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Exactly your boiler should be at 30kw when doing 12lpm
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Discuss New Boiler to replace a old Worcester 35CDi II in the Boilers area at Plumbers Forums
If you had a very (very) large bath tap that gave a unrestricted flowrate of say 30LPM then a 30kw boiler will only give a temperature of 22C (21.8), not much use to anyone, if you want to heat it to 60C then you must restrict the flow rate to 8.2LPM and then add cold water (at 7.5C) to give the required bath temperature. The boiler output required will still be 30kw.
You could be caught out this way with a stored combi where you get lovely roasting hot water for a few minutes and then lukewarm thereafter except you throttle in the hot tap.
What can lead to a wrong diagnosis is say in the case above where you have a flowrate of 4.68LPM at a temperature of 39.4C despite having the DHW temp set to 60C. If this tap is a hot water tap only (no chance of mixing) then you can be 100% sure that it is a boiler problem.
But if the tap is a mixer and even though set to give full hot water flow, due to a problem with the mixer, you could end up with a flow of 2.84LPM at 60C and a flow of 1.84LPM at 7.5C to give that mixed flow of 4.68LPM at 39.4C. and the boiler will be running perfectly correctly at 60C with a output of 10.42kw. If you had a suspicion that this may be the case then just measure the hot water outlet temperature at the boiler.
You could be caught out this way with a stored combi where you get lovely roasting hot water for a few minutes and then lukewarm thereafter except you throttle in the hot tap.
What can lead to a wrong diagnosis is say in the case above where you have a flowrate of 4.68LPM at a temperature of 39.4C despite having the DHW temp set to 60C. If this tap is a hot water tap only (no chance of mixing) then you can be 100% sure that it is a boiler problem.
But if the tap is a mixer and even though set to give full hot water flow, due to a problem with the mixer, you could end up with a flow of 2.84LPM at 60C and a flow of 1.84LPM at 7.5C to give that mixed flow of 4.68LPM at 39.4C. and the boiler will be running perfectly correctly at 60C with a output of 10.42kw. If you had a suspicion that this may be the case then just measure the hot water outlet temperature at the boiler.
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Fine for a shower but a bit cool for a bath as it will cool down from 43C fairly rapidly, throttling the hot tap to give say 9LPM should give a temp of 55C, add cold to give required bath temperature but I suppose that's what most people do anyway.
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Yes but the bath taps are only going to 23kw ish you would expect 30kw as these are full flow / non limiting normally
Yes, I think the bath tap flow was measured at ~ 15.5LPM so will definitely look for full boiler output as the flow temp at 30kw will only be 35C.
It seems a strange fault as it allows approx 77% (23kw) output at higher flowrates but only approx 61% at the lower flow rates, it only gives 10.42kw at a flowrate of 4.68LPM & 39.4C,should be 17.14kw & 60C.
It seems a strange fault as it allows approx 77% (23kw) output at higher flowrates but only approx 61% at the lower flow rates, it only gives 10.42kw at a flowrate of 4.68LPM & 39.4C,should be 17.14kw & 60C.
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Could be modulated but until they fix full flow it could be effecting everything else and no point with speculation atm
Until the experts arrive,
You could just reduce the DHW temperature setting by a division or two, to approx 50C, and see if it makes any difference to your very low flow taps, there's one there at 2.74LPM. If it results in the boiler firing at 30kw on the bath tap then you will still only achieve ~ 35C, you got, I think, 31.2C, but you were getting reasonable but not max boiler output on the higher flow rates.
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So I have finally got onto Baxi and they are due to come out on Thursday, not ideal but hey ho. I have been on holiday all over xmas/new year so was unable to do anything. I had a shower last night and whilst it wasn't hot, it was enough to wash but definitely wasn't the 38C which the thermostatic tap was set at. I tried turning it to 50C to see if anything increased, it didn't in terms of hot, but went cold a few times, then back warm again. In the meantime, I emailed Baxi to ask the question re hot water and I got this response from their "technical team".
Good morning
Thank you for your email
The 830 has a 5KW less output to the Worecester 35CDii so that is where the issue lies.
The Baxi 836 Combi would have been the ideal replacement for it.
Good morning
Thank you for your email
The 830 has a 5KW less output to the Worecester 35CDii so that is where the issue lies.
The Baxi 836 Combi would have been the ideal replacement for it.
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It’s the fact your kitchen / basin taps and shower which are normally regulated don’t get hot could understand if it was just a bath as these are full flow
Their reply wouldn't exactly fill one with confidence but at least they are following it up with a visit.
Make sure you emphasize that the boiler, isn't giving the required temperatures, more obvious at the lower flowrates.
