Residential Hydronic Heat Boiler Odd Temp Readings

Discussion in 'Boiler Forum' started by JohnnyA, Sep 13, 2019.

  1. JohnnyA

    JohnnyA New Member

    Sep 13, 2019
    New jersey
    If a T&P gauge is too close to expansion tank could this cause the temp READING to fall during flame on and rise during flame off? Scenario...Cold boiler, turn on. Flame on, temp reading rises to 180 then flame off. During flame off temp reading rises to 190, then falls to 180 and boiler relights. Temp reading falls to 170 then flame off. During flame off reading rises to 190, then falls to 180 and boiler relights. So on and so on. Circulator pump runs as normal. (On while thermostat is calling for heat.) Expansion tank is installed off of a tee on vertical supply pipe of boiler approximately 4' from boiler. Pump is installed on vertical boiler return piping, at boiler, with flow going towards expansion tank. (Through boiler and out of supply)
    Thanks much for any information!
  2. JohnnyA

    JohnnyA New Member

    Sep 13, 2019
    New jersey
    Hey folks. I inherited this boiler set-up. A year ago the circulation pump failed and I replaced it. It was the first time I had worked on a hydronic system so I spent months learning what I could from the world wide web and talking to folks in the industry. I know the system set up is...not right. Theres a chance that the heat exchanger is leaking. Although I have replaced some valves and installed a new expansion tank, Im not going to reconstruct the existing system if I have a bad heat exchanger. So, I'm checking it for an exchanger leak. While doing so, I realized the that according to the T&P gauge, the water temp was rising directly after flame off and then lowering before relight. How does the water temp rise when there is no flame, and fall when when there is? I'm 99% sure the gauge is good so the only thing I can think of is that the temperature fluctuation is only in the pipe coming off of the tee on the supply piping and connecting to the expansion tank, due to the expansion tanks location. But, I don't know. I'm thinking I'm going to at least need to replace the T&P gauge that is connected to the heat echanger. The original one. The broken one. At least I can continue to check the heat echanger for leaks. The system actually seems to run well. Don't know about efficiency yet.
    Anyway, if any of you folks have any ideas about this temperature issue, I'd really appreciate hearing what you think.
    Thank you.
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  4. Dana

    Dana In the trades

    Jan 14, 2009
    Some pictures might be useful for figuring out what's going on with the temperature overshoot. If the circulator pump is turning on/off with the flame it could be completely normal.

    While the placement of the expansion tank relative to the pump is sub-optimal for protecting the pump from cavitation issues it's usually not a show-stopper or disaster. If you can hear the pump cavitating raising the system pressure a bit is usually enough to quell that in most residential systems.

    If the heat exchanger is leaking the system will lose pressure over time unless there is an auto-fill valve. With an auto-fill there is usually a separate valve for isolating it from the potable supply. By isolating it and monitoring the system pressure it should be possible to determine if there is truly a leak.

    Since you're running the boiler when it's not that cold out, can I presume it's because you're heating the domestic hot water with the boiler too? If yes, is it a tankless coil embedded in the boiler, or is it an indirect tank operated as it's own zone? With either, pinhole leaks in those heat exchangers will cause the system to gain pressure over time.

    At what pressure are you running the system? Did you pre-charge the new expansion tank to that pressure (or a tiny bit higher) before installing it?

    What is the high-limit aquastat set to?

    How much radiation is there (zone by zone, if broken up into zones)? Is it all fin tube baseboard, or is it cast iron (rads or baseboard)?

    What boiler model/make /size is this?

    Since you have a heating history on the place, it's useful to run a fuel-use based heat load calculation on it to

    A: determine the oversize factor


    B: determine the needed capacity for any replacement solution, if that becomes necessary or desirable.

    Most cast iron boilers out there are 3-4x oversized for their actual loads, which becomes an efficiency issue. If the boiler looks like it has more than just one or two seasons left in it, installing a heat-purging economizer control can be worthwhile.

    BTW: Paragraph breaks make it easier to read.
  5. JohnnyA

    JohnnyA New Member

    Sep 13, 2019
    New jersey
    Hi. The circulator pump on my system does not turn off during flame off. Stays running until thermostat reaches user set temperature.

    Expansion tank was pre-filled but I topped it off to 13.5 lbs. before installation.

    Only running boiler at this time for testing purposes. Have separate gas hot water heater.

    Hi limit is set to 180°.

    Fin tube baseboard units only.

    The house is a rancher. One floor about 860 sq. ft.

