Navien CH210ASME- burner cuts out for an instant

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Jac04

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It's not a matter of "need all that". The fact is that your propane supply system isn't sized properly. The Navien CH series units are VERY sensitive to fuel supply. Like I said, I've had a system like yours previously and experienced all sorts of issues because of it. Is it the cause of your short-cycle issue? Maybe, maybe not. Is it the cause of the 'shift gears' flame-out? I say no, since mine does the same thing and my propane delivery system is correct. Is it wrong and does it need to be fixed? Absolutely. I know this isn't something that you want to fix, but I'm trying to help you make sure this unit runs properly, and the first step is making sure the propane supply system is correctly sized.
 

MikeG88

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Got it. I've questioned the gas people before- my supplier installed the system and obviously owns the tank. They insisted it was correct but I've read what you have said here and elsewhere, as well as others have stated. I will add this to my list to discuss with the Navien trained plumber coming out. If that is the case I need somebody with some professional clout to call them up and say they "undersold" me and should upgrade the system taking without charging me full retail.
 

Dana

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Dana- in answer to the question in you last post (remember I have the the basic round Honeywell T-stat set to 3 CPH) a "normal" burn lasts about 6 to 8 minutes on average. When it short cycles it will burn for several minutes, cool for a few minutes, reignite- but sometimes the call for heat end before the re-ignition, or shortly after the re-ignition. So when it happens I am getting the 3 ignitions per hour based on the T-stat's 3 CPH and another short-cycle induced ignition per cycle, for a worst case scenario of 6 ignitions per hour, based on what I have observed.

Oddly earlier tonight while running my electric dryer, I timed a burn for 10 minutes, then the burn stopped, but the call ended before re-ignition. Another time it reignited but the call ended shortly there after. The dryer is on the heated level and must be sucking a lot of heated air out of the house - the burn lasted a bit longer than average and I would have expected it to short cycle earlier than 10 minutes into the call. I have noticed this before because the bedrooms, away from the area where the dryer is, actually get warmer (you can feel it and see a degree or two increase on a thermometer).The T-stat is closer to the laundry closet. Maybe the bedrooms have more baseboard per SF than the more open living room, kitchen & dining, and laundry closet. (Or are more isolated from the areas having the heat sucked out of them).

Jac- I'll try lowering the heating capacity further when it's a little warmer and I have time to observe it over a period. Some start ups seem fairly smooth now and some involve a bit more ramping and shifting, and the occasional flame restart. (And I think sometimes you are not sure if there is a restart or just a "shift" because I think the remote controller and the green burner light on refresh only every so often).

A T-stat does not and can not control the length of the burns- it can only determine the length &/or number of calls for heat per hour. The lengths of the burns during a continuous call for heat are a function of the boiler's internal controls, primarily how far above the boiler's temperature setpoint it allows before turning the boiler off, and how much below the setpoint it allows before re-ignition. With some boilers that temperature difference is user-tweakable, but SFAIK that's not the case with this series. If it's normally burning for more than 3-4 minutes per BURN cycle during a continuous call for heat, it's not a short cycle.

If as you say cycling on/off during a call for heat but the burns are always 6-8 minutes during a continuous call for heat that's cycling, but not short-cycling. When the burns are under 3 minutes and cycling at a rate way above 5 burns per hour it becomes an efficiency and longevity problem. If it's averaging burner-on for 7 minutes, burnr-off for 3 minutes and running an absolute maximum of 6 burns/hour it's less than ideal, but it's not a problem.

The difference in indoor to outdoor temperature determines the instantaneous heat load, which varies through out the day. That won't affect the cycling rates, but it does affect the length of time it takes for the thermostat to be satisfied. If the temperature of the air at the floor entering the baseboards is colder the amount of heat the baseboards put out is higher, which would affect the cycling slightly, but that won't change much with outdoor air temp.

If you replaced the Honeywell with a dumber T-stat that didn't limit the number of calls for heat per hour it would probably cut down on the number of ignition cycles per heating season by more than 10%, which will improve boiler performance & life, but not by a huge factor. (Recommended.) In your case since the min-fire output is higher than your 99% heat load and you're operating outside the condensing zone for temperature, you might get modest fuel savings using a cheap $25 programmable (eg Lux TX500E or Honeywell RTH2300 )
using modest overnight setbacks (not more than 5F or recovery ramps would take awhile), but a cheap $25 digital non-programmable would be fine too.
 

MikeG88

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This morning I was woke up early, it was about 19.5F outside and I decide to time the burns and time between calls. It averaged pretty close to 10 minutes burn, 10 minutes non-burn/non-call. I did see the burner shut down once during a call, but the call ended very shortly thereafter. I figure the burns were longer than the 8 minutes I've seen at even lower temperatures- because it has been fairly cold for a few days and we had a snow storm (limited solar gain) Monday, so the house didn't have as much stored up heat as it did some days when the temps might drop for a short time into the low teens but the highs and averages are much higher overall than we have seen for a few days. Based on this observation I did some math based on some assumptions.

