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Discuss New Boiler to replace a old Worcester 35CDi II in the Central Heating Forum area at PlumbersForums.net

j3oeff

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48
Hi All,

Last night i discovered my Worcester 35CDi II was leaking, I had a new expansion vessel fitted in may as the old one had failed along with a new PRV.


I think it is probably about time to replace the boiler given it was installed when the house was built in 2002 so is approaching 20 years old.

Can anyone please advise on something to replace it with as it is a minefield. I've seen the below on websites such as BOXT and Warmzilla but happy to look at others as I have no idea.

The below all are coming in around £1950 - £2250 Installed.

Ideal Logic Max C24
Vaillant EcoTec Plus 825
Worcester Bosch Greenstar 4000

thank you!
 

ShaunCorbs

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Go with a local heating engineer as there fitters get paid peanuts so just slam it in
 

j3oeff

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The guy i usually use for repairs on my current boiler quoted £2,000 for a baxi (7 year warranty).
 

ShaunCorbs

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Or 2200 for a baxi 830 ? As that one has a 10 year if you can fit the supplier filter
 

j3oeff

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48
I assume Baxi is as good as the others? I don't mind which brand I get. Just want one which is reliable and won't cause me hassle :)
 

ShaunCorbs

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From who your local heating engineer?
 

ShaunCorbs

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Good price just make sure it’s not a hot flush, it needs to include chemicals etc
 

ShaunCorbs

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X400 or x800, mc3 or mc5 but your engineer may have his one he uses
 

j3oeff

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Got the boiler installed yesterday. Took him from 8.45 till 6pm. All up and running however this evening I've discovered the hot water coming from the taps upstairs is not very hot. You can keep your hand under it for as long as you want. On the old broken 20 year old boiler that wasn't possible as it was too hot. I've checked all taps upstairs and they are all the same. The downstairs bathroom tap is also the same.

The HW on the boiler is turned to max. Any idea? I
 

ShaunCorbs

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Close the tap halfway/ reduce the flow
 

John.g

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Happy Christmas to all.

Your old boiler at 35kw gives 17% more flow than the new one at 30kw.
Your best bet to avoid any later arguments is to note the time in secs required to fill a 1 litre jug/bottle and convert to LPM.

Mains temp here just now is 10C.
HW temp 60C New Blr 8.6 LPM Old Blr 10.0 LPM
HW temp 50C New Blr 10.8 LPM Old Blr 12.5LPM
HW temp 40C New Blr 14.3 LPM Old Blr 16.7 LPM
 

j3oeff

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48
Close the tap halfway/ reduce the flow
Then it's just tricking out though? Still not 100
Happy Christmas to all.

Your old boiler at 35kw gives 17% more flow than the new one at 30kw.
Your best bet to avoid any later arguments is to note the time in secs required to fill a 1 litre jug/bottle and convert to LPM.

Mains temp here just now is 10C.
HW temp 60C New Blr 8.6 LPM Old Blr 10.0 LPM
HW temp 50C New Blr 10.8 LPM Old Blr 12.5LPM
HW temp 40C New Blr 14.3 LPM Old Blr 16.7 LPM

So is it now a case of running the taps slower to get the water at its hottest? Quite disappointing if so but too late now I guess. I've tried again this morning when getting a wash and the bathroom sink was cold for sometime then warmed up.

The wife is going to give me ear ache if I tell her she has to half open taps and judge it herself first. Could I not reduce the flow before it comes out of the taps or is that not possible?
 

John.g

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2,497
I would still do a actual test , you don, really know if iit has been installed properly, gas rate adjusted correctly etc, I see the Baxi is set to 20kw on CH, who knows maybe some confusion in setting up, I wouldn't, blame the wife if you discovered 5 years down the line that you only had a 20kw combi all along.
 

ShaunCorbs

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Is the hot water dial on the boiler fully hot eg max ?

Also Jon you can’t adjust baxi eg range rate
 

John.g

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You may be getting a greater flow (and lower temperature) through the HW taps for a few reasons, the heat exchanger might have been a bit fouled on the old boiler restricting the flow rate so the tap(s) will have to be throttled a bit with the new boiler to give the same flow even allowing for its lesser output, or there may have been a flow restrictor installed in the old boiler.
 

John.g

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Apart from doing a bucket test assume big label on the front giving the particular Model?. and a bit surprising that a 836 didn't replace the 35CDI.
 

j3oeff

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Apart from doing a bucket test assume big label on the front giving the particular Model?. and a bit surprising that a 836 didn't replace the 35CDI.
I know nothing about boilers. I just went off a boxt quote and found one based on similar to what they were recommending. I do remember having a plumber round though in the past to fix the 35cdi and I remember him saying it was overkill for our house.

