No hot water from Worcester 25Si COMBI

monkey64 · 6 Nov 2010

My Worcester 25Si COMBI is not supplying the piping hot water it used to.
It started playing up since I fitted a Salamander electrolytic limescale reducer. Possibly a red herring?
The boiler is 4 years old.

CH works fine but when I turn the tap on full, the boiler fires and I get red hot water for about 5 seconds.
After that, it gets colder until it is just lukewarm.
It would seem to be a flow sensing issue.

Any ideas?

ChrisR · 6 Nov 2010

WHat's the flow rate from the hot tap? SHould be 8 litres/minute.
If slow, it's badly limescaled .

StevenJames125 · 6 Nov 2010

DHW Thermistor ok?

if as suggested a flow issue, check any filter/cartridge in your boiler on the cold inlet (that supplies the hot water to your hot taps) has not clogged up and resitriced the flow.

monkey64 · 6 Nov 2010

The flow rate is about 8 litres / min and I don't think that there is a restriction.

How would I test the thermister? Do they fail.
Could it be the diverter valve failing?

onlyfitidealboilers · 6 Nov 2010

only if your rads come on when you run the tap yes!

run the tap really slow does it get hot? it may be down to cold wtr temp in the road mate

boilerdoktor · 6 Nov 2010

assuming this is the greenstar i note the following:
1) the 5 seconds worth of hot water sounds like it is a result of the preheated water!
2) when you say it goes lukewarm, is the heating on at the same time?
3) you should be getting 5c dc at the flow turbine
4) does the boiler fire on h/w only (green light illuminating)
5) Does this light come on and go off during h/w demand
6) do you get a flashing light!

This boiler gives 35 degrees c above the cold main coming in. today i typically measured 11deg on a new installation cold and 52 deg hot! this is about right! does heating pipes get warm when h/w is on??

ChrisR · 6 Nov 2010

Hang on, the 25Si I'm thinking of doesn't have a diverter valve, it has a double main hex instead, which scales up.

8l/min is OK though.

So, monkey64, exactly which boiler is it? Does it say Greenstar, or "II" ?

monkey64 · 7 Nov 2010

This is a Greenstar 25Si boiler.

In response to boilerdoktor:

1. There is no pre-heated water as I have Hot Water off and I'm running in Eco Mode.
2. When it goes lukewarm, GCH is not on or has not been run.
3. The boiler does fire on DHW mode (green light on)
4. During DHW mode, the green light stays on throughout. No flashing.
5. GCH flow does get quite warm to the touch when DHW has been run for a few mins. It's only warm for the first 8 inches of pipework. Would this indicate a faulty diverter valve?

BTW, did I mention that the only way to have a hot bath is to open the taps up half way. Open it further and it gets lukewarm.

Any ideas?

monkey64 · 9 Nov 2010

Okay, well I found the answer. My fault...

When I was referring to the flow rate in my earlier post, I took this from my installation notes which showed 5 litres / minute.

I've just tested and found a staggering 16 litres / minute which is way past the maximum 9 litres / minute limit shown in the manual. A simple turn of the the stop cock was all that was needed.

Exactly why the water pressure has risen so much is a mystery.
Thanks to everyone for their help.

sparkykink · 12 Dec 2010

Hello there,
I have a 25si which gives a great output for the heating, but the DHW never gets so hot that you have to take your hand from under the tap, even after it's been running for 2 minutes or so (and that is at the kitchen sink which is roughly 4 feet away from the boiler).
So it seems my problem doesn't seem to be with the flow rate, just whether or not the boiler is reaching it's pre set temperature.

Regarding the last comment /post, have I missed something here or are you saying that the problem was caused by the incoming water pressure being too high, and solved by turning the stop cock to restrict the mains flow?

According to the worcester bosch "find a bolier" app on it's website, this boiler is recommended for a 3 bed bungalow with 1 bathroom, which I have so I am looking for possible causes before I call them out on the warranty.

Cheers,
Dave

monkey64 · 13 Dec 2010

So it seems my problem doesn't seem to be with the flow rate.

As I found out, you need to test and record the flow rate from the taps. The 9 litres / minute is not a great deal. Exactly what is it?

I found that I was forcing too much water through the boiler. After I adjusted it, I got hot water again.

sparkykink · 13 Dec 2010

Hi,
Thanks for the reply, I will measure the rate tonight when I get home.
Hopefully mine will be the same problem, which can be sorted as yours was.

I would have thought that there would be some sort of restrictor in the boiler which would limit the incoming mains flow rate in the event of it being too excessive, rather than having to manually turn the stop-cock down, especially on what is regarded to be a boiler from a company market leader.

Hopefully this will sort the problem, if not I'll take it up with Worcester Bosch because if the boiler can't deliver hot water to an adjacent tap, in my opinion it isn't fit for purpose.
As I said in my last post, my interpretation of this being the correct boiler for the job was based on their website advice.

My main concern is that at present I have an electric shower, but plan to have a new bathroom installed with a shower fed from the boiler so
I need to be confident that the shower will get hot enough before I go ahead.

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No hot water from Worcester 25Si COMBI

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