Boiler not heating enough during cold times

[Cristina Micsa]

If the temperature outside is in the 40s, 50s and I set the thermostat at 75 degrees, the boiler heats the house to 75 degrees. If outside is really cold, under the 20s, the furnace keeps the room at 69 degrees.

why the difference? Do I need to call a technician?

The boiler is Solution Lochinvar.

 

[Dana]

For the record, which size? (CBN045 is the smallest, at about 37,000 BTU/hr output when not in condensing mode, the biggest CBN260 is good for 216,000 BTU/hr out, with five other sizes in between.)

Any number of things could be causing it to not heat the house, including something as simple as the outdoor reset curve being mis-programmed. Read the part of section 5 of the manual starting on p.36 on how to set up the boiler's output temp. This isn't rocket science to adjust, and when you've figured out how to do it you can fine-tune it for maximum efficiency and comfort without paying for multiple visits from the tech. The "reset curve" or "outdoor reset curve" adjusts the boiler's temperature up in response to colder outdoor temperatures.

The radiators/baseboards/convectors or whatever you have for heat emitters deliver the heat at a higher rate when the boiler's output temperature is higher, less when it's cooler. The boiler is most efficient when running cooler, but it's also more comfortable to have the radiation warm or hot all the time as opposed to cycling on and off with a hot flush followed by a chill. When the curve is dialed in to perfection the room temperature barely varies, the burn times on the boiler are very long, and the radiation is always warm.

BTW: "furnace" refers to a heating appliance that moves the heat to the rooms via hot air and a blower. What you have is a "boiler", or more specifically a "hydronic boiler", that moves the heat via hot water and pumps.

 

[Sponsor]

 

[james61]

Hello,
I'm new to the forum and I'm here because of a similar issue with our new Navien combi boiler (NCB420e). We have had the unit for a little more than a year (two winters) and it works well until the temperature really dips (right now it's 8f with a wind chill of -10f). I've got the thermostat target temp at 67 but the boiler struggles to reach that temperature, even with the supply temp at 178f. The result is a boiler that is almost constantly on.

I've read a lot of the threads regarding these units, which naturally makes me think that if something sounds to good to be true, then it probably is. Dana's replies are very informative but highly technical so there is a definite learning curve here.

Based on what I've read I have a couple of questions:
- I realize that short cycling can be an issue in regards to wear and tear, but what about continuous operation? Is it a problem if the Navien runs almost without stopping, as it tries to maintain or reach 67f target temp? While I'm at home during school break I can lower that target temp now and then for a 1/2 hour just to give the unit a break. Is that necessary?

- Our house is c.1,300sf and fairly well insulated although we do get the NW wind coming off a pond. (the basement is another issue, with no heat other than the exposed pex tubing. Needless to say, the basement is cold).

We have one zone, with roughly 45' of total baseboard slant fin emitters including bedrooms and kitchen. I had reservations about the lack of baseboard footage in the larger living area but the installer assured us it would be enough. It's definitely not enough in terms of heating. Am I correct in saying that the total baseboard footage is not enough to make the Navien run as efficiently as it could? I plan on adding another 10-12' of baseboard to the living area this spring/summer. Will that bring the baseboard footage more in line with the unit? Or...

- ...is this unit simply too big for a house this size?

- BTW, for the moment I'm raising the heating supply temp manually as the temperature drops. I realize that I'm sacrificing efficiency but at this stage I'm more concerned with keeping my family warm. Last question: Is there an outdoor sensor built into these units or is that something that has to be installed before I can use the outdoor reset curve?

Thanks in advance.

 

[tropostudio]

Can you confirm the Navien boiler model number? I think you mean an NCB-240E, which has input of 18,000-120,000 btuh for CH.

 

[djdavenport]

I'd love to hear from the experts. ( I have probably just enough knowledge to be dangerous.) I have a Navien 240E and a 1,800 sq ft house with in-slab radiant. Our unit merrily loafs along, the ODR keeping the supply temperature at about 115 degrees during the coldest weather. Long burn times, often 18-20 hours a day. It would seem that your unit, if the supply set temperature was 175 would either be nuking your 1,300 sq ft house with the capacity of 110,000 btu/h or, if there wasn't sufficient radiation, would be short cycling like crazy. The fact that it runs continuously but struggles to keep up with a 67 degree thermostat set point is baffling to me. When you say your boiler is running continuously, are you saying that it is firing continuously? Or just that the circulation pump is running continuously? Granted, I'm just a DIY homeowner, but I can't imagine, given my experience with the NCB 240E (which has been working great, BTW), that it would be firing continuously and struggling to maintain an modest indoor ambient temperature.

