower which uses the same amount of electricity but the water gets hotter the less the flow rate?

Undervalued

Helen28 said:

The boiler is 5 years old.
So reducing the flow of the shower does actually uses less gas, unlike an electric shower which uses the same amount of electricity but the water gets hotter the less the flow rate?

So that will be the newer type and the amount of gas burnt will reduce as the water flow reduces.

If you want to prove the point, set the shower running at full blast and go the the gas meter and see how much gas is used in 1 minute. Then reduce the flow and go back to the meter and see how much gas is now used in 1 minute.

That assumes your shower mixer has maintained the same temperature at both flow rates. If the shower is hotter at the lower flow then you won't save as much gas or maybe even none at all!

If you are happy to restrict the flow down to the level you would have got from a electric shower the saving could be significant.