40kw Navien NCB Combi install with v.poor DHW? Is this adequate?

[ecswtrav]

Hi all,
Just had a Navien NCB 40kw combi installed by an approved fitter. The DHW rates of the boiler seemed promising on paper - 16.4lpm at a 35 temp rise and 22.9lpm at a 25degree rise. More than adequate for what we need. The only issue now its installed is that I'm not getting any more than 9 or 10lpm using the boilers display. I knew it would be a compromise fitting a combi as I had an Ideal Istor previously (cylinder & system boiler all in one), but the reduction in DHW is quite noticeable.

I've a new 25mm mains - fitted years ago mind, the Istors 22mm pipe feed now goes to the Navien (although reduced to a 15mm connection). I have 25lpm at the outside tap off the kitchen wall and 3bar pressure (not sure if this is static or dynamic but it was a gauge screwed to the outside tap???). Gas pipe is 22mm probably 20 feet to boiler). There is a tee off at the stop tap where it enters the property leading off to the cold supplies for the house - the other leading to the Navien. The hot water supply pipes to a bathroom and ensuite and kitchen from the boiler are 22mm (again from the old Istor) which tee off in 15mm to the fittings, the cold water pipes seem to be in 15mm to the various outlets. I've a bathroom and an ensuite. Boiler is in the basement, the bathroom 2 floors up and ensuite in the loft conversion.

The fitter assures me he's seen much higher readings on 40kw Naviens. He just doesn't know why it only reads between 9-10lpm.

I just wondered whether the quoted figures related to the flow rate when hot water is mixed with cold? I guess then it probably would be a much higher flow rate which is why the showers seem ok. Apparently the incoming temperature is 9 degrees, so setting the boiler hot water temp to 45degrees should give approx 16lpm...but it's a mile off based on the display.

Just wondered if I was missing something as my fitter couldn't shine any light on it and said he would speak to Navien? I never expected it to match the Istor with multiple outlets on at the same time, but I did expect a slightly better flow rate with one outlet...particularly to my bath tub which we used to fill in no time, which now takes noticeably quite a bit longer to fill.

Is the reality that what I'm getting currently is actually normal for a 40kw combi? Thanks for any comments.

 

[Scott_D]

Any Pictures of the install?
Is there a filling loop at the boiler?

 

[ecswtrav]

Hi thanks for replying. Here is a pic of the underside of the boiler. Assume there is a filling loop in the photo although I'm no expert.

The boiler was commissioned with a gas pressure reading of 21mbar...I wondered whether it was undergassed so couldnt do its job? Between 17 and 25 mbar is recommended so I assume this ok. Although I don't remember being asked to turn on multiple outlets to get the boiler up to its max operation.

Puzzles me why it won't perform anywhere near its specs.

Thanks

 

[ShaunCorbs]

I would suspect low cold mains flow into the boiler if it’s only reading 9-10lpm

 

[Scott_D]

If you have 25lpm at an outside tap
You should have similar at other cold taps eg bath cold?
Hot is probably restricted through the boiler but as it’s a 40kw I would imagine a 15-16 LPM Restrictor.

I know nothing about nevian but it may have a filter that needs cleaning.
You can test at the filling loop pressure and flow to give an idea of what the cold supply just before the boiler is.

Any other valves been shut or touched?
I would ask the installer to return and have another look at it.

 

[ecswtrav]

I would suspect low cold mains flow into the boiler if it’s only reading 9-10lpm

I've got approx. 22lpm at the outside tap which makes me think surely the boiler must be receiving that? or does it need to be receiving more than that to achieve more than its 9-10lpm heated up?

 

[ShaunCorbs]

It might not be due to Pipework valves or other restrictions

or could be a restriction on the hot somewhere

 

[ecswtrav]

If you have 25lpm at an outside tap
You should have similar at other cold taps eg bath cold?
Hot is probably restricted through the boiler but as it’s a 40kw I would imagine a 15-16 LPM Restrictor.

I know nothing about nevian but it may have a filter that needs cleaning.
You can test at the filling loop pressure and flow to give an idea of what the cold supply just before the boiler is.

Any other valves been shut or touched?
I would ask the installer to return and have another look at it.