 

[tropostudio]

No expert, but have spent considerable time learning, designing, and installing a CH mod-con system in a totally renovated bungalow in Twin CIties that is working well.

Re James 61 system, and maybe pursuant to Cristina Micsa (but lacking info on her heat requirements) : In Mpls/ St. Paul we have to design for 68 F interior temp at -8 F outdoor air temp. A 'moderately tight' house in those conditions would require ~20 btuh/sq ft. If a 1300 sq ft house falls into that category, you need 20 x 1300 = 26,000 btuh to hold 68 F. Assume your mod-con boiler is at 85% efficiency, running in non-condensing mode (this efficiency level is a nominal guess). It would put out 102,000 btuh, which is 4x more than required for the 1300 sq ft house at the -8 F DD (design day). The Navien 240 E at low-fire setting is probably going to put out 16-17,000 btuh in condensing mode, which is - just guessing - about what our 1300 sq ft, moderately tight house needs at around 15-20 F outside temp.

Assumptions about James 61's house need to be checked with actual fuel consumption history or a Manual J calc. He says basement is unheated, which has a large effect on the Manual J calcs if there is no insulation between conditioned/unconditioned space, or if it is or isn't part of his square footage calcs. In any case, I would guess the boiler is grossly over-sized.

On the emiiter side: Slant Fin 30 baseboard should emit 500-550 btuh/lineal foot at 170F average water temperature. That gives 22,500 -24,750 btuh emitted with 45 lf baseboard. This is a deficit from the requirement of 26,000 btuh calculated above for a -8F DD. James 61 said it was tough to hold temp at 8F with -10F windchill. Maybe that's warmer than a -8F DD. Maybe not. But the fact that it struggles to hold 67 F at 178 F supply temp (which is about 170 average water temp (AWT) with a 20 F delta T (temperature drop) across emitters makes one think emitter sizing is too small.

In short, best guess: boiler too big, emitter sizing too small. YMMV.

 

[james61]

Can you confirm the Navien boiler model number? I think you mean an NCB-240E, which has input of 18,000-120,000 btuh for CH.

Yes, sorry. NCB 240e.

Thanks for your reply. I am still in the process of learning the technical specs needed to determine proper boiler size/settings, but I think you are right as far as the size of the boiler and the lack of baseboard footage. I will be looking to install more baseboard emitters this summer and have plans to finish the basement, which will add another zone to the heating set up. I'm also wondering if it would be possible to keep both the main floor and basement in one zone, which would add more emitters to the set-up.

 

[james61]

No expert, but have spent considerable time learning, designing, and installing a CH mod-con system in a totally renovated bungalow in Twin CIties that is working well.

Re James 61 system, and maybe pursuant to Cristina Micsa (but lacking info on her heat requirements) : In Mpls/ St. Paul we have to design for 68 F interior temp at -8 F outdoor air temp. A 'moderately tight' house in those conditions would require ~20 btuh/sq ft. If a 1300 sq ft house falls into that category, you need 20 x 1300 = 26,000 btuh to hold 68 F. Assume your mod-con boiler is at 85% efficiency, running in non-condensing mode (this efficiency level is a nominal guess). It would put out 102,000 btuh, which is 4x more than required for the 1300 sq ft house at the -8 F DD (design day). The Navien 240 E at low-fire setting is probably going to put out 16-17,000 btuh in condensing mode, which is - just guessing - about what our 1300 sq ft, moderately tight house needs at around 15-20 F outside temp.

Assumptions about James 61's house need to be checked with actual fuel consumption history or a Manual J calc. He says basement is unheated, which has a large effect on the Manual J calcs if there is no insulation between conditioned/unconditioned space, or if it is or isn't part of his square footage calcs. In any case, I would guess the boiler is grossly over-sized.

