Adjusting hot water flow rate when combi boiler ignites

Is the hot water flow rate at which a combi boiler ignites something that is normally adjustable?

Or do I just have to live with the fact that if I turn a hot water on a small amount all I will get is cold water?

Reply to
Michael Chare
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The water flow has to be sufficient to operate the flow detector but only b e flowing at a rate whereby the heat (gas/oil) input can raise the temperat ure of the flowing water to the desired temperature. You cannae beat the la ws of physics Captain - Mr Scott, chief engineer of the starship Enterprise .

Reply to
johnjessop46

Yes, but is it likely that I can adjust the rate at which the flow detector triggers? Ideally I would like hot water to come from a hot tap even if the tap is turned on a small amount. Not an issue when having a bath, but it would make it easier to wash dishes.

Reply to
Michael Chare

My inexpert view is that there will be a flow rate which matches the minimum output of the boiler and stops the water going above the selected temperature.

Below that flow rate the boiler will fire up, detect an overheat and turn the flame off again. This would probably be very inefficient and not acceptable to the manufacturer.

Depends on how low the boiler can throttle the heat source, but I would not (in my inexpert opinion) expect it to be adjustable.

It may be an issue of boiler sizing - if you had a smaller boiler then you could heat a slower flow of hot water but might struggle to heat the house and fill a bath or run a shower.

[Note: it is said to be cheaper in energy terms to wash dishes in a dish washer than by using the hot tap. No idea how long it takes to cover the initial cost of the dishwasher, though.]

Cheers

Dave R

Reply to
David

This is correct. If your boiler has a minimum burner rate of, say, 6kW, and you turn on the tap so it only dribbles, 6kW would result in boiling that flow rate, so boiler can't run safely and meet your max setting.

In theory, if you turn the hot water temperature to max (not safe if you have children/elderly or anyone else who is not going to expect it to change), then the boiler could sustain a lower hot water flow rate (with much hotter water), but I don't know that any of then actually measure the rate and compare it with the set water temperature to decide when to cut-in.

If your boiler has an option to keep a litre or two hot ready for use, this might help your case, depending how the boiler manages its small water store.

Reply to
Andrew Gabriel

No often...

On some combi's yes. The ones that store and maintain a small store of a couple of litres of hot water will work better in this respect, since they will deliver that, and will also fire to re-temper that in addition to the normal pressure drop triggering of the simpler models.

Reply to
John Rumm

Thanks, I suspect that I have the latter. It does have a hot water thermostat and I when I turned this up to near maximum I did get hot water even with a slow flow rate. It was at 3/4 max before!

Reply to
Michael Chare

It sounds more likely that previously it knew you were using water, but could not fire the main burner without exceeding the maximum water temperature you had specified.

The ones with a small amount of stored water, can't easily avoid giving you the hot water at low flow rates, since its just sat there waiting to be drawn off. (the boiler will also fire periodically for a few seconds every few hours to keep the store hot). Some allow this feature to be turned on or off.

Reply to
John Rumm

And the cost of the expensive detergent. 30p a wash for some branded products. Far more than any energy costs.

Reply to
Dave Plowman (News)

I've had two in my (larger, old house with inconvenient pipe runs) and neither were ever really satisfactory for DHW, even when new. I added a separate, electrically heated supply for the kitchen sink, then an electrically heated hot water cylinder for a shower. Problems solved with the third boiler, a conventional system boiler heating the hot water cylinder.

Reply to
newshound

In my case both the boiler and storage tank are in the bathroom. So get near instant hot water there. In the kitchen, it takes more time to run hot. If I had a combi in the kitchen, I'd expect hot water quite quickly but slower in the bathroom.

So basically how quickly you get hot water is going to depend on lots of things.

Reply to
Dave Plowman (News)

Thanks, my suspicion is that the boiler is not working properly. I really need to take the covers off and see if I can find anything with my meter. Unfortunately the boiler is under a worktop which has to be removed first.

Reply to
Michael Chare

I have experienced a few over the years... the first (when I was a kid) was not a combi, but a multipoint. That replaced a electrically heated cylinder. It was fairly crude and not passively powerful. You needed to turn a tap on fairly hard to get it to fire at all, and when running it could only deliver about 9 or so lpm of hot water. You had to learn to not turn taps on too hard, or the temperature would fall. So it had its limitations, but in one respect was a massive improvement on the system it replaced, it meant there was hot water available all the time, which was a big step forward. (the cylinder was manually controlled, and non thermostatic - so got turned on twice a week to heat water on "bath nights". It was also not lagged!)

The second was a combi made by Main. Slightly better than the multipoint for maximum flow rate but in other respects similar. However that was a

*massive* step forward since it was the first time we had CH, and the first time the house was properly warm in the winter.

First house I bought, had a stored hot water system heated from the CH boiler. It was poorly implemented and not very good. Showers were feeble, reheat times very slow. Bath filling was better, but not by much. When I did a loft conversion on that place, it meant all the stored water kit needed to go, so I fitted a 35kW modulating combi. That was the best so far, it would do one excellent shower, or two ok showers at a time. Bath filling was on par with the previous stored water system but without the limitation of continuously running out of water.

This house had an even more crap gravity hot water system. Not enough water, pressure, slow reheat - you name it, it failed doing it. That I replaced with a mains pressure 210L unvented cylinder heated by a system boiler. Certainly the best system yet, although three times the price of a decent combi.

Others I have encountered include a modern combi in a small semi, with one shower room only. Absolutely ideal - it meets every demand made of it, and saves space and uses far less gas than the Ideal Mexico cast iron lump it replaced. It meant the small bathroom could be adapted for its elderly occupant with a walk in shower and lots of other easy access facilities.

No system is perfect. but in the right circumstances, a combi can be the best compromise.

Reply to
John Rumm

Note that on the ones that do keep a small amount of water tempered, there is often a configuration option to turn this capability off - so just because the boiler supports it, does not mean its always used.

Reply to
John Rumm

Some makes have a setting that allows them to learn what your requirements are and will fire up occasionally just to heat up the primary circuit. Then when you want hot water the delay is shorter, but it uses more gas.

Reply to
Andrew

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