New Boiler to replace a old Worcester 35CDi II

[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

 

[j3oeff]

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

Baxi rather than installer?

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

 

[ShaunCorbs]

I would say so as I’m guessing your bath taps don’t have flow restrictiors in hence the correct flow rate and your only pulling on johns calculations 23kw

You can prove this by taking a gas meter reading then run the tap for 2 mins and take another reading and come back to us (aslong as it’s a metric meter eg not in ft/h

 

[John.g]

Measured Mains temp 7.5C

3.74 secs @ 31.2c 16.04LPM 26.52kw
3.86 secs @ 33.7c 15.54LPM 28.4kw
6.30 secs @ 36c 9.52LPM 18.92kw
10.16 secs @ 39.8c 5.91LPM 13.31kw
16.26 secs @ 41c 3.69LPM 8.62kw

Edit: Also corrected kw from post 31, mains are 7.5C, assumed originally at 9C.
4.13 secs @ 31.8c 14.52LPM, 23kw corrected to 7.5C mains = 24.62kw

 

[j3oeff]

I would say so as I’m guessing your bath taps don’t have flow restrictiors in hence the correct flow rate and your only pulling on johns calculations 23kw

You can prove this by taking a gas meter reading then run the tap for 2 mins and take another reading and come back to us (aslong as it’s a metric meter eg not in ft/h

Before and after

 

[ShaunCorbs]

Sorry they don’t update fast enough needs to be the gas meter

 

[Scott_D]

Could also be a mixer shower passing or diverter issue.
I would call the installer after the holidays.
You have some hot water and heating so not an emergency but needs looking at ASAP

 

[j3oeff]

Sorry they don’t update fast enough needs to be the gas meter

Gas meter doesn't show anything from the outside which I found odd. It was just blank which is why I resorted to smart meter

 

[ShaunCorbs]

Press any button ? Should light up

 

[j3oeff]

Could also be a mixer shower passing or diverter issue.
I would call the installer after the holidays.
You have some hot water and heating so not an emergency but needs looking at ASAP

Would a dodgy shower tap cause such issues? I noticed when my old boiler was in, the ensuite shower has one tap which turns left and right for 25 upto 50c with a little red button which needs to be pushed to go out 38c. This had been playing up and was red hot on any temp unless I turned it right down to 25c then it went cold. There was no in between.

I've just had a shower tonight and whilst it was on 38c it was nice but not overly hot but turning it up to 45/50c didn't really increase the temp much? Or is that nothing to do with my issues.

 

[j3oeff]

Press any button ? Should light up

Before 2 minutes hot tap run 380.264 and after 380.304

 

[ShaunCorbs]

And this was with the bath tap running just checking ?

 

[John.g]

If that's m3i in 2 mins then 0.04 in 2 mins = 1.2m3/hr approx 13kw or about 11kw at 85% efficiency. Bath tap 0n??

 

[ShaunCorbs]

11.59net (12.86gross)

 

[j3oeff]

And this was with the bath tap running just checking ?

No, kitchen one. Should it of been the bath tap? If so I'll do it in the morning now

 

[j3oeff]

If that's m3i in 2 mins then 0.04 in 2 mins = 1.2m3/hr approx 13kw or about 11kw at 85% efficiency. Bath tap 0n??

Kitchen tap on full.

 

[ShaunCorbs]

No, kitchen one. Should it of been the bath tap? If so I'll do it in the morning now

Yes please

 

[j3oeff]

Will in the morning. Thanks for helping with this chaps. Appreciate it and learning at the same time.

 

[John.g]

If these two (post31) then
12.09 secs @ 42.4c 4.96LPM, 12.07kw
18.10 secs @ 54.4c 3.31LPM, 10.83kw
Average 11.45kw

Very close to flow/temp calc??falls off with increasing temp

Might be something to do with combi temp control? as output decreases with increasing HW temp and boiler gets close to rated output at reducing temps.

 

[John.g]

Had a look at the Baxi830 manual, it states minimum flowrate 2 LPM but recommend 2.5 LPM and the minimum output is 6.1kw.
The lowest test flow reading was 2.74LPM (bathroom tap) which gave a measured temp of 41.1C from mains at 7.5C = 6.42kw.
With the DHW temp control to max, 60C, and if the measured HW temp was 60C, then 10.04kw required so no reason why the boiler shouldn't have achieved this.

 

[j3oeff]

Some more 2 minute hot tap runs with readings.