Is this something I could do or should I get the installer back? I've mentioned the issue and I've since seen he even recorded on his commissioning sheet 9LPM so he was well aware. His answer was that he'd phone Navien and ask. There are a few isolation valves next to the stopcock - one for the cold and one direct to the boiler but these are fully open and full bore as far as I know.

 

[ecswtrav]

It might not be due to Pipework valves or other restrictions

or could be a restriction on the hot somewhere

Do you mean a restriction within the boiler? I'm only assuming that as its the boiler that tells me what lpm its kicking out - the hot tap in kitchen for example seems to match this rate too.

 

[ShaunCorbs]

Don’t think so would need testing

best option would to put the hot into a bucket From the valve and see what you get coming out of the boiler if it’s 13-14lpm you know your problem is on the outlet / hot pipe work

but if your only getting 9lpm you know it’s probably cold pipework

 

[ecswtrav]

Don’t think so would need testing

best option would to put the hot into a bucket From the valve and see what you get coming out of the boiler if it’s 13-14lpm you know your problem is on the outlet / hot pipe work

but if your only getting 9lpm you know it’s probably cold pipework

Great - apologies though - which valve are you referring to as I'm not overly familiar with all of this? So supposedly if I release the hot water from this valve I should hope the lpm reading on the digital display would increase and show a true reflection of what the boiler is capable of? Looking at the commissioning sheet the temperature of the cold is 9degrees? Thanks

 

[ShaunCorbs]

Tbh the installer should be doing this else there was no point in fitting a 40kw when you could of used a 28kw etc

 

[ecswtrav]

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!?)

 

[John.g]

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.

 

[ecswtrav]

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.

 

[John.g]

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.

.

 

[ecswtrav]

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

 

[ecswtrav]

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.?

 

[Scott_D]

Any mention of a flow restrictor or filter in the boiler mnual?

 

[John.g]

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.

 

[ecswtrav]

Any mention of a flow restrictor or filter in the boiler mnual?

Don't think so although I'll have another good read tomorrow. I've kept an eye out for flow restrictors but not filters so will have a good check. Thanks

 

[John.g]

Yes, there is a restrictor and a filter........

Navien NCB-180 Installation & Operation Manual (Page 22 of 77) | ManualsLib

www.manualslib.com

 

[ecswtrav]

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

 

[ecswtrav]

Yes, there is a restrictor and a filter.******.

Navien NCB-180 Installation & Operation Manual (Page 22 of 77) | ManualsLib

www.manualslib.com

I'll check for that in my manual tomorrow and ask the fitter to reclean the filter assuming the flow restrictors can't be cha he'd or removed. Much appreciated

 

[John.g]

I would think, if installed, that they can certainly be removed.
It shows a differential pressure of ~ 2 bar is required for a flow rate of 20 LPM so would probably need 3 bar mains pressure to give 1 bar at the boiler inlet,
at 12 lpm the deltaP required is ~ 0.9 bar so the mains pressure would be ~ 2 bar at this flowrate to give 1 bar at the boiler inlet., these restrictors, if fitted, can have a very significant effect on the final flowrate IMO.

 

[ecswtrav]

I would think, if installed, that they can certainly be removed.
It shows a differential pressure of ~ 2 bar is required for a flow rate of 20 LPM so would probably need 3 bar mains pressure to give 1 bar at the boiler inlet,
at 12 lpm the deltaP required is ~ 0.9 bar so the mains pressure would be ~ 2 bar at this flowrate to give 1 bar at the boiler inlet., these restrictors, if fitted, can have a very significant effect on the final flowrate IMO.

Had a quick look at the manual I have here and there's no mention of flow restrictors just something about cleaning the return adaptor filter on page 51 which I assume isn't it? I've attached the manual it would be great if you had a sec to see if there's anything obvious that could have been missed. I'm going to try at some point today to measure the mains pressure just after the stopcock just so I can be sure what pressure is coming in. Thanks

 

[John.g]

I don't see any reference to cold water restrictors but on page 23 it states "After installing the boiler clean the cold water inlet filter", this of course may only refer to a typical installation but it is mentioned.

