New Boiler to replace a old Worcester 35CDi II

[Scott_D]

Did you get the 825, 830 or 836?

 

[John.g]

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]

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]

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.

 

[ShaunCorbs]

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

 

[j3oeff]

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]

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

 

[j3oeff]

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

Each time represents me filling a 1 litre jug if that's what you mean. Sorry, all new to me this

 

[ShaunCorbs]

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

 

[j3oeff]

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

Have a look at that video. The taps seem fairly quick to me but not at the required temp.

 

[John.g]

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]

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. Output	21.2 kW
Mounting	Wall Mounted
Flow Rate	12.2 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]

Can you take a picture of the boiler please ?

 

[j3oeff]

Can you take a picture of the boiler please ?

 

[ShaunCorbs]

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]

Central Heating Max. Output	21.2 kW
Mounting	Wall Mounted
Flow Rate	12.2 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]

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

 

[j3oeff]

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

 

[ShaunCorbs]

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

 

[j3oeff]

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