On the emiiter side: Slant Fin 30 baseboard should emit 500-550 btuh/lineal foot at 170F average water temperature. That gives 22,500 -24,750 btuh emitted with 45 lf baseboard. This is a deficit from the requirement of 26,000 btuh calculated above for a -8F DD. James 61 said it was tough to hold temp at 8F with -10F windchill. Maybe that's warmer than a -8F DD. Maybe not. But the fact that it struggles to hold 67 F at 178 F supply temp (which is about 170 average water temp (AWT) with a 20 F delta T (temperature drop) across emitters makes one think emitter sizing is too small.

In short, best guess: boiler too big, emitter sizing too small. YMMV.

The basement was not calculated into the square footage and needs insulation between floors.

When you mention "low-fire setting," does that mean setting a low supply temperature or is it a separate setting entirely? Thanks.

 

[tropostudio]

James 61: Low-fire means the lowest the burner can throttle down to. On a Navien NCB-240E, that is 18,000 btuh input, and probably somewhere between 15,000 and 17,000 output (85% efficiency for non condensing mode, up to 95% at peak efficiency in condensing mode - nominal figures pretty typical for a mod-con boiler)

With an over-sized boiler, the high output at the low end means you won't often be able to operate in condensing mode with warmer outdoor temperatures. Our renovated house is now about 2,900 conditioned sq. ft. 1915 Crafstemen bungalow gutted, 2nd story added, and now with finished basement. Our mod-con boiler is an 80,000 btuh unit, with indirect domestic hot water (DHW). With a 10: turndown ratio, It can run consuming as little as 8,000 btuh. Calculated heat load at a -15F DD was 58,000 btuh, but is probably less because of extra insulating and air sealing we did. Our emitters are sized 2X what they needed to be for 170 AWT, in order to run in condensing mode at low outdoor temps.

The outdoor re-set curve is not dialed in, and we don't even have radiators installed on the 2nd floor yet. But the house is holding 68F day and night, albeit running higher boiler temps, and outdoor temps dropping as low as -10F right now.

Out of curiosity, what is your location and climate? Insulate your basement ceiling, and you'll be even further over-sized on the boiler, but may not need to add more emitter length. If the basement could be made usable cost-effectively, it might be worth insulating the walls and doubling the square footage of your place.

 

[james61]

I'd love to hear from the experts. ( I have probably just enough knowledge to be dangerous.) I have a Navien 240E and a 1,800 sq ft house with in-slab radiant. Our unit merrily loafs along, the ODR keeping the supply temperature at about 115 degrees during the coldest weather. Long burn times, often 18-20 hours a day. It would seem that your unit, if the supply set temperature was 175 would either be nuking your 1,300 sq ft house with the capacity of 110,000 btu/h or, if there wasn't sufficient radiation, would be short cycling like crazy. The fact that it runs continuously but struggles to keep up with a 67 degree thermostat set point is baffling to me. When you say your boiler is running continuously, are you saying that it is firing continuously? Or just that the circulation pump is running continuously? Granted, I'm just a DIY homeowner, but I can't imagine, given my experience with the NCB 240E (which has been working great, BTW), that it would be firing continuously and struggling to maintain an modest indoor ambient temperature.

Thanks dj davenport. It's baffling to me as well but I am a total novice with this technology. It seems to be firing (the combustion icon is on). The baseboards do seem a little hotter at times than others. Does that suggest that the boiler is at times firing and at other times circulating water? I've seen elsewhere that these boilers are supposed to stay on for hours but I just don't know for how long is normal. It is definitely not short cycling.

 

[james61]

James 61: Low-fire means the lowest the burner can throttle down to. On a Navien NCB-240E, that is 18,000 btuh input, and probably somewhere between 15,000 and 17,000 output (85% efficiency for non condensing mode, up to 95% at peak efficiency in condensing mode - nominal figures pretty typical for a mod-con boiler)

With an over-sized boiler, the high output at the low end means you won't often be able to operate in condensing mode with warmer outdoor temperatures. Our renovated house is now about 2,900 conditioned sq. ft. 1915 Crafstemen bungalow gutted, 2nd story added, and now with finished basement. Our mod-con boiler is an 80,000 btuh unit, with indirect domestic hot water (DHW). With a 10: turndown ratio, It can run consuming as little as 8,000 btuh. Calculated heat load at a -15F DD was 58,000 btuh, but is probably less because of extra insulating and air sealing we did. Our emitters are sized 2X what they needed to be for 170 AWT, in order to run in condensing mode at low outdoor temps.