Bath start 383,730 finish 383,812
Main bathroom sink tap start 383,812 finish 383,894
Ensuite sink tap start 383,894 finish 383,973
Ensuite shower set at 38c start 383,973 finish 384,028

 

[j3oeff]

If these two (post31) then
12.09 secs @ 42.4c 4.96LPM, 12.07kw
18.10 secs @ 54.4c 3.31LPM, 10.83kw
Average 11.45kw

Very close to flow/temp calc??falls off with increasing temp

Might be something to do with combi temp control? as output decreases with increasing HW temp and boiler gets close to rated output at reducing temps.

They are both kitchen tap readings which is directly next to the boiler. That tap is certainly the hottest and the only one where you wouldn't be able to keep your hand under at the correct flow to achieve the hottest water.

 

[ShaunCorbs]

Bath 23.76 kw
Mbst 23.76 kw
Est 22..89 kw
Ess 15.94 kw

And your getting around 12 lpm out of the bathroom and sink taps if you are boiler fault as the top two should be 30kw

 

[j3oeff]

Bath 23.76 kw
Mbst 23.76 kw
Est 22..89 kw
Ess 15.94 kw

And your getting around 12 lpm out of the bathroom and sink taps if you are boiler fault as the top two should be 30kw

Am I right in reading that as.... There is a fault with the boiler if those readings I give are correct?

 

[ShaunCorbs]

If you have 12 lpm coming out of the tap then yes

 

[j3oeff]

If you have 12 lpm coming out of the tap then yes

All taps were on full when tests were done so it will be correct. Will a boiler reset achieve anything or am I best off calling baxi as you said previously. ta

 

[John.g]

Some more 2 minute hot tap runs with readings.

Bath start 383,730 finish 383,812
Main bathroom sink tap start 383,812 finish 383,894
Ensuite sink tap start 383,894 finish 383,973
Ensuite shower set at 38c start 383,973 finish 384,028

From post#31 (below) My calcs in bold.
Bathroom ensuite
12.82 secs @ 39.4c 4.48LPM 9.97kw
20.31 secs @ 43.1c 2.96LPM 7.35kw
Main bathroom sink
12.51 secs @ 35.1c 4.79LPM, 9.22kwkw
21.83 secs @ 41.1c 2.74LPM, 6.44kw
From post #62 (Shaun)
Bath 23.76 kw
Mbst 23.76 kw
Est 22..89 kw
Ess 15.94 kw

Which, if any, of the above (Shaun) taps correspond to the post #31 taps (Ignore power outputs)

Edit: Also, if the shower temp was actually 38C then a boiler output of 15.94kw gives flowrate of 7.5LPM

 

[j3oeff]

From post#31 (below) My calcs in bold.
Bathroom ensuite
12.82 secs @ 39.4c 4.48LPM 9.97kw
20.31 secs @ 43.1c 2.96LPM 7.35kw
Main bathroom sink
12.51 secs @ 35.1c 4.79LPM, 9.22kwkw
21.83 secs @ 41.1c 2.74LPM, 6.44kw
From post #62 (Shaun)
Bath 23.76 kw
Mbst 23.76 kw
Est 22..89 kw
Ess 15.94 kw

Which, if any, of the above (Shaun) taps correspond to the post #31 taps (Ignore power outputs)

Now....in simple terms what does this mean? I get the LPM but don't understand the kw readings and what it all means in comparison to my iasue. But I assume the LPM is irrelevant if we are not getting the required HW output?

 

[ShaunCorbs]

You should be getting 12lpm and 30kw

 

[j3oeff]

You should be getting 12lpm and 30kw

Crikey, so some way off then on all of the upstairs outlets.

 

[John.g]

One is a check on the other.

IF the measured flowrates and temperatures are correct then the calculated boiler output can a is very accurately calculated.

Now....in simple terms what does this mean? I get the LPM but don't understand the kw readings and what it all means in comparison to my iasue. But I assume the LPM is irrelevant if we are not getting the required HW output?

A bit long winded but....
From one of your readings above of 1 litre fill in 12.82 secs and 39.4C then this equals to a flowrate of 60/12.82, 4.68LPM and the water must be raised in temperature from 7.5C to 39.4C, a dT of 31.9C.
The well know calc for the energy required is kw= LPMX60XdT/860, in this case, 4.68X60X31.9/860, 10.42kw. this will be IMO almost100% correct, as a check you can take the gas meter readings, these readings in M3 can be converted to the boiler output required or visa versa in our case, I don't know the exact conversion factors but if you use 10.8kw/M3 of gas and a boiler efficiency of say 88% then you won't be far out .
So, gas consumption in M3/hr is 10.42/10.8/0.88, 1.096M3/hr or 0.0365M3/2mins. So if both of these calcs marry up then you are happy but either one will give be almost 100% correct.