Did you get the 825, 830 or 836?
830 buddy.
 

John.g

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Your plumber was probably thinking of the CH requirement but a combi must satisfy the DHW demand as well which, to obtain a decent flowrate, requires a much greater output. That said, 30kw output on DHW is still fairly decent IMO. A showering flowrate of 14LPM at 40C is still pretty good. Even in the debths of winter, a mains temp of say 6C will still result in a flowrate of 12.6LPM at 40C.
Going back to your original post re apparently poor performance from upstairs tap, if you assume 55C as being too hot to hold your hand under with old boiler then the flowrate was 11.1LPM, the new boiler, assuming it's giving its rated output of 30kw means the HW temp will now be 48.7C, not exactly roasting but still very hot IMO and don't think would be that pleasant to hold your hand under it all day.
Even if you havn't a household thermometer to actually measure the temperature I would still measure the time taken to fill a 1 litre or 1 pint container or whatever to hand to remove any doubts whatsoever you may have re this boiler's performance.
 

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Is it all taps or just maybe eg bath taps if you have a bath ?
 

j3oeff

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So I've just done some jug filling tests with times and temps. Not one room upstairs gets above 42c

Kitchen tap (closest to boiler)
12.09 secs @ 42.4c
18.10 secs @ 54.4c

Bathroom ensuite
12.82 secs @ 39.4c
20.31 secs @ 43.1c

Main bathroom sink
12.51 secs @ 35.1c
21.83 secs @ 41.1c

Bath
4.13 secs @ 31.8c
11.43 secs @ 39.1c
13.07 secs @ 39.9c
13.47 secs @ 41.2c
17.68 secs @ 41.7c
18.94 secs @ 41.6c
21.62 secs @ 41.2c
 

ShaunCorbs

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And that’s with 10 lpm coming out the tap ?
 

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Are you sure as your bathroom sink is around 3lpm which is bad
 

John.g

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So I've just done some jug filling tests with times and temps. Not one room upstairs gets above 42c

Kitchen tap (closest to boiler)
12.09 secs @ 42.4c
18.10 secs @ 54.4c

Bathroom ensuite
12.82 secs @ 39.4c
20.31 secs @ 43.1c

Main bathroom sink
12.51 secs @ 35.1c
21.83 secs @ 41.1c

Bath
4.13 secs @ 31.8c
11.43 secs @ 39.1c
13.07 secs @ 39.9c
13.47 secs @ 41.2c
17.68 secs @ 41.7c
18.94 secs @ 41.6c
21.62 secs @ 41.2c

Flow Rates & Boiler Outputs based on data above: assuming cold mains at 9C

12.09 secs @ 42.4c 4.96LPM, 11.6kw
18.10 secs @ 54.4c 3.31LPM, 10.48kw

Bathroom ensuite
12.82 secs @ 39.4c 4.68LPM, 9.93kw
20.31 secs @ 43.1c

Main bathroom sink
12.51 secs @ 35.1c 4.79LPM, 8.72kw
21.83 secs @ 41.1c 2.74LPM, 6.14kw

Bath
4.13 secs @ 31.8c 14.52LPM, 23kw
11.43 secs @ 39.1c 5.24LPM, 11kw
13.07 secs @ 39.9c 4.59LPM, 9.9kw
13.47 secs @ 41.2c 4.45LPM, 10kw
17.68 secs @ 41.7c 3.39LPM,7.73kw
18.94 secs @ 41.6c 4.68LPM, 9.93kw
21.62 secs @ 41.2c 3.17LPM. 7.12kw

Reply
Looks like the flows are too low or there is a flow switch problem as the only output that looks like a constantly firing boiler is 4.13 secs @ 31.8c 14.52LPM, 23kw
I think the minimum output of this boiler is 8kw?? so requires a minimum flowrate of 2.25LPM to avoid temperature exceeding 60C, flow switch possibly set to 3/3.5LPM???perhaps faulty??. There is certainly a boiler problem somewhere IMO.
 

j3oeff

Messages
48
Flow Rates & Boiler Outputs based on data above: assuming cold mains at 9C

12.09 secs @ 42.4c 4.96LPM, 11.6kw
18.10 secs @ 54.4c 3.31LPM, 10.48kw

Bathroom ensuite
12.82 secs @ 39.4c 4.68LPM, 9.93kw
20.31 secs @ 43.1c

Main bathroom sink
12.51 secs @ 35.1c 4.79LPM, 8.72kw
21.83 secs @ 41.1c 2.74LPM, 6.14kw

