Navien NCB-240E possible purchase

[Dana]

You will always have long burn times if you bump the thermostat 4 degrees- that experiment tells you nothing.

Leaving the thermostat at a single temperature for 10+ hours and THEN observing the number of burns in last 2 yours, as well as their length will give you a better idea of how it will behave at the water temperature you're giving it. If it satisfies the thermostat quickly (less than 15 minutes) you can lower the programmed water temperture (unless it's short-cycling during the call for heat.)

When it's only 29-30 F outside you probably shouldn't need water temps anywhere NEAR 150F, or even 140F. With 100' of baseboard and 150F water out/130F back (140F average water temp) the baseboard is delivering about 30,000 BTU/hr, which is WAY above your actual heat load is at 30F.

 

[DR-DEATH]

You will always have long burn times if you bump the thermostat 4 degrees- that experiment tells you nothing.

Leaving the thermostat at a single temperature for 10+ hours and THEN observing the number of burns in last 2 yours, as well as their length will give you a better idea of how it will behave at the water temperature you're giving it. If it satisfies the thermostat quickly (less than 15 minutes) you can lower the programmed water temperture (unless it's short-cycling during the call for heat.)

When it's only 29-30 F outside you probably shouldn't need water temps anywhere NEAR 150F, or even 140F. With 100' of baseboard and 150F water out/130F back (140F average water temp) the baseboard is delivering about 30,000 BTU/hr, which is WAY above your actual heat load is at 30F.

So I just ran a test. Thermostat was 68. I clicked it to 69. Went down and starting timing as soon as boiler fired. 3:30 sec into burn the flame went off and water temp went from 135 to 116 pretty quickly. Thermostat was still 68. Water down to 105 f 9 mins in. Burner has not kicked back on but thermostat still has not been satisfied. 13 mins in water temp is 103. Burner still not back on but thermostat not satisfied yet. 13:30 burner kicked back on. Burner ran for 3:20 seconds and shut off. Water temp got to 135 again. 21 mins in and thermostat is still not satisfied. Water temp has gone down to 121. 23:30 mins in thermostat says 69 but has not clicked off. 27:30 in burner kicked on again. So it looks like three burns on one single call trying to move up 1 degree. Outdoor temp is: 43 f. That doesn't seem so bad or like short cycling but shouldn't it have modulated down and burned really low to keep a constant temp until thermostat was satisfied?

The upstairs zone was not used during this. Our main floor has a bedroom with radiation, hallway where thermostat is and then kitchen, living room and small dining room all open together with half of that being 15 foot vaulted ceilings that pretty much lead directly into upstairs bedrooms and bathroom. Those doors were open so I am wondering if a lot of the heat went up into those rooms as the majority of radiation is in the open area below the vaulted ceilings.

Would speeding up my pump help? If I understand correctly that will allow hotter water to get to radiators faster but the down side is the return temp will be hotter and maybe not condense as much?

 

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[Dana]

The experiment should be passive: Just leave the thermostat alone and observe it's behavior & duty cycle, and how long (if ever) it takes to satisfy the thermostate. Bumping it up to force the burn puts it into a recovery ramp.

 

[DR-DEATH]

Ok that makes sense. I changed the settings and turned on step modulation and limited the heat to 60 % boiler output and that is def allowing it much longer burn times. Also changed the temp differential from 30 to 20f as Rob suggested.

 

[NY_Rob]

You should limit your burner capacity based on your heatloss calculation.... every home will be different.

According to my heatloss calculations on my house- at 15F with 55 heating deg I need about 30K BTU's to maintain 70F inside my house.

The HTP UFT-80W can reduce capacity down to 50% of 80K BTU's, so if I set it at 50% capacity I basically have a boiler with 40K BTU output. Limiting the output prevents the boiler from applying 80K BTU's to the CH water then not being able to modulate down fast enough as the DT narrows once the water has made a full circuit through your system. It's a nice feature on low mass systems like fin-tube baseboard.

Step modulation helps too as it makes the boiler start at a low fire rate then ramp up the fire in six one min. steps rather than starting off at full fire rate and modulating down from there. It's helpful on shorter zones that heat up quickly.