Re monitoring pressure, you have a 3 bar PG at the outside tap so when no flow through it but with flow through the combi should tell you something as it should reflect any pressure drop at the mains, just switch in the DHW and watch it, you really want another PG somewhere at the end of hot water supply to the house. Also even though it shouldn't have any effect on the flow through the boiler turn up the DHW temp control knob to max while doing the tests.

What was the kw output of your old Istor? did flow rate feel ok?

 

[ecswtrav]

I don't see any reference to cold water restrictors but on page 23 it states "After installing the boiler clean the cold water inlet filter", this of course may only refer to a typical installation but it is mentioned.

Re monitoring pressure, you have a 3 bar PG at the outside tap so when no flow through it but with flow through the combi should tell you something as it should reflect any pressure drop at the mains, just switch in the DHW and watch it, you really want another PG somewhere at the end of hot water supply to the house. Also even though it shouldn't have any effect on the flow through the boiler turn up the DHW temp control knob to max while doing the tests.

What was the kw output of your old Istor? did flow rate feel ok?

I will clarify the page 23 with the installer so thanks. I've just attached the PG to the point just after the stopcock....today around 15 mins ago it measured 2.2bar. Assuming it must fluctuate if I got 3 bar previously an outside tap. (The outside tap was the same 2.2bar btw) cranked up the boiler temp to maximum 65 degrees and opened a few outlets. The PG changed to 2bar lowest...it didn't move much at all. I'm not sure what this means I'll be honest, I just can't figure a way of measuring the pressure beyond the boiler although hopefully my fitter will have something for this. If I felt confident at removing the filling loop I'd try and measure the flow rate before it hits the boiler.
The Istor I believe was 24kw but with the pressurised cylinder we had much better flow from the taps....particulalry the hot bath tap...a massive difference...and the showers were more powerful.

 

[John.g]

It means basically that the mains pressure is fluctuating between 3 bar and 2.2 bar due to other users or whatever but your demand of 12 LPM or whatever is only dropping it a further 0.2bar (no problem from that point of view but 2 bar is IMO a pretty poor pressure for combi boiler flow rate performance). The outside tap (when not in use) will measure the mains pressure. The filling loop PG can only be utilized if the cold supply to it is from the same cold supply as to the combi, you must then be able to shut off the PRV outlet to the boiler, then screw down the PRV adjuster to reflect the mains pressure, a bit messy IMO, if you can use your own PG and attach it to the cold take off then open up your hot taps, you will then measure the pressure loss in the 40ft of cold pipe between the mains and the boiler inlet which I expect to be very low but worth taking to rule it in/out. I would then remove the gauge and re install it on the HW outlet, as close to the boiler as possible but this will have to be done with a Tee, then remove it and Tee in in to one one of HW users with the greatest flow and repeat the the test, your plumber might have to do these tests for you.

 

[gmartine]

I looked at the data tables and they specify 12.71 l/m @ 45 degrees for the "kitchen outlet" that happens to be what the OP is getting as a flow rate. Might not be relevant but thought it was odd.

 

[John.g]

That was from two taps..... "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" so kitchen was 5.4 LPM.

Would like to know the deltaP through its plate Hx or through any combi for that matter.
The old boiler, not sure if a combi but with a HW cylinder as well was giving much better flow rates through the showers and HW taps and it was the same cold water line supply so the only big change is the boiler itself?.

 

[ecswtrav]

That was from two taps.***. "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" so kitchen was 5.4 LPM.

Would like to know the deltaP through its plate Hx or through any combi for that matter.
The old boiler, not sure if a combi but with a HW cylinder as well was giving much better flow rates through the showers and HW taps and it was the same cold water line supply so the only big change is the boiler itself?.

Was hoping to reply this eve with some info from Navien as I was contacted today by the boiler fitting company saying the installer had spoken to Navien earlier today. No phonecall as yet so I can't really update.
Yes previously the ideal istor was an all in one unit of a 24kw system boiler (an icos or something similar) attached to an unvented cylinder. It had a prv at the mains set to 2bar which has since been replaced with a straight pipe and the flow from taps was far superior. I never did take any measurement but a bath would fill up in no time....now I'm sitting around for a while longer. Cold tap is no different than previously so no issues with the cold supply.
All the same pipework other than the 22mm pipe serving the boiler has been reduced to 15mm where it connects and the prv removed so pressure now makes its way up to 3bar. I've attached a pic...assuming it all looks ok?
The gas supply crossed my mind just simply with it being a bigger KW boiler but probably thinking on a bit of a simplistic level for that justification.
Thanks

 

[ShaunCorbs]

Have you tested the lpm at the boiler yet ?

