Combustion Problem Lochinvar Noble

Peregrine

i am having a combustion Problem. It is a Lochinvar Noble Combi NKC 150 nat gas 150KBTU high altitude (8000 ft) modcon fire tube boiler. it is a 2017 build which was at the beginning of the product cycle. the unit is 3x oversized as a heatload calc yielded a 45KBTU design day demand. the install specs for hydronics piping ask for 1" copper pipe but it is piped with 3/4" pex. the stack is 18' of 3" PVC with three 45 degree couplings. there are no obstructions. static inlet gas is 8.25 w/c and dynamic is 7.75 w/c. there are no other factory supplied specs. it is a negative regulation gas valve. the gas valve and burner are new. the analyzer has been recently calibrated and the unit has been recently serviced.

the Problem is that at High fire the CO2 levels are too low and O2 too high. to get CO2 higher into spec range throttle screw must be turned out and pressure must be increased but then the CO and COAF go Bananas as do stack temps which eventually shut the boiler down at limit. XAIR is 36, Efgc 88.5%.

at Low fire (10%) all is dead nuts right on spec. COAF is 8 and stack temps are 100. its Beautiful. at "mid fire" (50%) the CO2 and O2 levels are to spec but CO and COAF are already at levels (>100) indicating poorer combustion. stack temps are 120F.

presently, max fire rate is limited to 10% as it satisfies "cold day" (15F) design demand at extended cycle times. it is only december so we will see.

could it be that the thing is so oversized and that there is nowhere for the heat to go?

please help and have a great day

EBEBRATT-Ed

Combi's are a problem . Why install a 150k boiler for a 45k load? Makes no sense. Probably has something to do with the altitude your at.

Are the people servicing it familiar with Lochinvar equipment?

It's been there for 7-8 years. Is this a new problem or has it always existed?

If so how has it lasted 7 years? Has it been serviced regularly and cleaned?

SuperTech

Usually if the low fire is correct and the high fire isn't the bias on the gas valve needs to be adjusted. I'm not familiar with that particular combi but that's what the installation instructions for other models have shown. Usually you have to open all the hot water fixtures and flow as much water through the combi as possible when doing this to keep it in high fire. But if you are that badly oversized and on a well with low water pressure it will be almost impossible to keep it running in high fire for very long.

I agree, why install a combi at all, especially something so badly oversized? A properly sized boiler and indirect would work so much better. It needs 1" copper but has 3/4" pex? You realize that 3/4" pex=1/2" copper, right?

hot_rod

you can limiting the heating firing rate as low as 10%, 15,000btu/hr.

So limit it to 45,000 and you have both a perfectly sized boiler for your heat, yet 150k for DHW production. On a design day the boiler should run non stop.

As a bonus, efficiency goes up when you down fire them. As you have a 150k boiler HX exposed to a 45k firing so you get more condensation. Viessmann has a performance chart showing the effect of the de rated firing rates. I have it on another computer.

You should really explore all the control options to maximize that boiler. Eliminate excessive firing, wear and tear.

And your combustion numbers may fall in line better at the limited firing rate.

You don’t have any of these unique options on a fixed fired atmospheric boiler, by the way🤔

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Peregrine

Hot Rod i feel like im getting a baseball signed by Mickey Mantle. “Space heat limiting” as in your pdf presented is already being used and limiting firing to 10% and combustion is perfect. It is running and condensing constantly.At the 45% firing rate it fires to spec but CO is low to mid100s;it will get set there when it gets super cold. At 100% It wont spec and CO is bananas. Combustion analysis was done with all zones open to dissipate heat. And the bias or offset as this gas valve manufacturer (SIT848) calls it, adjusts low fire and thats perfect. The throttle is the problem. There are no leaks and no flame impingement.
the install is what it is. Is there anything else that can cause poor combustion?

So my question remains is the oversize the reason for the high CO levels and poor combustion?

Thank you for your attention.

GroundUp

The oversize has no bearing on the combustion issue. Has the unit been cleaned? 7 years without cleaning and likely a lack of combustion tuning at install, the exchanger and burner are probably filthy which would certainly cause the problem you are seeing. Also I know you mentioned the gas pressures but would like to verify- at 100% firing rate the pressure drop is only .5" from static? I see you'd commented on my post from a couple years ago and yes, the issue with that one was the gas regulator. I've also had several of them since then where I ran out of adjustment, and it's always been a fuel supply issue. Are you turning the same 4.5MM screw for both high and low fire adjustment, or are you using the 2.5MM screw for high fire in the opposite direction like you're supposed to?

Peregrine

the unit has been cleaned; heat exchanger, condense trap new gaskets the whole 9 yards and combustion analysis. new burner, new gas valve. w/c at full fire is 7.75 w/c a .5 drop from static. the 4.5mm screw is the low fire which is great; turn in to raise CO2. 2.5mm screw is high fire; turn out to raise CO2 which is the problem. i dont run out of room. the system has not been flushed but then high temps would not impact combustion.

thanks for the attention.

have a great day

GroundUp

Sounds like a call to tech support is in order

Peregrine

thanks have a great day.