I assumed since burner is running 50% of the time you get 1/2 of the BTUs you would in an hour. I didn't compensate for the lower temperature for the first few minutes of the burn as it ramps up. I didn't figure an average water temperature based on the theory that I have a low delta, and if anything I be on the high side. I used the BTU per LF from SlantFins base line at 1 GPM (Unit is set for 160F). Based on the calculation Dana did earlier using simple division to convert from stated BTU at a specific differential to what it would be at a different differential (simply stated if something requires 5 btu at 50F I assumed it would require 10 BTU at 25F). Attached is my "napkin math" exercise, actually I had to use a standard sheet of paper a I couldn't fit it on a napkin. Please excuse me if it's incorrect (but let me know) as I don't come from a math or engineering background.
 

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Jac04

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Maybe I missed it, but have we established if the burner is shutting down due to overshooting the set point?
If 160F is your supply setting, the burner would shut down around 175F - 180F (I think). What are you actually seeing?
 

MikeG88

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Hi Jac04- not sure if that is what is happening. I am set for 160F and it really doesn't go more than a few degrees higher than that. When it happens it seems to run for several minutes hoovering around the 160F mark, then the burner goes out. Often the heat call will end before the burner comes back on, but the few times when call didn't end- the burner seemed to come back on after the supply temperature had dropped by 15F, maybe that's just by chance or maybe it has something to do with the Delta-T getting too small? I've never really seen the unit pass the set supply temp by more than a few degrees -and then it usually drops to set or a few degrees below.
 

Jac04

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Interesting. The more I think about it the more I think it could be a gas supply issue. When the burner tries to modulate down after hitting the set point, something happens to the gas supply to cause it to go out (that twin stage regulator is most likely the culprit, as it caused similar problems on mine during transient loads). It then won't re-fire because the computer wants to see about 15F under setpoint before ignition.
 

Dana

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These things are designed to turn off the burner when the system temp rises a few degrees above the setpoint. As long as there's still a call for heat the circulator keeps running until the water in the fin-tube drops below the set point temp a few degrees. With some boilers that differential is programmable, but apparently not this one. Heat is still being emitted out of the baseboard when the burner is off, which is why the temperature eventually drops below the boiler's setpoint and it re-lights the burner. The average temp of the baseboard or the rate at which heat is being emitted into the room doesn't change a whole lot whether the burner is on or off unless the designed in or programmed temperature differential is large.

If you're getting 10 minute burns with 10 minute off intervals that's three burn cycles per hour, which is just fine.

Regarding the napkin math, most of the baseboard manufacturers charts use average water temp (AWT) not entering water temp (EWT). De-rating BTUs by 50% for the duty cycle during calls for heat is illegitimate, since the baseboard is still hot and still emitting heat during the off cycles. The duty cycle of the calls for heat from the T-stat is what you should be using, not the burner's duty cycle. There will still be some error, since the thermostat doesn't post-purge the system down to room temp, and as it cools the baseboard is still emitting heat at a slowly declining rate until it reaches room temp.
 

Jac04

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These things are designed to turn off the burner when the system temp rises a few degrees above the setpoint.
Mine will sometimes get about 5 - 10 degrees above the setpoint briefly, but will then modulate down. It still seems weird to me that the burner shuts down within a few degrees of the set point. Maybe it depends upon where the unit is on the turndown curve. Maybe it's that the unit is already at minimum modulation, so it shuts down the burner instead of modulating down.
 

Dana

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That's right- if the AWT of the radiation has it emitting less than the min-fire output of the boiler at the given setpoint, the AWT rises and the boiler turns off the burner until the temperature drops lower than the setpoint. The size of that temperature swing between turn-off and re-fire is usually a bit less than 10F, but on some boilers it's programmable.
 

MikeG88

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I had a Navien trained plumber out late last week who seems very knowledgeable in modulating-condensing systems etc. He went through the unit with tech support on the phone and it does seem to have a low Delta-T, especially after its been running for several minutes. Measured the difference as nine degrees scrolling the the readouts accessible from the momentary switch on the circuit board. (Was running it at a supply temp of 160 and had 151 return).

Over the weekend I wimped out and turned the supply up to 180 degrees. I saw -10.5 Sunday morning and -6.5 Monday morning. The unit obviously kept up, I probably could have left it at 160 (or maybe gone lower, but am not really sure how well unit would respond to a truly continuous burn for a multi-day period?)