The outdoor re-set curve is not dialed in, and we don't even have radiators installed on the 2nd floor yet. But the house is holding 68F day and night, albeit running higher boiler temps, and outdoor temps dropping as low as -10F right now.

Out of curiosity, what is your location and climate? Insulate your basement ceiling, and you'll be even further over-sized on the boiler, but may not need to add more emitter length. If the basement could be made usable cost-effectively, it might be worth insulating the walls and doubling the square footage of your place.

We are just south of Boston.

James 61: Low-fire means the lowest the burner can throttle down to. On a Navien NCB-240E, that is 18,000 btuh input, and probably somewhere between 15,000 and 17,000 output (85% efficiency for non condensing mode, up to 95% at peak efficiency in condensing mode - nominal figures pretty typical for a mod-con boiler)

With an over-sized boiler, the high output at the low end means you won't often be able to operate in condensing mode with warmer outdoor temperatures. Our renovated house is now about 2,900 conditioned sq. ft. 1915 Crafstemen bungalow gutted, 2nd story added, and now with finished basement. Our mod-con boiler is an 80,000 btuh unit, with indirect domestic hot water (DHW). With a 10: turndown ratio, It can run consuming as little as 8,000 btuh. Calculated heat load at a -15F DD was 58,000 btuh, but is probably less because of extra insulating and air sealing we did. Our emitters are sized 2X what they needed to be for 170 AWT, in order to run in condensing mode at low outdoor temps.

The outdoor re-set curve is not dialed in, and we don't even have radiators installed on the 2nd floor yet. But the house is holding 68F day and night, albeit running higher boiler temps, and outdoor temps dropping as low as -10F right now.

Out of curiosity, what is your location and climate? Insulate your basement ceiling, and you'll be even further over-sized on the boiler, but may not need to add more emitter length. If the basement could be made usable cost-effectively, it might be worth insulating the walls and doubling the square footage of your place.

We are 20 miles south of Boston, on the south shore.

 

[tropostudio]

James61 - look for a copy of your install and use manual here:

http://wholehousecombi.com/wp-conte...allation__Operation_Manual_English_131002.pdf

It appears the outdoor re-set temperature sensor is optional. See page 74. Page 64 shows where it would be connected in the boiler, or see if the unit is mounted on the exterior of the house. This is a good feature, because it lets you map outdoor max and min temepratures to supply water max and min temperatures. The outdoor temperature is a proxy for the heat load of your home, and tells the boiler how hot it should allow the supply water to be for that outside temperature.

Your 99% outdoor design temperature would be ~10F. Thats 25F warmer than we have to design for in St. Paul!

Your emitters are hot then cold because you are running high water temps and the system is short-cycling. The boiler dumps a lot of heat into the house at 180F supply temperature. The thermostat says turn off, so call for heat ends. Rooms cool back down to thermostat set point until it signals a call for heat again, and the cycle repeats. It's called 'Bang-on, Bang-off' control. Any boiler benefits in longevity and heating efficiency by putting out just the amount of heat needed to keep up with the load, so that runs more or less continuously at a constant firing level. Same as driving your car at a steady speed is easier on the engine and uses less fuel than starting and stopping constantly.

There are parameters for max firing % that can be adjusted on the Navien, as well as anti-short cycling time intervals. They don't solve a lack of emitter surface area, but might help. Check with your installer if you are uncomfortable programming the unit.

You said you are currently set up with 1 zone for the main level. If the temps across that level are consistent room-to-room, using one central thermostat, you are lucky or have an open floor plan with open doors and good air movement. You will want to set the basement up on a separate zone with it's own thermostat when you renovate. If you need finer control of temperatures room-to-room, you set up additional zones with thermostats at each area that needs control. Flow of hot water to each zone is typically controlled via electric zone valves or separate, small circulation pump at each zone. Another option, which I used, is to install thermostatic radiator valves (TRVs) - 1 at each radiator, with each radiator having its own supply and return lines back to distribution manifolds near the boiler. This allows precise temperature control at each radiator, but gives up central temperature control.