    Two zones. Approximately 35 linear ft. of copper tube/baseboard units in one zone, and 40 linear ft. in the other.

    Boiler is a 72200 BTU/H Utica boiler model M-100-AGB 2I manufactured in 1983.

    When I replaced the pump, the existing one was installed to flow through the 1" return piping, Y off into the 3/4" pipe and baseboard units, and into the supply side of boiler. I can only imagine it was installed that way due to the placement of the expansion tank. I installed the new pump to flow into return side of boiler, through heat exchanger, out supply side, through 2 zones of 3/4" pipe/baseboard units, converge into the 1" return line via the Y, and into return side of boiler. I assumed that the return line has a larger diameter for a reason. Two zones converging into one pipe?

    The reason I thought it was a leaking heat exchanger is because end of last winter when boiler wasn't running often, I noticed that the flame rod assembly was rusty. Then I realized there was water dripping onto the assembly when boiler was cold, and auto-fill valve supply is open. If I closed the supply to the auto-fill valve, no drip. At all. It took a few hours before the drip began after opening supply to the auto-fill valve.

    The leak appeared to be coming from the heat exchanger.

    Pressure hasn't dropped all summer while boiler was off. Also, no water leaking.
    I decided to remove the boilers outer shell to get a good look at heat exchanger while I run the boiler to cause pressure and expansion in heat exchanger, and look for a leak while operating and after cool down. This is when I noticed the temperature fluctuations.

    At the moment, I have not been able to recreate the leak. Although, when I moved my attention away from the heat exhanger, I noticed a malfunctioning Hy-Vent air bleeder that may have been leaking water down the copper piping, being soaked up by the insulation under heat exchanger, and dripping onto the flame rod/igniter assy. Kind of makes sense why it would take hours for drip to occur. I disassembled and thoroughly cleaned the Hy-Vent and it is not leaking at this time. Waiting Until tonight when it's cooler to run boiler again. I will raise the pressure a bit as you recommended as I believe I can hear pump cavitation and the gauge is only showing operating pressure of 15-17 lbs. although I'm not sure I trust the gauge due to its placement.

    So, my questions at this point are if the Hy-Vent was leaking enough to cause the dripping, does it make sense that it only does so when there is supply to Auto-fill valve?

    Does circulator pump flow direction matter with this set-up?

    If there is pump cavitation would this cause the odd temperature readings?

    And lastly, being the boiler is 36 yrs. old with an outdated/discontinued White Rodgers control, is it worth spending the time and money to reconfigure the piping system, or is it time for a new boiler?

    After I post this I will go to the link you provided to read about running a fuel-use based heat load calculation on it. I will also look into the heat purging economizer control.

    Thanks much for the info Dana AND for the heads up on using paragraphs!

    Hope attached pic helps.

  6. Dana

    Dana In the trades

    Jan 14, 2009
    The M-100-AGB is a 100KBTU- in/ 83KBTU-out boiler, probably similar to the MGB series (?) and ridiculously oversized for a house that size. The I=B=R net 72K rating would only be relevant if the boiler is fully outside of conditioned space, say, in a garage separated from the conditioned heated space by an insulated wall, or in a ventilated crawlspace under an insulated floor. It looks like yours is in a basement(?), where at least some of the standby losses still accrue to offsetting some of the heat load. (If the basement walls are insulated to the current code minimum virtually all of those standby losses would be heating the house, not the ground, and not "the Great outdoors". Check back on how to do it without creating a mold farm if that's a project you want to take on.)

    Is the boiler also heating the domestic hot water with an embedded tankless coil or indirect fired tank? (I'm guessing not if the boiler was off for the summer.)

    A dripping vent will only keep dripping if there's enough pressure on it. If the auto-fill is isolated from the potable supply the pressure will drop, and so will the drip, as the system pressure drops.

    The circulator direction isn't going to affect the leak, only the amount of cavitation if it's mis-located relative to the expansion tank. If the pipe run into the circulator is pretty straight for at least a foot or three the cavitation issues can be pretty low even if pumping the "wrong" way .

    The way the expansion tank is mounted in the picture looks like a disaster waiting to happen unless there is something mechanically supporting the weight of the expansion tank. That's quite a moment-arm of torque bearing down on that iron tee, and over time with temperature cycling & vibration it's only a matter of "when", not "if" it's going to break.