I also borrowed one of those non-contact laser aimed thermometers and checked the Delta with that. I still see the <10 degrees delta when running at 160F supply, but when it was at 180F I witnessed closer to 15 degrees delta. I surmise some of that is due to what I learned in high school physics that the hotter the water -the faster/greater the initial drop in the cooling curve. Secondly since it was actually so cold- the btu were actually being "used" more. Also I didn't notice any short cycling during the extreme cold, but I suspect it did as it got warmer outside, so I returned it to 160.

At this point I am planning to have the plumber install an ECM based circulator (as previously suggested). Currently the major electrical suppliers in CT offer a $100 rebate if you install one, so that takes a chunk of the sting out of the cost.
 

Jac04

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...but am not really sure how well unit would respond to a truly continuous burn for a multi-day period?
You shouldn't have any problem at all running continuously. As a matter of fact, the 'perfect' system would run continuously all winter long and modulate supply temp to satisfy indoor temperature. During those few really cold nights we had, I had some really long burns - like 12-14 hours. I probably could have gone longer, but I manually bumped up the supply temp in the morning for a faster recovery from my night setback (I usually set back to 68 at night, but kept it at 69 for the 2 really cold nights).

For a point of reference, I'm running the CH-180 for a 2100sf house near BDL. On the coldest past night (-11F), I only had my supply temp set at 140F. T-stat was set to 69, and my actual temp at 6am was 68. I'm lucky that I have a decent amount of baseboard.
 

MikeG88

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Dana- I noticed in the instructions for the Taco Bumble Bee that it states: "NOTE: When set in “dE” or “SP” variable speed operating modes, the circulator will always run at full speed for the first 3 minutes to ensure consistent water temperature for the sensors. " My concern is that 3 minutes at full speed might be enough to make it short cycle before it gets the chance to do it's thing?

Jac- thanks for the info on "long burn times"
 

MikeG88

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Dana (and/or anyone else with knowledge of using a "smart" circulator to tame short cycling due to low Delta-T) Looking at other pumps due to my concern over the Bumblebee running 3 minutes full bore at start up- possibly exacerbating the short cycling- I noticed the Taco Viridian VT2218 instructions state: "In Delta-T mode, the VT2218 will start at the lowest speed. If it reads a temperature differential (∆T) LESS than its target, it will continue to gradually increase speed through a 3 minute “Start Cycle” to its highest speed, until target ∆T is reached. “Start Cycle” will be indicated on the LCD screen in place of “Watts”. If ∆T is GREATER than target, the pump will immediately go into Variable Speed operation and vary speed to maintain the target ∆T. "

That sounds like the pump runs faster to decrease the Delta-T, sound backwards from what I am looking for? I believe my small Delta T is caused by my 3 speed Taco not being able to slow down enough for my small demand.

My plumber had mentioned the Bell & Gossett Eco Circ and the Grundfos Alpha as possibly being able to throttle down the speed maybe low enough to tame the Delta-T.
 

Leon82

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I have the 2218, 2 of them,and it does start at low speed. It rarely goes above 8 watts unless its very cold out because my fin tube is running about as low as it goes. Right now the water temp is 110 and I have 7 delta t for the long zone and 4 for the shorter one. I have both zones jumped in the relay so they always run together. 9 delta t at the boiler.

The alpha I think needs constant power. The 2218 remembers its settings
 

MikeG88

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Thanks Leon- my concern is I think I want the pump to slow down when the Delta T is low- not speed up!? I am trying to tame a low Delta T causing short cycling, If the pump runs faster when there is a low Delta- I only think that will make the problem worse- not better....
 

MikeG88

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I called Taco Tech support today to ask about my concerns with the Bumblebee and Viridian VT2218. I was told that none of their pumps was really the cure for a low Delta-T caused by too fast a flow through the secondary loop. The rep told me they do have a pump "slower" than the 3 speed I have now, but he recommended I try restricting the valves a bit to cut the flow down. That makes some sense but seems easy to bump the handle and screw it up (or when purging or cleaning etc), or you could take the handle off, but you may need to tweak for different seasons and/or when you run at different supply temps? Someone else told me flow restrictors are available, are their any that can be user adjusted as necessary, and maybe easy to go back to a certain setting. Sounds more accurate than twisting the handle on a valve.
 

Leon82

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If want a pump you can run at minimum speed the vt1816 has a dial where you can set an infinite amount of speed(assuming you can turn a screwdriver a trillionth of a degree). It also has pressure modes. But it covers just about all the taco pumps in its chart.

I have never seen my 2218 rev up from low delta t. It has 4 fixed speeds but the first one is faster than the minimum speed in delta t mode.
 

MikeG88

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Thanks Leon- the VT1816 also looks interesting. It has a wide range like the Bell & Gossett Eco-circ the plumber who came out had first recommended. I'd rather spring for a good pump than play with manually restricting the flow unless it's adjustable/calibrated...
 
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