 

[jokabri]

James61, same issue I have with a Navien NCB 180e when it gets down below say 15 deg (I am in Cambridge, MA) - I have to crank the "space heating water supply temp" way up to 180 (MAX) to get the thing to maintain 69 or 70 deg. Never gets above 69 or 70 even tossing the thermostat to 90. I obviously don't want 90 deg but it's odd the unit CANNOT supply it when the temps get low (< 15 deg). Supply at 180 (A reading at say 178) and return (B reading) is still good at 169 or so which tells me it's not losing a lot travelling around the baseboard.

In this unit I have only ~1600 sq ft, single zone (1st floor) and 75' of finned baseboard, so real odd this Navien can't keep the pace. Not talking heating the Boston Garden here. Now I don't have the Outdoor Temp Sensor (so no Outdoor Reset Control) unit on mine and the house is old, so insulation is weak at best. Question to experienced Navien installers - does it make sense that the NCG 180e would struggle heating a smallish unit? If so, would the Outdoor Sensor solve the problem of manually cranking things up to 180?

2nd winter with the unit and the tenants I had in here last year complained heat was weak on cold days. I didn't believe them, but do now since I moved in myself. Cold days... cold unit. Karma is a ....

 

[djdavenport]

James--Way above my pay grade here. I agree with the other comments. Your boiler has the capacity to heat a much larger space. 199,ooo btus input. A lot of fire power. But that heat has to go somewhere, and if you don't have sufficient emitters, the heat won't be released into the house. Typically, the boiler will short cycle. The boiler will hit the upper limit and shut down. Then, if the thermostat is still calling for heat, the circulator pump will continue to run until enough heat has been distributed and the return temperature falls sufficiently that the burner fires back on. That's the part I don't understand. You indicate that the boiler is firing for long periods of time. But, the ambient temperature in the house is barely up to the thermostat set point. Where is that heat going?

I can address the ODR question. Yes, it is an optional item. Not particularly expensive. Once you have it, you can indicate what your type of radiation is, and it will compare the outside temperature with what it thinks is the appropriate supply temperature to keep the house static--just enough heat to match the loss. When it works, it's awesome. My energy bills are about 25% less, year over year. It does take some tweaking, but not hard once you understand the principle.

Just baffled a little by your initial problem. Big, probably oversized boiler. Probably undersized emitters. Lots of heat with nowhere to go. And still the boiler runs constantly. What am I missing? Dana?

 

[Dana]

The NCB 240 is overkill for your actual space heating load, but probably not for the domestic hot water load.

Burning almost continuously is what you WANT the thing to do for best efficiency and least wear & tear on the unit. The fact that it's burning continuously it means you have enough baseboard or radiator to not be short-cycling (=lots of burns per hour, less than 3 minutes each). It's fine to use thermostat setbacks whenever you're going to be away for 8 hours or more (but that rarely saves fuel if it would otherwise be modulating in condensing mode if left alone.) It does NOT need to "rest", ever.

But if it's not heating the house even though the burner is on continuously, the issue is most likely going to be either the programmed output temperature is not high enough, or you don't have enough radiation to emit enough heat at the maximum temperature of 180F. The 45' of fin-tube it should have enough baseboard to not short-cycle at 180F, but at condensing temperatures you'll have to time the burns and count them to see how it's doing.

At the beginning of the condensing zone (~135-140F out, average water temp 130F) the 45' of baseboard would only be emitting ~10-11,000 BTU/hr, while minimum fire at condensing temperatures it delivers ~17,000 BTU/hr into the system. You'd need on the order of at least 70' of baseboard to hit the low-90s for combustion efficiency without cycling at all, but as long as the burn cycles are long enough, with five or fewer burns per hour it's not a disaster. The NCB 240 definitely oversized for your radiation, it's only a matter of how much, so time & count the burn cycles.

Most houses will have enough radiation to stay warm even at 0F or lower with the boiler set to something less than 180F, but go ahead and bump it to the max until the indoor temperature is high enough, then start backing it off 5F at a time to see where it stops keeping up. With the outdoor reset option implemented you can set it up to do the temperature adjustments for you.

Most "well insulated" reasonably tight 2x4 framed 1300' houses with no foundation insulation would have a heat load of about 24-26,000 BTU/hr @ 0F, dropping to 17-19K once the band joist and foundation walls are insulated. So yes, the NCB 240 is ridiculously oversized for your actual heat load, and won't modulate much even if you have enough radiation to operated in condensing mode.