 

[John.g]

Was hoping to reply this eve with some info from Navien as I was contacted today by the boiler fitting company saying the installer had spoken to Navien earlier today. No phonecall as yet so I can't really update.
Yes previously the ideal istor was an all in one unit of a 24kw system boiler (an icos or something similar) attached to an unvented cylinder. It had a prv at the mains set to 2bar which has since been replaced with a straight pipe and the flow from taps was far superior. I never did take any measurement but a bath would fill up in no time....now I'm sitting around for a while longer. Cold tap is no different than previously so no issues with the cold supply.
All the same pipework other than the 22mm pipe serving the boiler has been reduced to 15mm where it connects and the prv removed so pressure now makes its way up to 3bar. I've attached a pic...assuming it all looks ok?
The gas supply crossed my mind just simply with it being a bigger KW boiler but probably thinking on a bit of a simplistic level for that justification.
Thanks

The gas supply has nothing to do with the flow rate, you could have the boiler switched off and it won't affect the flow rate. A 20kw combi can give the same flowrate as a 40kw, it just wont be as hot.

 

[ecswtrav]

Have you tested the lpm at the boiler yet ?

I'm going to ask the fitter to do this for me as I'm a bit uncertain how to go about it. Funnily enough I did ask him to do this during the installation before connecting up the boiler but he didn't have a flow cup on the van!

 

[John.g]

Basically what is required is to measure the flow coming out of the boiler to see if there is some fault/restriction in the boiler so simply put, disconnect the outlet from the boiler, turn the temperature control to minimum, (despite what I said you do I think need the boiler diverter valve to DHW) and let the water flow into a bucket for a timed period of one minute and just measure this with anything handy like a one litre jug.

 

[ShaunCorbs]

I'm going to ask the fitter to do this for me as I'm a bit uncertain how to go about it. Funnily enough I did ask him to do this during the installation before connecting up the boiler but he didn't have a flow cup on the van!

5l bucket or 1l bottle work well just time hoe long it takes to fill either and work it out how many you can fill in 60 seconds

for example if a 1l bottle takes 10 seconds to fill your running 6lpm etc if it takes 5 seconds it’s 12lm etc

you can do this via the filling loop will give an approximate gauge

eg turn the black lever off turn the blue lever off

disconnect the flex from the black lever side on the silver nut

put this into a 5l bucket or aim get your timer ready when ready open the blue lever fully and start timer etc

 

[ecswtrav]

5l bucket or 1l bottle work well just time hoe long it takes to fill either and work it out how many you can fill in 60 seconds

for example if a 1l bottle takes 10 seconds to fill your running 6lpm etc if it takes 5 seconds it’s 12lm etc

you can do this via the filling loop will give an approximate gauge

eg turn the black lever off turn the blue lever off

disconnect the flex from the black lever side on the silver nut

put this into a 5l bucket or aim get your timer ready when ready open the blue lever fully and start timer etc

Just followed your instructions nice and straightforward thanks. Although the boiler has a dedicated feed in 22mm pipe using that method I get spot on 9lpm....which corresponds nicely with the boiler readouts.
I gather this is why I'm not getting the desired flow rates?

 

[ShaunCorbs]

Correct you have a restriction on the cold main somewhere need to follow this pipe all the way back to the main

 

[John.g]

Just followed your instructions nice and straightforward thanks. Although the boiler has a dedicated feed in 22mm pipe using that method I get spot on 9lpm....which corresponds nicely with the boiler readouts.
I gather this is why I'm not getting the desired flow rates?

Don't want to be awkard but its strange that a open ended pipe via that flexible hose is only giving you 9LPM when you can get up to 12.5LPM from two HW taps, one of which is upstairs, and the water must also pass through the boiler heat exchanger.

