Combi Boiler - hot water when tap half on?

Hi All,

I have a WB Greenstar 25si which is really undersized for my property (came with the house, 5 bed, 2 bathroom). I have been considering for a while upgrading it to something more meaty to increase the hot water flow (as I was renovating my house I paid to upgrade the mains to 32mm, cold and hot pipes to 22mm, and gas pipes to 28mm. Pressure from the mains is great as I am very close to the Thames Water treatment facility for the whole of London).

The question that I cannot find the answer to is about getting hot water when taps are half off. At the moment if I turn the kitchen tap half on, I don't get any hot water (I guess the flow switch does not trigger). I can live with it for the moment, but if I upgrade my boiler and nearly double my water flow, I want to make sure that I can still get hot water if for example 1 tap is on and the flow is significantly below the maximum stated flow rate.

Put another way, do the larger boilers, have any automatic adjustment that allows them to slow down the heating of water, when only 1 tap is on, and speed it up when there is a much higher demand? Also, I cannot seem to find anything that lists the minimum flow rate/flow switch trigger so that I can compare models and understand what flow is required to turn the boiler on, is this listed anywhere?

Thanks in advance,

Jon

Yes, and some boiler manuals at least list the minimum flow which would only be a couple of litres a minute. You also need enough water to keep the burner on the minimum rate which could be 6+ kw.
More expensive boilers generally have higher modulation ratio and better flow turn on, cheaper ones don't bother with nice to have like that.
Eg vaillant ecotec pro needs 2l min but ecotec plus only needs 1.5l so the pro needs a third more water.
Some boilers it's probably possible to switch the point it turns on.

jonwestuk said:

The question that I cannot find the answer to is about getting hot water when taps are half off.

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Using a tap or a mixer tap like that is very 'hit&miss' it could work but depends if the tap has washers or discs and it has to be a fairly decent tap,your boiler should fire at 2 l/min but that depends on how well the water flow sensor/turbine is operating,filter partly blocked etc it may need several seconds or more to fire at that very reduced flow rate.
If you want to control the tap flow you can have flow regulators fitted or if you have compatible tap spouts these adjustable aerators.https://www.screwfix.com/p/strom-maxi-flow-tap-flow-restrictor-aerator/3344r

jonwestuk said:

Put another way, do the larger boilers, have any automatic adjustment that allows them to slow down the heating of water, when only 1 tap is on, and speed it up when there is a much higher demand?

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edit,Is this to do with the boiler controlling the actual hot water flow or temperature ? Havent come across a combi that automatically varies water flow ,others may have.

jonwestuk said:

I paid to upgrade the mains to 32mm, cold and hot pipes to 22mm,

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you have already done a lot of work to improve your water system,it would be beneficial to have an unvented cylinder fitted and heated by your worcester,

Old&Bold said:

Havent come across a combi that works like that,others may have.

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Yeah ours does, we just choose the temperature we want and then it modulates the burner to hit that regardless of flow rate and incoming water temp.
Subject to minimum flow and maximum burner power.

Old&Bold said:

it would be beneficial to have an unvented cylinder fitted and heated by your worcester,

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Good advice, and fit an immersion heater for hot water when the boiler is out of action

bernardgreen said:

Good advice, and fit an immersion heater for hot water when the boiler is out of action

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That was my ideal solution, but the limitation of the house means that there is no location to put the cylinder. The attic is a no go owing to height, none of the cupboards are large enough, and we do not want to lose space in a room to allow for it. I did put one in a garage in a previous house, but my garage is converted to a room and there is no room there either. Overall, this is a compromise across the board and our ideal option is to get a decent Combi that can trigger at a reasonable flow rate - at minimum I dont want it to get worse.

John D v2.0 said:

Yeah ours does, we just choose the temperature we want and then it modulates the burner to hit that regardless of flow rate and incoming water temp.
Subject to minimum flow and maximum burner power.

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I had heard of this in some of the latest Worcester Bosch boilers.

I also dont understand the whole eco mode thing on the Worcester Bosch boilers. As I understand it, the function should keep some water warm ready for the demand from a tap. Intuitively I cant understand why the boiler does not recognise that the water has dropped temperature when the flow is low, and then fire up intermittently to bring the stored water back to temp.

Thanks for all the replies by the way. I guess I need to work with my installer and maybe call some boiler companies to work out the minimum flow rates etc.

jonwestuk said:

the whole eco mode thing on the Worcester Bosch boilers. As I understand it, the function should keep some water warm ready for the demand from a tap.

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The eco mode means let the boiler cool completely to save energy. Comfort will keep the heat exchangers warm so you don't have to run water waiting for the boiler to fire up.
But the overall energy in that water is very low, so to get below the minimum water flow you would have to have the burner cycling every second or two which is not possible.