Below grade basements have low heat loads that don't change very much with outdoor temperature, unlike the above grade floors. For that reason they don't work well when operated as a single zone when combined with an upstairs zone. To get around that you could install a bypass branch on the basement radiation, and control the flow through the basement radiation with a zone valve operating off a separate basement thermostat. The basement would only get heat then the upstairs zone is calling for heat, but the separate thermstat, zone valve & bypass branch would keep the basement temperatures from overshooting. It's not perfect, but it's not bad. Once the foundation is insulated to the code-minimum R15 continuous insulation or better the basement will probably stay above 65F when the upstairs is 70F even without adding basement zone radiation.

Insulating between the joists would knock off some of the heat load, and might be enough to allow your existing radiation to keep up (or not), but it's always better (= more effective, and lower energy use) to air-seal and insulate the foundation instead.

MassSave won't subsidize foundation wall insulation, but in MA there are multiple vendors of reclaimed rigid foam board that make doing it as a DIY pretty cheap. MassSave WILL subsidized band joist & foundation sill insulation however. A couple or three inches of closed cell spray polyurethane would work. (2" of the more environmentally friendly HFO-blown foams would be R14. The high global warming potential HFC blown foams would be R12-R13 @ 2".)

 

[Cristina Micsa]

For the record, which size? (CBN045 is the smallest, at about 37,000 BTU/hr output when not in condensing mode, the biggest CBN260 is good for 216,000 BTU/hr out, with five other sizes in between.)

Any number of things could be causing it to not heat the house, including something as simple as the outdoor reset curve being mis-programmed. Read the part of section 5 of the manual starting on p.36 on how to set up the boiler's output temp. This isn't rocket science to adjust, and when you've figured out how to do it you can fine-tune it for maximum efficiency and comfort without paying for multiple visits from the tech. The "reset curve" or "outdoor reset curve" adjusts the boiler's temperature up in response to colder outdoor temperatures.

The radiators/baseboards/convectors or whatever you have for heat emitters deliver the heat at a higher rate when the boiler's output temperature is higher, less when it's cooler. The boiler is most efficient when running cooler, but it's also more comfortable to have the radiation warm or hot all the time as opposed to cycling on and off with a hot flush followed by a chill. When the curve is dialed in to perfection the room temperature barely varies, the burn times on the boiler are very long, and the radiation is always warm.

BTW: "furnace" refers to a heating appliance that moves the heat to the rooms via hot air and a blower. What you have is a "boiler", or more specifically a "hydronic boiler", that moves the heat via hot water and pumps.

The boiler is a CBN075 at 61500BTU. The Max Set Point was set at 180 and I moved it to 230. I will wait and see if it works. Thank you for your suggestions.

 

[Cristina Micsa]

The house still does not get heated enough. Can anyone tell me what the right settings are for a Zero degree temperature outside? What is supposed to be the Set Point, Diff and High-Fire offset? Right now they are at 230, 5 and 10. I have 3 zones, it keeps the temperature at 70, 67 and 64. Please help!

 

[Reach4]

I am not familiar with this stuff. However I am thinking that if you need more heat during an unusually cold snap, directing a small fan through a convection radiator may make it act as a bigger radiator temporarily.

 

[Dana]

The house still does not get heated enough. Can anyone tell me what the right settings are for a Zero degree temperature outside? What is supposed to be the Set Point, Diff and High-Fire offset? Right now they are at 230, 5 and 10. I have 3 zones, it keeps the temperature at 70, 67 and 64. Please help!

The most important setting for getting the most heat out of it is the high-temp- the rest can be opened up to improve efficiency if it's short cycling, but at (230F it probably isn't short cycling.)

Is the boiler cycling on/off during continuous calls for heat from all three zones?

Is it actually hitting 230F?

If yes to both you are SERIOUSLY under-radiated for the boiler (and for being able to heat the house at 0F). How much radiation/baseboard do you have, zone by zone?

At an entering water temp of 230F most fin-tube baseboard puts out about 800 BTU/hr per running foot, so at ~60,000 BTU/hr of high fire output it would take 75 feet of baseboard to deliver the entire high-fire output, and ~40' of baseboard to deliver the entire low-fire output. At lower boiler temperatures it takes more baseboard to deliver the heat into the rooms. A rough guide to what to expect from fin-tube baseboard can be found here (those are average water temperature, not entering water temperatures. With 230F water the average water temp will be 210-220F.)