    At 180F out the average water temp (AWT) through the radiation would be about 170F (unless way over-pumped, which it might be. At an AWT of 170F typical fin-tube baseboard delivers about 500 BTU/hr per running foot. So with 75' of baseboard you only have enough heat emitter for 500 x 75' = 37,500 BTU/hr, which makes that boiler more than 2x oversized for the radiation even if it were operated as a single zone, but 4x oversized for any single zone, which means even at the coldest outdoor temps it's doing lot of unnecessary on/off cycling taking a toll on both efficiency and putting wear & tear on the ignition components.

    A heat purging economizer would reduce the total number of cycles and lower the average operating & idling temperature of the boiler, saving something like half the standby & distribution losses, cutting fuel use by something like 10-20%. But if the boiler is actually leaking between the heat exchanger plates it may not be "worth it".

    The design heat load of a 2x4 framed 860' rancher with single pane windows + clear storm windows over a full basement at at a typical +10F inland-NJ type 99% outside design temp would be 13-15,000 BTU/hr if it's pretty tight and the basement walls are insulated or 18-20,000 BTU/hr with no foundation insulation and/or leaking excessive air. That makes even the zone radiation 2x oversized for the design load, and the boiler 8x oversized for any given zone, which means a very low duty-cycle with lots of standby loss.

    With that low duty cycle the fact is it has relatively few burner hours compared to the same boiler serving a design heat load of ~60K, (which would have made it right-sized.) But it's also probably seen 4-5x as many ignition cycles as a right-sized boiler would have over the same number of years. From an ongoing reliability issue point of view the basic boiler probably still has a lot of life left to it, but the controls and ignition stuff hanging on it may have to be repaired/updated. But there may be other reasons for changing it out even if it's otherwise in good shape:

    Run the fuel-use load numbers, but if the 75' of baseboard only needs to deliver 20,000 BTU/hr even at design condition, that's only 268 BTU/hr per foot, which it could deliver at an AWT of 130F (or less), which means a right sized modulating condensing boiler or a condensing water heater (isolated from the potable water by a heat exchanger) would improve the overall efficiency by quite a lot! There are some pretty good easy-to-retrofit stainless fire-tube boilers out there for under $2K. Bricking up the old flue port would also reduce a 24/365 stack-effect outdoor air infiltration driver that raises both the heating and cooling loads for the house.

    Do you have central air? If yes, are the ducts in the basement, or did some idiot (as is all too common) install them in the attic above the insulation, where it increase both the heating and cooling loads for the house?

    If not, do you WANT central air? The typical cooling load of a house that size is about 3/4 ton- 1 ton, and there are modulating ducted cold-climate heat pumps in the 3/4-1.5 ton range that are capable of delivering enough heat for a house that size (if sufficiently tightened up) at good to excellent efficiency. If it's an open floor plan even higher efficiency (and capacity) can be had with ductless solutions (often at an even lower upfront cost), but heat distribution to doored-off rooms has to be looked at carefully. In most markets high efficiency heat pumps still have a somewhat higher operating cost than condensing natural gas boilers, but not all. In low-priced electricity markets it can be less expensive.

    With the fuel-use heat load calculation numbers we'll have a better idea of what makes sense in the near term. Most boilers can tolerate even 210F without undue stress, so even though the reason for the temperature overshoot to 190F isn't clear (could be an aquastat calibration issue, could be the aquastat's sensor isn't well coupled to the sensor well- some need to use a thermal goop work well), it's not doing anything bad to the boiler. The bigger issue is the constant on/off cycling.

    What is the boiler's low-limit temperature set (or the differential, if it's the type of aquastat that is set to refire when xx degrees below the high temp)?

    When fire up the boiler, start measuring the burn times and duty cycle when it's cycling on/off during an extended call for heat.

    In the near term it's fine to just lower the boiler temperature and keep the high/low difference as wide as possible to keep the burn times as long as possible, utilizing the thermal mass of the boiler to the best extent possible with dumb aquastat controls. For low money (really low, as a DIY) a smart heat purging economizer control would take that a step further and fully optimize the use of the available thermal mass, if you decide you'll be keeping the boiler for several more years.
  7. JohnnyA

    JohnnyA New Member

    Sep 13, 2019
    New jersey
    Definitely very similar to the MGB series. I took care of the issue of the expansion tank hanging off of the piping with no support. Thanks! I'm ninety nine percent sure that what I thought was an exchanger leak was actually the faulty Hi-Vent. I'm very grateful for that. I will of course keep an eye on it.

    And yup, the central air ducts are indeed in the attic. I've seen it many times and I thought it was ridiculous, I'm glad I wasn't too far off on that. They are wrapped with padded aluminum but, come on..

