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Discuss 40kw Navien NCB Combi install with v.poor DHW? Is this adequate? in the Gas Engineers Forum area at PlumbersForums.net

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31
Tbh the installer should be doing this else there was no point in fitting a 40kw when you could of used a 28kw etc
That's exactly my issue. He told me 40kw was ideal. Had I have know I'd get 9 or 10lpm I would never have gone this route or saved a few hundred quid and got a smaller boiler. Just to be certain, when/if he returns what should I be suggesting he do exactly (without telling him how do his job!?)
 
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1,448
If you open multiple hot water taps can you measure this and compare it with the boiler display?, a 22kw boiler at 9 LPM will give you a deltaT of 35C, they don't make combis much smaller than this.
 
Messages
31
If you open multiple hot water taps can you measure this and compare it with the boiler display?, a 22kw boiler at 9 LPM will give you a deltaT of 35C, they don't make combis much smaller than this.
Just opened two hot taps - one in a sink next to the boiler and one in a sink upstairs in the kitchen (boiler in basement). kitchen was 5.4lpm and basement tap 7.2lpm - the display on the boiler was just passing 12lpm which was progress in some ways. the water temp is 47 degrees so a 37 degree rise if the 10 degree water temp the fitter recorded on his commissioning sheet is correct.
 
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1,448
Is the DHW temperature set to 47/50C ?

If you split the difference then 12.3 LPm at a deltaT of 37c = boiler output of 32kw which is equivalent to 40 kw at 16.3 LPM and a 35C deltaT so I think nothing wrong with the boiler thermally except its range rated to below 40kw.. the problem seems to be with the cold water supply pressure or restriction somewhere, I suppose there could be a restrictor fitted in the boiler set to say 16 LPM and it would take a mains supply pressure drop of almost 45% to drop the flow to 12 LPM.

.
 
Messages
31
Is the DHW temperature set to 47/50C ?

If you split the difference then 12.3 LPm at a deltaT of 37c = boiler output of 32kw which is equivalent to 40 kw at 16.3 LPM and a 35C deltaT so I think nothing wrong with the boiler thermally except its range rated to below 40kw.. the problem seems to be with the cold water supply pressure or restriction somewhere, I suppose there could be a restrictor fitted in the boiler set to say 16 LPM and it would take a mains supply pressure drop of almost 45% to drop the flow to 12 LPM.

.
Thanks for that. In terms of the cold water supply pressure and potential restriction, the run from the incoming mains tees off two ways - one in 22mm that runs direct to the boiler . Probably covers 40 feet or so. The other again 22mm runs off to the cold outlets (but turns into 15mm after 15 feet of so....this supplies the outside tap that gets 22lpm, 3bar. So are you saying the restriction of some form could potentially be in the 22mm that feeds off the mains to supply the boiler? Thanks
 
Messages
31
Thanks for that. In terms of the cold water supply pressure and potential restriction, the run from the incoming mains tees off two ways - one in 22mm that runs direct to the boiler . Probably covers 40 feet or so. The other again 22mm runs off to the cold outlets (but turns into 15mm after 15 feet of so....this supplies the outside tap that gets 22lpm, 3bar. So are you saying the restriction of some form could potentially be in the 22mm that feeds off the mains to supply the boiler? Thanks
I've also noticed the boiler has a read out for central heating pressure. This might be irrelevant but it reads at 1.25 bar.?
 
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1,448
No, the boiler pressure has nothing to do with the problem.
The pressure drop through 40 ft of 22mm piping even at a flow rate of 20 LPM is only ~ 0.15 bar, as someone pointed out the problem may be on the hot water outlet to the users, you or somebody will have to take pressure readings before/after the boiler to narrow down the problem.
 
Messages
31
No, the boiler pressure has nothing to do with the problem.
The pressure drop through 40 ft of 22mm piping even at a flow rate of 20 LPM is only ~ 0.15 bar, as someone pointed out the problem may be on the hot water outlet to the users, you or somebody will have to take pressure readings before/after the boiler to narrow down the problem.
I'll try and do this. Think there is a point where the mains enters the property where I could attach a gauge and take a reading. Not sure about after the boiler though in terms of attaching to taps etc but will look into it. Thanks
 

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