The DOE output of the thing even at LOW fire is about 31-32,000 BTU/hr, which is enough to fully heat ~1500' of tight 2x4 framed insulated houses @ 0F. At high fire it should be able to heat a reasonably tight 2x4/R13 ~3000' house. But if there isn't enough heat emitter such as baseboards or radiators for the zone loads you're kind of stuck at whatever they can deliver.

 

[james61]

The NCB 240 is overkill for your actual space heating load, but probably not for the domestic hot water load.

Burning almost continuously is what you WANT the thing to do for best efficiency and least wear & tear on the unit. The fact that it's burning continuously it means you have enough baseboard or radiator to not be short-cycling (=lots of burns per hour, less than 3 minutes each). It's fine to use thermostat setbacks whenever you're going to be away for 8 hours or more (but that rarely saves fuel if it would otherwise be modulating in condensing mode if left alone.) It does NOT need to "rest", ever.

But if it's not heating the house even though the burner is on continuously, the issue is most likely going to be either the programmed output temperature is not high enough, or you don't have enough radiation to emit enough heat at the maximum temperature of 180F. The 45' of fin-tube it should have enough baseboard to not short-cycle at 180F, but at condensing temperatures you'll have to time the burns and count them to see how it's doing.

At the beginning of the condensing zone (~135-140F out, average water temp 130F) the 45' of baseboard would only be emitting ~10-11,000 BTU/hr, while minimum fire at condensing temperatures it delivers ~17,000 BTU/hr into the system. You'd need on the order of at least 70' of baseboard to hit the low-90s for combustion efficiency without cycling at all, but as long as the burn cycles are long enough, with five or fewer burns per hour it's not a disaster. The NCB 240 definitely oversized for your radiation, it's only a matter of how much, so time & count the burn cycles.

Most houses will have enough radiation to stay warm even at 0F or lower with the boiler set to something less than 180F, but go ahead and bump it to the max until the indoor temperature is high enough, then start backing it off 5F at a time to see where it stops keeping up. With the outdoor reset option implemented you can set it up to do the temperature adjustments for you.

Most "well insulated" reasonably tight 2x4 framed 1300' houses with no foundation insulation would have a heat load of about 24-26,000 BTU/hr @ 0F, dropping to 17-19K once the band joist and foundation walls are insulated. So yes, the NCB 240 is ridiculously oversized for your actual heat load, and won't modulate much even if you have enough radiation to operated in condensing mode.

Below grade basements have low heat loads that don't change very much with outdoor temperature, unlike the above grade floors. For that reason they don't work well when operated as a single zone when combined with an upstairs zone. To get around that you could install a bypass branch on the basement radiation, and control the flow through the basement radiation with a zone valve operating off a separate basement thermostat. The basement would only get heat then the upstairs zone is calling for heat, but the separate thermstat, zone valve & bypass branch would keep the basement temperatures from overshooting. It's not perfect, but it's not bad. Once the foundation is insulated to the code-minimum R15 continuous insulation or better the basement will probably stay above 65F when the upstairs is 70F even without adding basement zone radiation.

Insulating between the joists would knock off some of the heat load, and might be enough to allow your existing radiation to keep up (or not), but it's always better (= more effective, and lower energy use) to air-seal and insulate the foundation instead.

MassSave won't subsidize foundation wall insulation, but in MA there are multiple vendors of reclaimed rigid foam board that make doing it as a DIY pretty cheap. MassSave WILL subsidized band joist & foundation sill insulation however. A couple or three inches of closed cell spray polyurethane would work. (2" of the more environmentally friendly HFO-blown foams would be R14. The high global warming potential HFC blown foams would be R12-R13 @ 2".)

Thanks to all for the replies to my original post. I appreciate it. The boiler is definitely not short cycling and I'm getting used to the fact that it is a different animal from what I'm used to (typical oil furnace/cast iron radiators). I've been spending lots of time insulating the walls of our basement, which is partially below grade. It has made a difference. That and a little heat in the basement as well, especially during this deep freeze. Thanks again for all the information and insights.