If you assume cold mains now at 6C then any 30kw boiler should give a flow rate of 8LPM @ 60C, 8.5LPM @55C, 9.5LPM @ 50C, 11LPM @ 45C & 12.5LPM @ 40C.
If you want to, ensure DHW temp set to 60C and measure the mains temperature, the flow temperature and the flow rate (once again) from one of the Hot (only) taps that gives around 5 or 6 LPM, you can then show him/her up to date info which may convince them that you will be quite happy with a 30kw boiler once its fixed.
I will do my best. I have no issues with the boiler, it heats up the rads nice and toasty and is very efficient and quiet. They have sent this back this afternoon.
Attachments
That technical information applies to any Combi boiler, for example any 30kw boiler will give a flowrate of 30*860/60/30, 14.3LPM, with a temp rise of 30C and 30*860/60/35, 12.2LPM, with a temp rise of 35C. This is not in dispute but you are not getting the correct temperature rise at any flowrate as explained above several times, the challenge is to make them believe this. I can only suggest again to re take one or two of the tests that you carried out so well a few weeks ago. The one I suggested above at the lower rate of ~ 6LPM or whatever that hot tap flowed, it doesn't matter as long as you take your measurements accurately, again.
You might also consider taking the hot bath tap flow again with mains and hot water temps in both cases and you can then present these readings and calcs to them, they can't have any excuses for not fixing your boiler then.
You might also consider taking the hot bath tap flow again with mains and hot water temps in both cases and you can then present these readings and calcs to them, they can't have any excuses for not fixing your boiler then.
This should be enough to demonstrate the boilers problems mainly at lower flow rates as, at the higher flow rates, your first two tests, the calculated boiler output is quite close to the rated boiler output of 30kw, but the last three clearly show a problem.
Mains Temp: 7.5C. DHW temp setpoint 60C, Boiler Rated Output 30kw.
3.74 secs @ 31.2c 16.04LPM 26.52kw Achievable Temp 34C Req output 30kw Actual output 26.5kw
3.86 secs @ 33.7c 15.54LPM 28.4kw Achievable Temp 35C Req output 30kw Actual output 28.4kw
6.30 secs @ 36c 9.52LPM 18.92kw Achievable Temp 53C Req output 30kw Actual output 18.9kw
10.16 secs @ 39.8c 5.91LPM 13.31kw Achievable Temp 60C Req output 21.6kw Actual output 13.3kw
16.26 secs @ 41c 3.69LPM 8.62kw Achievable Temp 60C Req output 13.5kw Actual output 8.6kw
Mains Temp: 7.5C. DHW temp setpoint 60C, Boiler Rated Output 30kw.
3.74 secs @ 31.2c 16.04LPM 26.52kw Achievable Temp 34C Req output 30kw Actual output 26.5kw
3.86 secs @ 33.7c 15.54LPM 28.4kw Achievable Temp 35C Req output 30kw Actual output 28.4kw
6.30 secs @ 36c 9.52LPM 18.92kw Achievable Temp 53C Req output 30kw Actual output 18.9kw
10.16 secs @ 39.8c 5.91LPM 13.31kw Achievable Temp 60C Req output 21.6kw Actual output 13.3kw
16.26 secs @ 41c 3.69LPM 8.62kw Achievable Temp 60C Req output 13.5kw Actual output 8.6kw
Hi John,
Thanks for this and advising me what to say. I am going to carry out the above tests again this afternoon. Just to confirm, this is seeing how long each tap takes to fill a 1 litre jug and then what temp the filled jug is at?
Am I correct in saying the measuring of gas readings are 2 minute tap runs?
Correct re jug, just rinse out a few times to ensure hot, also please take the mains cold water temp.
No need for gas meter readings.
Also ensure DHW temp setpoint set to 60C.
Set to 60c on boiler.
Cold water was 12c out of the tap but bare in mind inside the house is 21c with heating having been on.
Kitchen tap
10.81- 47c
10.66 - 43.5c
10.55 - 42.9c
10.35 - 42.3c
Downstairs bathroom tap
6.23 - 37.7c
6.69 - 36.3c
Bathroom upstairs tap
5.96 - 35c
5:40 - 34.9c
Bath tap
4.12 - 31.7c
4.27 - 34.6c
19.5 - 41.2c
25.6 - 41.4c
Ensuite sink tap
6.67 - 37.8c
6.37 - 39.3c
All filled to approx 1 litre and each tap was left running for 30 seconds or so before starting.