 

[ecswtrav]

Correct you have a restriction on the cold main somewhere need to follow this pipe all the way back to the main

The Istor never suffered from this issue so could this issue possibly be installation related? Seems strange how all of a sudden the flow rate has dropped after installation of a new boiler. The Istor even had a prv limiting the pressure....this doesn't and have anything limiting on the supply line now. Just for reference too, I also popped outside and tried the cold tap again....approx 21/22lpm getting to that still.

 

[ecswtrav]

Don't want to be awkard but its strange that a open ended pipe via that flexible hose is only giving you 9LPM when you can get up to 12.5LPM from two HW taps, one of which is upstairs, and the water must also pass through the boiler heat exchanger.

Yeah I agree....the only time I've managed to get a higher flow rate reading on the boiler screen is when I've opened a few hot outlets. A single outlet stays around the 9-10 mark.

 

[John.g]

Is the blue isolating valve just attached to the cold supply to the boiler and is that "1.5 bar" PG just monitoring the boiler system pressure?

Also check for a PRV installed somewhere on the cold feed to the boiler, it may be faulty because in order for the flow rate to increase from 9LPM to 12.6LPM would require a pressure increase from 2.2 bar to 4.3 bar for any fixed restriction/obstruction in the cold water feed.

 

[ShaunCorbs]

Don't want to be awkard but its strange that a open ended pipe via that flexible hose is only giving you 9LPM when you can get up to 12.5LPM from two HW taps, one of which is upstairs, and the water must also pass through the boiler heat exchanger.

but that would of had 22mm pipework and not 15mm so there’s a joint somewhere maybe an iso valve etc

 

[John.g]

Maybe my thinking is wrong but a test was carried out on the cold feed supply only to the boiler by breaking a pipe (hose) so would expect the maximum possible flow rate from this as the cold feed is now open at one end, the same supply is then feeding the boiler, the hot water pipe or pipes to the hot taps and gives 40% more flow than flowing out the open end of the supply pipe.
The only other explanation is that the hot taps have mixers.

 

[ecswtrav]

Well, no contact from the fitter as yet although an electrician was here fitting some opentherm controls as part of the deal. Checked again at the outside tap and confirm it was still 22lpm. I did try the washing machine inlet hose and that didn't quite match the 22lpm and was around 17.5 but I think by looking it was really restrictive with a very tiny hole at the end. Either way it exceeded 9lpm.
Just a few feet upstream from the stopcock are the two isolation valves....one on the cold supply and the other on the boiler supply....so as suggested maybe the boiler supply one is faulty? I believe these were full bore when fitted. From looking they're both open it would seem fully.
The prv has been removed that was used to limit the istor boiler feed to 2 bar so there is nothing else on this feed to the boiler....just a left hand turn which then leads straight to the boiler.
The fitters have pieced back some plasterboard behind the boiler so I wonder if there is something restrictive they've fitted beyond this hidden out of sight?
John.g you are correct - the bathroom tap is a mixer tap.
I will keep this updated anyway until we get this resolved as I appreciate everyone's help.

 

[John.g]

Sometime, you might try this.
What flowrate does the boiler show (1) if you ran the shower only, with the boiler temp at minimum of 35C and (2) with boiler temp at max temp where you will also have mixing.

 

[ecswtrav]

Sometime, you might try this.
What flowrate does the boiler show (1) if you ran the shower only, with the boiler temp at minimum of 35C and (2) with boiler temp at max temp where you will also have mixing.

I just gave that a try....ran the shower in the ensuite in the loft as that is the furthest away of the two. There was minimal difference for some reason and both temps hovered around the 5.8lpm mark on the boiler display....the lower temp exceeding it intermittently. The shower itself has a rain head and is perfectly usable at those flow rates but just not what we were used to.

 

[John.g]

A rain shower should give a torrent of water.
If the shower is maintaining ~ 45C showering temp (thermostaic mixer?) then it should have flowed ~ 9LPM with boiler at max, still very poor.

 

[John.g]

You could also measure the actual shower flow rate with the shower temp control to minimum and on full flow, if temp setting is low enough ie a "cold" shower then no water will flow through the boiler, if this works then measure the actual shower flow rate with the shower at say 50C (or max) and the boiler at its minimum temp setting, this should then force all the water flow through the boiler and compare both flow rates.