Do Combi Boilers only provide hot water at 100% Open Tap?

[Renier]

Recently had boiler issues heat exchanger was replaced but water would still go Cold Randomly especially in the shower.

Engineer says Combi boilers can only provide hot water if the turbine is spinning at max speed so you have to open the tap at 100% to get hot water. So he removed(broke) the flow restrictor out of the shower head and now water is 100% hot 100% of the time with the water 100% open. Now taking a shower is not cold but like being in a sandstorm having my tenders part cooked and part pressure washed.

It also goes completely cold and stops producing any hot water when I mix in cold water to bring the temp down to bare able.

Tap in Kitchen is the same I have to open to near 100% before it starts giving me hot water can run the tap for 10 minutes at 80% flow and get nothing but ice cold water. Imagine trying to wash a spoon at 100% pressure and you turn it the wrong way and have scalding water fly everywhere.

It just sounds like the engineer was lying or inept and I`m starting to think both. He even asked why would I need hot water at anything other than 100%.

Since I`m new at this forum I have refrained from using expletives(as I am quite frustrated) so you are more than welcome to add you favourite four letter word where you would like.

 

[ShaunCorbs]

Yes

Let’s say the boiler requires 13lpm to stay on heating hot water

And let’s say your taps have restrictors bring this down to 8lpm the boiler first heat everything up as normal and then when it reaches its max temp it will shut off producing hot water until it drops and goes below max all this time your running colder and colder water through

The same can be said the other way if your boiler has a min flow of 5 lpm and at max your water mains is 6lpm etc

Hope that makes sense

 

[John.g]

Assuming a gas boiler then it should be able to modulate down to around 6kw if say a 30kw combi.
30kw should give a flowrate of 9LPM at 60C fron cold at 12C but it should give as low as 2LPM @ 60C at 6kw, there is a flowswitch but this should allow a flowrate down to at least 2.5LPM before shutting down the burner, so get another engineer.

 

[John.g]

Should add that most combi boilers have settable DHW temperatures, range may be from 35C to 60C so if your required temperature is 40C then 30kw will give a flowrate of 15.4LPM and 6kw will give a flowrate of 3.1LPM (from mains at 12C), again you should have no problems in running at these requirements.
You might post a link to make/model/output of the boiler.

 

[Renier]

Should add that most combi boilers have settable DHW temperatures, range may be from 35C to 60C so if your required temperature is 40C then 30kw will give a flowrate of 15.4LPM and 6kw will give a flowrate of 3.1LPM (from mains at 12C), again you should have no problems in running at these requirements.
You might post a link to make/model/output of the boiler.

Thank you for the answer really appreaciate it.

It is a gas boiler the model is: greenstar 24i junior combi mk 111
Made by Worcester bosch

A condensing boiler if that clears up more.

I`m not sure what our flowrates are I`m afraid.

I know just about nothing about boilers and would like to know some bits.

 

[John.g]

If you care to do a HW tap test then turn the DHW control on the right of the Boiler panel fully anti clockwise which is a DHW setting of 40C.
Open the HW tap say 1/2 way and hold a dish under it for exactly 30 secs then measure this into a 1 litre milk bottle or whatever to hand, X2 and post back your findings. As long as the flow is between 3.5LPM & 15.5LPM then the HW temperature should be 40C (showering temperature).
The flow on that boiler has to be < 3.0LPM to shut down the burner.

 

[king of pipes]

Thank you for the answer really appreaciate it.

It is a gas boiler the model is: greenstar 24i junior combi mk 111
Made by Worcester bosch

A condensing boiler if that clears up more.

I`m not sure what our flowrates are I`m afraid.

I know just about nothing about boilers and would like to know some bits.

The Worcester junior has a preset hot water temperature at 55°c and is not adjustable by the user

If you care to do a HW tap test then turn the DHW control on the right of the Boiler panel fully anti clockwise which is a DHW setting of 40C.
Open the HW tap say 1/2 way and hold a dish under it for exactly 30 secs then measure this into a 1 litre milk bottle or whatever to hand, X2 and post back your findings. As long as the flow is between 3.5LPM & 15.5LPM then the HW temperature should be 40C (showering temperature).
The flow on that boiler has to be < 3.0LPM to shut down the burner.

There is no domestic hot water temperature adjustment on a Junior it preset at 55°c John ?? So what your stating above is misleading information please correct this statement.

