Worcester Bosch Greenstar 24i, hot water not very hot or even cold sometimes

[talktalk]

This was installed about four and a half years ago, and apart from a leaking plastic hydraulic block which had to be replaced last year it has been fine. CH is fine, and the shower water is hot. But for the last few months the bath water is only warm and the hot water supply in the kitchen is sometimes hot, sometimes warm and sometimes cold. I had the Worcester Bosch engineer out last week, he replaced what he called the plate (I think it was the heat exchanger) which he said was full of grit. There is no improvement to the hot water supply. He blamed the warm bath water problem on the boiler not being powerful enough (only 24kw) except for a shower. He also blamed the weather - the cold water supply temperature is low and the boiler can only heat the water by 30 degrees. But the hot water supply problem only started a few months ago - last winter was just as cold if not colder and we had hot bath water! The condensate pipe froze last December, that's how cold it got! Has anyone got any ideas that I can pass onto the next engineer? It is installed in the loft of a ground-floor bathroom but so far no one has blamed that.

 

[wazzer]

slow the flow down and wait with a 24 it should take all night to get a hot bath

 

[diamondgas]

Talktalk - You'll need a thermometer, stopwatch, a litre measuring jug, and your manufacturers instructions. Measure the temp of the cold water at your cold tap, measure how hot the hot water is getting too! If it's plus 30 degC then you need to measure how much water is flowing out the hot tap hence the measuring jug and stopwatch. Measure 1Ltr, in seconds, divide into 60 to give Ltr/min. Check against MI's reccomendations

If it isn't performing as per mi's then you start looking for the reasons why and not before!! 24kW boilers are renowned for the hot water issues mentioned.

 

[talktalk]

Thanks Diamondgas, I have got all that equipment. Regarding the technical data for the 24i, the manual gives two sets of values:
Domestic Hot Water specific rate – 30 degrees rise – 11.5 litres/minute
Maximum Domestic Hot Water flow rate – 40 degrees rise +/- 15% - 8.6 litres/minute
Is it the 11.5 litres/minute that applies in this case?

Regarding measuring how hot the hot water is getting, is that with the tap(s) fully open? If so, the water is running so fast that it is barely getting warm. The only way to get hot water is to open the tap(s) partially to slow the flow down, and doing that with the bath hot water tap results in the boiler shutting the water supply down. Also, with the hot tap in the kitchen and bathroom basin fully open the water is so fast that it splashes out of a 2 litre jug so it isn’t possible to know when the jug is holding a litre.
The Worcester Bosch engineer said the hot water was about 45 degrees which he said was very good for that boiler, but I didn't see how he got the water flowing at that temperature. The only way I can get really hot water is to have the tap partially open so that the flow rate is reduced, with the exception of the shower.

 

[wazzer]

that is to fast slow it down

 

[GrahamM]

As wazzer mentioned. If you are running the hot water that fast that it is splashing everywhere then that is too fast.
The water is heated as it runs through the boiler. If too fast the water will never get to temperature, especially on a less powerful boiler.
You need slow down the tap flow.

 

[wazzer]

If you want it that fast you will have to buy a 38-42 kw boiler

 

[talktalk]

The cold water is coming in at 12 degrees. I tried the kitchen tap with the ceramic disc tap open about an eighth of a turn and got hot water at 45 degrees with a flow rate of about 12 litres per minute. This is fine, and if it was always like this I would be pleased, but sometimes the boiler doesn’t light at that flow rate so the water is cold. I don’t understand why the boiler worked fine for four years, with no cold/lukewarm hot water problems, and now has an intermittent problem which seemingly can’t be fixed.

 

[wazzer]

If the boiler thinks the water is hot it will reduce the gas to the burner ...this will be the thermister.......or if it dose not know the water is flowing it wont light .....this will be the flow switch

 

[diamondgas]

The cold water is coming in at 12 degrees. I tried the kitchen tap with the ceramic disc tap open about an eighth of a turn and got hot water at 45 degrees with a flow rate of about 12 litres per minute. This is fine, and if it was always like this I would be pleased, but sometimes the boiler doesn’t light at that flow rate so the water is cold. I don’t understand why the boiler worked fine for four years, with no cold/lukewarm hot water problems, and now has an intermittent problem which seemingly can’t be fixed.

