hot water cutting out from a combi at low flow rate

mymedi

We have a Worcester boiler 28i junior combi. It has a very annoying habit of cutting hot water at low flow rates - when you turn the tap down from a normal level, but only about half way to closed or less. After a short while the water just comes out cold and the boiler turns off. To make it heat the water again you have to open the hot tap more to increase the flow. I was wondering if anyone has any suggestions as to whether this is a design "feature" or there is a particular cause of this?
Thanks very much!

Mr_Ted

;)Its the way they work;) without enough pressure/flow the internal operational part will not do as required:)same as a car stops when you take your foot of the gas:(

mymedi

But... hang on. Unless you're pulling my chain, this is ridiculous - the water is coming through the boiler, so the temp sensor on the outlet should be able to tell that the output has turned to cold and switch the thing back on... no? :eek:

powerflush

The boiler will have to some kind of flow switch. This flow switch is activated when a hot tap it turned on and the water begins to flow.
Depending on exactly what boiler model and is age will depend on what type of device is used.
For the flow switch to activate it will normally need a minimum amount to pass through it.
When you are open the tap on full the flow switch is activated and the boiler recognises that the tap has been opened.
However when you reduce the flow rate the boiler thinks that the tap has been turned off.

This could be because of no enough water is passing through the boiler or the device that is detecting the flow is faulty.

When you reduce the flow down and the boiler is not firing, you should time a minute with the plug in the sink and try to judge how many litres per minute the flow rate is.

Financallychallenged

We have the same boiler, same cr*p performance. Had Worcester engineer out, who says it's designed like that, says you just have to waste the water until it runs hot. In my opinion the tank in the boiler is not big enough, or the boiler is too small for the water requirments. It's even worse if you have the ECO on. Works fine when the heating is running.

mymedi

powerflush wrote: »

When you reduce the flow down and the boiler is not firing, you should time a minute with the plug in the sink and try to judge how many litres per minute the flow rate is.

My quess would be quite low - 2-4 liters at the point where this happens.

Is it a reasonable assumption to think that modern higher end boilers would not have this issue, or is that wishful thinking...?
(no, I'm not thinking of changing my boiler just because of this...)

Mr_Ted

Financallychallenged wrote: »

We have the same boiler, same cr*p performance. Had Worcester engineer out, who says it's designed like that, says you just have to waste the water until it runs hot. In my opinion the tank in the boiler is not big enough, or the boiler is too small for the water requirments. It's even worse if you have the ECO on. Works fine when the heating is running.

:rotfl:But that applies to any DHW system other than a sink heater, and even that has to reheat when the already heated water runs out, there is no such thing as instantaneous hot water from whatever source, it has to be heated and get to the outlet point;) thats pure logic?

The same would apply if you had tank stored DHW, its hot in the tank but the pipework water isnt, so it has to get from source to outlet;)

Mr_Ted

mymedi wrote: »

But... hang on. Unless you're pulling my chain, this is ridiculous - the water is coming through the boiler, so the temp sensor on the outlet should be able to tell that the output has turned to cold and switch the thing back on... no? :eek:

The sensors are not on the outlet, if you mean the tap, tho are they, and water from any appliance does not go instantly from cold to hot, its takes time to transfer the heat:rotfl:
What ever the means of activating the heating process, be it sensor, switch or a diaphragm, it has to have the correct designed pressure/flow to activate it;) unless something is faulty?

You need to check the appliances minimum flow rates in comparison to the flow rate you are having a problem with, but i think its just the way you use the supply and expectations of instant hot water that are the problemTBH:)

Mr_Ted

mymedi wrote: »

My quess would be quite low - 2-4 liters at the point where this happens.

Is it a reasonable assumption to think that modern higher end boilers would not have this issue, or is that wishful thinking...?
(no, I'm not thinking of changing my boiler just because of this...)

2-4 litres per what, second, minute, hour, it cant be assessed on a guess:(

ANY appliance will have it minimum flow rate but still wont give instantly hot water:D

mymedi

Mr_Ted wrote: »

2-4 litres per what, second, minute, hour, it cant be assessed on a guess

The original quoted post asked for "per minute" which is what I provided.

Mr_Ted wrote: »

ANY appliance will have it minimum flow rate but still wont give instantly hot water

You seem to be labouring under a misunderstanding.
I'm NOT after instant hot water, nor did I ever even mention that. What my problem is - I open the taps and the water is flowing nice and hot/warm, when I then reduce the flow the hot water stops. This doesn't have anything to do with instant hot water, this has to do with the boiler cutting out.

Mr_Ted wrote: »

The sensors are not on the outlet, if you mean the tap

Obviously, I mean the outlet from the boiler, not the tap.

I would assume that whatever the minimum flow rate of an appliance is, this would be the rate that lets the water through. So, if it's flowing, it should be heating. That's what I'm trying to understand - is that a flawed assumption?

Ruski

In simple terms the combi needs a minimum flow of water to heat, e.g. 4 l/min. If it's less than the minimum the boiler could well overheat the water in the heat exchanger and cause all sorts of nastiness, so it's a bit of a safety measure too, for your benefit, honest!

If you want hot water from a 3/4 closed tap, then the only way it to have a cylinder, be it pressurised or gravity.

It's a bit of a pita at times, but as the engineer said - it's designed that way.

Hope that clears it up a bit for you

Russ