Just to point out, I did the ensuite sink tap last and when I initially turned the hot tap on I put my hand under it as soon as water commenced and it was scolding for 3 seconds then cooled down. That's the first time since having the new boiler in that I've had to move my hand from a hot tap. Very odd!!!
Very similar to your previous readings, don't know what these two readings mean from the Bath Tap but not needed, the other two are fine.
19.5 - 41.2c
25.6 - 41.4c
I would also venture that the mains temp after a good run is ~ 7C so the actual output of the boiler with the bath flow rate of 14.6 LPM is more like 28kw but that doesn't in any way explain the problems at lower flow rates which will have to be addressed.
19.5 - 41.2c
25.6 - 41.4c
I would also venture that the mains temp after a good run is ~ 7C so the actual output of the boiler with the bath flow rate of 14.6 LPM is more like 28kw but that doesn't in any way explain the problems at lower flow rates which will have to be addressed.
| Mains Temp: 12.0C. DHW temp setpoint 60C, Boiler Rated Output 30kw. | ||||||
| | |||||
| Measured | Achievable | Required | Actual | | |
Kitchen Tap | Temp | Temp | Output | Output | | |
LPM | DegC | DegC | KW | KW | | |
5.6 | 47 | 60 | 18.6 | 13.6 | | |
5.6 | 43.5 | 60 | 18.8 | 12.4 | | |
5.7 | 42.9 | 60 | 19.0 | 12.3 | | |
5.8 | 42.3 | 60 | 19.4 | 12.3 | | |
| | |||||
Downstairs | | |||||
Bathroom | Measured | Achievable | Required | Actual | | |
Tap | Temp | Temp | Output | Output | | |
LPM | DegC | DegC | KW | KW | | |
9.6 | 37.7 | 56.6 | 30.0 | 17.3 | | |
9.0 | 36.3 | 59.9 | 30.0 | 15.2 | | |
Upstairs | | |||||
Bathroom | Measured | Achievable | Required | Actual | | |
Tap | Temp | Temp | Output | Output | | |
LPM | DegC | DegC | KW | KW | | |
10.1 | 35.0 | 54.7 | 30.0 | 16.2 | ||
11.1 | 34.9 | 50.7 | 30.0 | 17.8 | ||
| ||||||
Ensuite | ||||||
Sink | Measured | Achievable | Required | Actual | ||
Tap | Temp | Temp | Output | Output | ||
LPM | DegC | DegC | KW | KW | ||
9.0 | 37.8 | 59.8 | 30.0 | 16.2 | ||
9.4 | 39.3 | 57.7 | 30.0 | 17.9 | ||
Bathroom | Measured | Achievable | Required | Actual | ||
Tap | Temp | Temp | Output | Output | ||
LPM | DegC | DegC | KW | KW | ||
14.6 | 35.0 | 41.5 | 30.0 | 23.4 | ||
14.1 | 34.9 | 42.6 | 30.0 | 22.4 |
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Wow.....thanks for this John. It certainly makes it easier to understand what's going on now and cI an certainly put my point across to the engineer when he comes and even show him to experiment's I've conducted.
Atleast we can see there is a clearly issue with the actual output not matching the required output which the boiler is capable of.
A bit easier to see the problem all right, I have just cleaned it up a bit more here.
DHW | Boiler | ||||
Mains | Setpoint | Rated | |||
Temp | Temp | Output | |||
DegC | DegC | KW | |||
12 | 60 | 30 | |||
| Measured | Achievable | Required | Actual | |
Kitchen | Temp | Temp | Output | Output | |
LPM | DegC | DegC | KW | KW | |
5.6 | 47 | 60 | 18.6 | 13.6 | |
5.6 | 43.5 | 60 | 18.8 | 12.4 | |
5.7 | 42.9 | 60 | 19.0 | 12.3 | |
5.8 | 42.3 | 60 | 19.4 | 12.3 | |
5.7 | 43.9 | 60.0 | 19.0 | 12.6 | Average |
Downstairs | |||||
Bathroom | Measured | Achievable | Required | Actual | |
Tap | Temp | Temp | Output | Output | |
LPM | DegC | DegC | KW | KW | |
9.6 | 37.7 | 56.6 | 30.0 | 17.3 | |
9.0 | 36.3 | 59.9 | 30.0 | 15.2 | |
9.3 | 37.0 | 58.3 | 30.0 | 16.2 | Average |
Upstairs | |||||
Bathroom | Measured | Achievable | Required | Actual | |
Tap | Temp | Temp | Output | Output | |
LPM | DegC | DegC | KW | KW | |
10.1 | 35.0 | 54.7 | 30.0 | 16.2 | |
11.1 | 34.9 | 50.7 | 30.0 | 17.8 | |
10.6 | 35.0 | 52.7 | 30.0 | 17.0 | Average |
Ensuite | |||||
Sink | Measured | Achievable | Required | Actual | |
Tap | Temp | Temp | Output | Output | |
LPM | DegC | DegC | KW | KW | |
9.0 | 37.8 | 59.8 | 30.0 | 16.2 | |
9.4 | 39.3 | 57.7 | 30.0 | 17.9 | |
9.2 | 38.6 | 58.7 | 30.0 | 17.1 | Average |
Bathroom | Measured | Achievable | Required | Actual | |
Tap | Temp | Temp | Output | Output | |
LPM | DegC | DegC | KW | KW | |
14.6 | 35.0 | 41.5 | 30.0 | 23.4 | |
14.1 | 34.9 | 42.6 | 30.0 | 22.4 | |
14.3 | 35.0 | 42.1 | 30.0 | 22.9 | Average |
Perfect. You certainly can't get away from these facts and if he wants to conduct the same tests then he'll get the same results.