 

[John.g]

The Worcester junior has a preset hot water temperature at 55°c and is not adjustable by the user
There is no domestic hot water temperature adjustment on a Junior it preset at 55°c John ?? So what your stating above is misleading information please correct this statement.

Thanks KOP, quite happy to correct as above.

The DHW temperature is fixed at 55C
There is no DHW temperature adjustment control.

Above, you are showing 55C, 11.5LPM & 8.6LPM, what does the 8.6LPM refer to?, is it the minimum flow?? it shows a minimum heat input of 7.38kw (DHW) which should allow a flowrate as low as ~ 2LPM at 55C and if flow rate kept > 3LPM then should not run cold?.

Can't find a link to this boiler version but presume it should still modulate down to 3LPM or so at 55C??, the OP says he cannot.

In view of this info suggest running tap at full flow until water hot then gradually reduce the flowrate until the water runs cold, measure this flow.

"Engineer says Combi boilers can only provide hot water if the turbine is spinning at max speed so you have to open the tap at 100% to get hot water. So he removed(broke) the flow restrictor out of the shower head and now water is 100% hot 100% of the time with the water 100% open. Now taking a shower is not cold but like being in a sandstorm having my tenders part cooked and part pressure washed.
It also goes completely cold and stops producing any hot water when I mix in cold water to bring the temp down to bare able.
Tap in Kitchen is the same I have to open to near 100% before it starts giving me hot water can run the tap for 10 minutes at 80% flow and get nothing but ice cold water. Imagine trying to wash a spoon at 100% pressure and you turn it the wrong way and have scalding water fly everywhere."

 

[king of pipes]

I would suggest its seasonal summer and winter temperature figures , first thing to check is the stats there's a reading at a set temperature they should be ? It could be as simple as a faulty ntc , the exchange of heat into the hot water plate doesn't seem the the issue as the water very hot ? . But I would expect it to be able to modulate down to blend hot and cold via a shower valve without issues ?
Regards Kop

 

[GazzaH]

Hello, combi boiler's don't need to have the hot tap run at full speed to kick the boiler in, if that was the case, combisave valves wouldn't work. The boiler will, however, have a minimum flow rate required to operate it. It should be in the manufacturer's instructions data section or the manufacturer will be able to tell you if you phone them,

Years ago, there were some combi's that could create a demand from a small but constant drip from a hot tap or hot pipe. I remember a boiler wouldn't let the heating work as it was in a hot water demand all the time but no taps dripping or running. After lifting the floor I found a slight drip from a hot pipe and on that old combi, it was enough to create a hot water demand and prevent the heating from working. I can't remember the boiler, it may have been the old Myson Combi which used thermistors to monitor the cold inlet pipe to tell if there was a flow. Myson Midas.

I think I've had similar with other boilers too (many years ago) but most boilers these days seem to need a good flow to operate them.

 

[GazzaH]

Also, funnily enough, one of the only modern boilers I've had a low mains flow not operating the boiler situation was a Worcester 24i junior combi.

Is there not a removeable flow restrictor disc in the boiler near the flow switch?

This was a couple of years ago but I think one I had was below the operating flow required rate for the boiler to kick in but was higher than the minimum pressure and flow rate that the water company by law has to provide.

In the end, I didn't charge them anything and advised them to pester Severn Trent to see if anything could be done or get the mains upgraded somehow.

 

[John.g]

I think I've had similar with other boilers too (many years ago) but most boilers these days seem to need a good flow to operate them.

The figure I see quoted quite often is a minimum flow rate of 2.5LPM, the above boiler has a minimum output of ~ 8kw so the temperature rise is 45.9C so in winter with a mains temp of 6C will give a HW temp of 51.9C and in summer with a mains temp of 18C, a HW temp of 63.9C. The fixed DHW temp is 55C but don't know if the burner tripsif the DHW temp is more than 5C above 55C, at 60C?.
More modern combis DHW temperatures can be selected 35C/65C and can modulate down to ~ 5kw., the DHW temperature from mains at 18C and 2.5LPM will be 46.7C, again don't know if the burner will trip if one had say a DHW temp of 35C or even 40C selected.

Just saw your latest post, the restrictor is fitted I think so that the DHW temperature is not too low (flow too high) which will happen if no restrictor fitted and the flowrate isnt restricted at the user end, people were probably inclined to blame the boiler for this.
The minimum flowrate is to ensure that the boiler isnt cycling on/off if it was set too low.