    Windows are only a few years old. Last year we had the attic insulated with cellulose loose fill insulation.

    The boiler's hi-limit I have set to about 170 right now. There's no setting for lo-limit, but the differential is 8°. This morning it was 60° ambient inside temperature, boiler cold. I set thermostat to 73°. First burn was 10 Minutes and 4.5 mins until relight. After that it was Flame on for 2.5 minutes and 4.5 minutes between relights. After 6 or 7 cycles, thermostat was at 73°. I stopped there because after 45 minutes the ambient room temperature was still at 73°.

    I was going to lower Hi-limit to 165° or 160° and see if it increases burn time. Do you think that is too low? Also I'm wondering if the burn time or cycle time will change in the winter when it's much colder, or just the number of cycles to reach thermostat set temp?

    Meantime, I'm looking into a heat purging economizer as well as the other information you provided me. I hear you about the over-sizing, heat load, fuel-use load etc., and I am researching, ciphering and calculating.

    Thanks you very much for taking the time to help me out Dana. I really appreciate it. If you ever find yourself needing information about welding, metal fabrication or metallurgy, I can definitely return the favor.
  8. Dana

    Dana In the trades

    Jan 14, 2009
    A differential of only 8F isn't enough for the minimal amount of thermal mass you have to work with. The (2.5m/7m=) 36% duty cycle isn't terrible, but the 2.5 minute burns are.

    For the average efficiency of a cast-iron boiler to approach it's steady-state thermal efficiency needs minimum burn times on the order of 10 minutes. Even 5 minute burns aren't too bad, taking a ~1% hit in average efficiency, but at 2.5 minutes (150 seconds) it's slipping over an efficiency cliff. It takes at least 15 seconds from the initiation of the ignition cycle for the boiler hit steady state- that's on the order of 10% of the total burn time. Every ignition cycle wastes some amount of fuel.

    If you can adjust the differential with the existing controls, set it to the maximum to deliver longer burns. The 4.5 minutes between burns will increase, as will the burn times. The duty cycle will be about the same, but ever so slightly shorter- a measurable amount if using a data logger with resolution down to 1 second.

    A heat purging economizer would automatically adjust the differential swings and the high limit, and with most at the beginning of a call for heat would purge heat out of an already-warmed boiler down to wherever you set the low limit before firing. That way when the house is already warm and it's just raising the temperature a half-degree or whatever to satisfy the thermostat, it guarantees the longest and most efficient possible initial burn. Sometimes the thermostat may be satisfied with no burn at all. Other times the thermostat may interrupt that initial burn shortening it, but when that happens it means the boiler's temperature isn't as high during the idle period, and thus incurring lower standby losses. The algorithms employed are all proprietary and differ slightly, but the gist of it is the same across most vendors.

    The 36% duty cycle of your burns during a long call for heat is a function of burner size and radiation size, and pretty much fixed- it won't change appreciably during the winter. As it gets colder outside and there is more heat loss the biggest change is the amount of time between calls for heat from the thermostats. During milder weather once the house is up to temp it could be hours before the next thermostat call to just maintain room temp within the dead-band range of the thermostat. During colder weather it might only be a few 10s of minutes. But with your current radiation and measured duty cycle, with an 83,000 BTU/hr boiler it means even at temperatures well below the 99% temperature bin the actual heat load can't be more than 0.36 x 83,000 = 29,880 BTU/hr, since that's the most this particular boiler + radiation combination delivers. A ratio of 29,880/860'= 35 BTU/hr per square foot, which is still crazy-sized for a reasonably tight 2x4 framed house, probably getting onto twice what the house actually needs even during cold snaps.

    With the high limit set to 170F and an 8F differential you're looking at an average water temp of about 160F, and it's delivering 29,880 BTU/hr. Lowering the high limit would lower the heat emitted, and the burns would be come shorter, and the time it takes to satisfy the thermostat is longer. But if my estimate is right that you only need half that heat rate out of the heating system, it means you have enough radiation to deliver the heat at an average water temp of 130-135F, which is cool enough that a condensing boiler would be running at condensing efficiency the vast majority of the time. It also means there is enough radiation to heat the house nicely with a condensing water heater, since it's unlikely you'd ever need heating system water hotter than 140F.

    Too bad the ducts and air handler are in the attic- there are some really great modulating ducted mini-splits out there that can still deliver more than 15,000 BTU/hr @ 0F with reasonable efficiency. The ducts in the attic would be an even bigger efficiency problem during the heating season than it is in the cooling season, but it's indeed a pretty stupid (and most common) way to do it.
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