Bath
4.13 secs @ 31.8c 14.52LPM, 23kw
11.43 secs @ 39.1c 5.24LPM, 11kw
13.07 secs @ 39.9c 4.59LPM, 9.9kw
13.47 secs @ 41.2c 4.45LPM, 10kw
17.68 secs @ 41.7c 3.39LPM,7.73kw
18.94 secs @ 41.6c 4.68LPM, 9.93kw
21.62 secs @ 41.2c 3.17LPM. 7.12kw

Reply
Looks like the flows are too low or there is a flow switch problem as the only output that looks like a constantly firing boiler is 4.13 secs @ 31.8c 14.52LPM, 23kw
I think the minimum output of this boiler is 8kw?? so requires a minimum flowrate of 2.25LPM to avoid temperature exceeding 60C, flow switch possibly set to 3/3.5LPM???perhaps faulty??. There is certainly a boiler problem somewhere IMO.

Flow Rates & Boiler Outputs based on data above: assuming cold mains at 9C

12.09 secs @ 42.4c 4.96LPM, 11.6kw
18.10 secs @ 54.4c 3.31LPM, 10.48kw

Bathroom ensuite
12.82 secs @ 39.4c 4.68LPM, 9.93kw
20.31 secs @ 43.1c

Main bathroom sink
12.51 secs @ 35.1c 4.79LPM, 8.72kw
21.83 secs @ 41.1c 2.74LPM, 6.14kw

Bath
4.13 secs @ 31.8c 14.52LPM, 23kw
11.43 secs @ 39.1c 5.24LPM, 11kw
13.07 secs @ 39.9c 4.59LPM, 9.9kw
13.47 secs @ 41.2c 4.45LPM, 10kw
17.68 secs @ 41.7c 3.39LPM,7.73kw
18.94 secs @ 41.6c 4.68LPM, 9.93kw
21.62 secs @ 41.2c 3.17LPM. 7.12kw

Reply
Looks like the flows are too low or there is a flow switch problem as the only output that looks like a constantly firing boiler is 4.13 secs @ 31.8c 14.52LPM, 23kw
I think the minimum output of this boiler is 8kw?? so requires a minimum flowrate of 2.25LPM to avoid temperature exceeding 60C, flow switch possibly set to 3/3.5LPM???perhaps faulty??. There is certainly a boiler problem somewhere IMO.
Central Heating Max. Output21.2 kW
MountingWall Mounted
Flow Rate12.2 l/min

If I was to fill a bath at 14.52lpm does that mean the bath would be 31.8c ?

I think I'll have to get the installer back in the new year for further investigation.
 

ShaunCorbs

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Can you take a picture of the boiler please ?
 

ShaunCorbs

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There should be a slide out tab on the left or right underside of the front if you look under you will see it
 

John.g

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2,497

Central Heating Max. Output21.2 kW
MountingWall Mounted
Flow Rate12.2 l/m

If I was to fill a bath at 14.52lpm does that mean the bath would be 31.8c ?

I think I'll have to get the installer back in the new year for further investigation.
That's right but if the boiler was at its rated output of 30kw then the HW temperature would be almost 39C (38.6C). If you required a bath filling temp of say 45C then you would have to reduce the flowrate to just under12LPM (11.94) again assuming boiler at its rated 30kw output.

The 21.2kw output above is the output when on CH duty only.

If you like sometime you might take one more test of that bath flowrate and temperature and also the cold mains temperature to really tie down the actual boiler output very accurately.
 

ShaunCorbs

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That's right but if the boiler was at its rated output of 30kw then the HW temperature would be almost 39C (38.6C). If you required a bath filling temp of say 45C then you would have to reduce the flowrate to just under12LPM (11.94) again assuming boiler at its rated 30kw output.

The 21.2kw output above is the output when on CH duty only.

If you like sometime you might take one more test of that bath flowrate and temperature and also the cold mains temperature to really tie down the actual boiler output very accurately.

Or a 25kw which I think might be the issue maybe
 

ShaunCorbs

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Best call baxi out as your having low kw rates out of your bath taps
 

j3oeff

Messages
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That's right but if the boiler was at its rated output of 30kw then the HW temperature would be almost 39C (38.6C). If you required a bath filling temp of say 45C then you would have to reduce the flowrate to just under12LPM (11.94) again assuming boiler at its rated 30kw output.

The 21.2kw output above is the output when on CH duty only.

If you like sometime you might take one more test of that bath flowrate and temperature and also the cold mains temperature to really tie down the actual boiler output very accurately.

Various tape flows

Cold

2.11 secs @ 7.5c
2.17 secs @ 7.5c

Hot
3.74 secs @ 31.2c
3.86 secs @ 33.7c
6.30 secs @ 36c
10.16 secs @ 39.8c
16.26 secs @ 41c
 

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