The only thing I noticed was the bathroom measured temps were the same which was a typo I assume as they were slightly different in temps.
These,
Bathroom upstairs tap
5.96 - 35c
5:40 - 34.9c
Bath tap
4.12 - 31.7c
4.27 - 34.6c
Think you just forgot to change the temps as the bath tap was the same temperature as bathroom upstairs in the table you created but different LPM. Honestly not an issue as you've been more then helpful to spend time doing such things.
Oh Yes, quite right, corrected now and important to get it as your tests show, actually makes a better case as the average boiler output is lower again, at 21.1kw.
As a matter of interest, you seem to have shut the bath tap in a bit? to get the temperature up for the last two tests but the (flow) times make no sense, do you have them?.
Bath tap
4.12 - 31.7c
4.27 - 34.6c
19.5 - 41.2c
25.6 - 41.4c
As a matter of interest, you seem to have shut the bath tap in a bit? to get the temperature up for the last two tests but the (flow) times make no sense, do you have them?.
Bath tap
4.12 - 31.7c
4.27 - 34.6c
19.5 - 41.2c
25.6 - 41.4c
DHW | Boiler | |||||
Mains | Setpoint | Rated | | |||
Temp | Temp | Output | | |||
DegC | DegC | KW | | |||
12 | 60 | 30 | | |||
| Measured | Achievable | Required | Actual | | |
Kitchen | Temp | Temp | Output | Output | | |
LPM | DegC | DegC | KW | KW | | |
5.6 | 47 | 60 | 18.6 | 13.6 | | |
5.6 | 43.5 | 60 | 18.8 | 12.4 | | |
5.7 | 42.9 | 60 | 19.0 | 12.3 | | |
5.8 | 42.3 | 60 | 19.4 | 12.3 | | |
5.7 | 43.9 | 60.0 | 19.0 | 12.6 | Average | |
Downstairs | | |||||
Bathroom | Measured | Achievable | Required | Actual | | |
Tap | Temp | Temp | Output | Output | | |
LPM | DegC | DegC | KW | KW | | |
9.6 | 37.7 | 56.6 | 30.0 | 17.3 | | |
9.0 | 36.3 | 59.9 | 30.0 | 15.2 | | |
9.3 | 37.0 | 58.3 | 30.0 | 16.2 | Average | |
Upstairs | | |||||
Bathroom | Measured | Achievable | Required | Actual | | |
Tap | Temp | Temp | Output | Output | | |
LPM | DegC | DegC | KW | KW | | |
10.1 | 35.0 | 54.7 | 30.0 | 16.2 | ||
11.1 | 34.9 | 50.7 | 30.0 | 17.8 | ||
10.6 | 35.0 | 52.7 | 30.0 | 17.0 | Average | |
Ensuite | ||||||
Sink | Measured | Achievable | Required | Actual | ||
Tap | Temp | Temp | Output | Output | ||
LPM | DegC | DegC | KW | KW | ||
9.0 | 37.8 | 59.8 | 30.0 | 16.2 | ||
9.4 | 39.3 | 57.7 | 30.0 | 17.9 | ||
9.2 | 38.6 | 58.7 | 30.0 | 17.1 | Average | |
Bath | Measured | Achievable | Required | Actual | ||
Tap | Temp | Temp | Output | Output | ||
LPM | DegC | DegC | KW | KW | ||
14.6 | 31.7 | 41.5 | 30.0 | 20.0 | ||
14.1 | 34.6 | 42.6 | 30.0 | 22.2 | ||
14.3 | 33.2 | 42.1 | 30.0 | 21.1 | Average |
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