You're saying here that you got a 33 DegC rise at 12lt/min, mi's state 30 Dec at 11.5Ltr/min. That looks about perfect IMO! Now if the engineer has replaced the Plate Heatex he/she may have left out the flow restrictor which minimises the hot water flow through the plate heatex. Just a thought If you hot water is delivering excess through the boiler it will never get hot!!

 

[wazzer]

"but sometimes the boiler doesn’t light at that flow rate so the water is cold"

 

[talktalk]

You're saying here that you got a 33 DegC rise at 12lt/min, mi's state 30 Dec at 11.5Ltr/min. That looks about perfect IMO! Now if the engineer has replaced the Plate Heatex he/she may have left out the flow restrictor which minimises the hot water flow through the plate heatex. Just a thought If you hot water is delivering excess through the boiler it will never get hot!!

I seem to remember that the boiler used to restrict the flow to ensure hot water but instead of that it now lets a load of lukewarm/cold water through unless the taps are used to throttle the flow down. The impression I got from the WB engineer was that this model wasn't up to the job, it was working as it was meant to work, and I just had to put up with its limitations.

 

[wazzer]

if you turn your stop tap so 11.5l of water are coming in the water should work to the best ability of the boiler 24kw are not for baths I never fit 24s always 28 and up

 

[dancinplumba]

reduce flow through boiler with cold feed iso

 

[diamondgas]

I seem to remember that the boiler used to restrict the flow to ensure hot water but instead of that it now lets a load of lukewarm/cold water through unless the taps are used to throttle the flow down. The impression I got from the WB engineer was that this model wasn't up to the job, it was working as it was meant to work, and I just had to put up with its limitations.

From what you're saying it does sound like the flow restrictor may be missing or damaged? Be mindeful of the advice from wazzer regards restricting the flow at the boiler isolation valve. use to do that a lot before restrictors were fitted as standard! Check the mi's also as there should be a mention of where the flow restrictor is situated and may also mention what Worcester restrict the flow to! It's not to difficult a job to get hold of a new restrictor and fit it I'd imagine

 

[wazzer]

Wasnt me !!! I was at the mains

 

[dancinplumba]

it was me !!!lol

 

[tweakyboy]

Pretty sure the flow restrictor is in the flow turbine assembly in the Junior Greenstar, along with the filter. Then again I could be wrong!

 

[diamondgas]

I didn't see your post till I posted Dancinplumba I must be psychic I'd read wazzers and moved where he'd suggested to the boiler .... lol

Personally without checkin out the mi's I wouldn't know where the restrictor is! Can't say I've done much more than a safety check on a Greenstar 24i

 

[tweakyboy]

Yeah I just checked the M.I's the flow restrictor and filter is in the flow turbine assembly, so I can't see how the plate heat exchanger can be filled with grit. Unless it wasn't fitted in the factory, but thats pretty unlikely.

 

[talktalk]

Update: I reduced the cold feed so the valve is only open a quarter of a turn and that seems to have fixed the problem. Hot water is coming out at 45 degrees C with the hot taps fully open and the flow rate is like it was a few months back before this problem developed. Thanks for everyone's help and info.

 

[SILKCUTZ]

Our plumber explained this to us really well. This is a good boiler but not the largest. It heats water by firing on cold water being pushed through it. In summer it's fine. The temp of the water going through it is higher so raising up the temp is easy. In winter the cold is very cold. Raising it Up 30% still doesn't make it warm enough. The answer is to not turn the taps on to full but back a few notches. Reducing the flow allows it to heat a smaller amount of water to a higher temp.

Watch out though to small a flow will not kick the hot water in. The flow has to be powerful enough to turn the hot water on.