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Community and Q&A

New Boiler Problems

J Chesnut | Posted in Mechanicals on

I replaced a 30 year old boiler with a high efficiency modulating, condensing boiler. My radiators and piping are the original cast iron. Above the boiler the supply pipe tees running a line to 3 radiators in the front of the house and the other side running to 5 radiators in the back. With the new boiler in operation the 3 front radiators get much warmer than the back radiators so apparently most the water is running the path of least resistance.
Question is will it be simply enough to insert pressure balancing valves on each side of the tee (which is piping in 1″ copper) to divert more water to the back or might there be other variables involved?

Also, I used to use a programmed thermostat but with the new mod-com boiler the lag time to heat back up from the lower setting takes hours and hours. This may be in part due to the unbalanced distribution but my impression is that with a high efficiency mod-con boiler using the outdoor reset function setting the thermostat back during the day and at night with a programmable thermostat does not work well because of the slow response times. Have others had the same experience?


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  1. Michael Chandler | | #1

    I would suspect an air bubble creating a partial airlock in the back radiators and would isolate and flush those
    lines and the entire system to purge any residual air. If you have an air elimination device I would go ahead and replace it too. They are definatly what we charitably call a "service item" or failure point.

    I don't see why a mod-con boiler would have such a slow response time but if it is feeding less than half of its design load the water could be returning to the boiler at too high a temperature creating low flame complications by forcing it to modulated to the very bottom of its range.

    We've seen sooty burns at low BTU output creating soot accumulation on the heat exchanger to the point of causing combustion air starvation due to exhaust back pressure. Can you put a data logger on it to see if its short cycling?

  2. J Chesnut | | #2

    I have the old freestanding cast iron radiators with bleed valves. I've open all the bleed valves and immediately get water released. Could I still have an air bubble? I do get some heat in the back radiators but the temp difference is very noticeable.

    An air eliminator is installed per the Buderus instructions. Just installed it a couple of months ago is there a way to tell if it failed?

    Thanks for the explanation about the data logger and the possibility of checking for short cycling. I wasn't impressed with the guy I had out to commission the start up. I'll know more to ask the next guy if I have someone out.

  3. Michael Chandler | | #3

    Yes you need to circulate water through the lines to dislodge any bubbles that may be adhering to the walls of the pipes creating a flow restriction. Fill a bucket with water with the rust inhibitors etc that you may have in your system (Sierra anti-freeze?) And run a pump from there into the system out through the radiators and back into the bucket holding the hoses under water the whole time until you run a good rapid stream with no bubbles coming out for three minutes or so. The boiler manual should possibly have details on how to do this.

    The air bleeder is probably not the problem. They generally take at least six months to freeze in the closed position.

    An error of commission?

  4. Riversong | | #4


    The supply piping should always run slightly uphill to the radiator. In older houses, if there is any settling of the floors, there may develop a high spot in the lines creating an air trap. Radiators, too, should be tilted slightly upward toward the bleeder side.

    Sometimes increasing the system pressure can help dissolve trapped air.

    Are the radiators piped in series or with monoflo or venturi tees? If in series, there will be a significantly larger pressure drop through the five that are farther away than the three that are closer to the circulator. If it's a monoflow parallel system, make sure the tees are installed in the right orientation.

  5. J Chesnut | | #5

    Thanks Micheal and Robert for your responses.

    Robert, the radiators are piped as a two-pipe hydronic system.
    I'm assuming the piping is original to the house (built in 1907) and that it used to function as a gravity system (without pump).
    I'm not familiar with the word 'monoflo' but the tees I believe are just simple tee connectors that don't function to restrict or direct flow.

    My brother (a plumber) and I installed the system ourselves. When we flushed the system we didn't use a pump to push water through it, we just filled it up several times and "gravity flushed" it till the water came out clean.

    A reducing valve now keeps the system at 20psi. When I had the old boiler I used to have to add water to the system every month or so to bring the water pressure back up.

  6. J Chesnut | | #6

    I wanted to mention also that I'm hearing a new noise emitting from the pipes every once in awhile that I had not heard with the old boiler.
    It sounds similar to something metal being dragged on concrete but not very loud. The noise lasts for a couple of seconds then stops.
    Does not sound like pipes rattling or water hammer.

  7. Riversong | | #7

    It's a poltergeist, dragging its chains. No wonder its chilly inside. Perhaps you need an exorcist or the Ghost Busters.

  8. j chesnut | | #8

    Funny thing too Robert is that I'm the only one who hears these sounds. I probably need more help than an exorcist alone can provide : )

  9. Roy Harmon | | #9

    J, Is it possible that your house was originally heated by a steam system~ later converted. Piping for a steam system would have been larger in diameter, and would pitch back toward the boiler so that as the steam cools to condensate, the water flows back to the boiler rather than circulate in one direction. Sometimes when this type of system was being converted, not all of the larger diameter pipe got replaced. The sound that you describe reminds me of a project that I was involved with. It required the removal of original large diameter pipe entirely and replacing it. Still not exactly sure how the noise was being made by this configuration, but the pipe replacement did solve the problem.

  10. aj builder | | #10

    J, the boiler rep for your area if he would do a site visit is the way to go. You need full flow ball valves on your two loops to adjust their flow. Also need your whole system checked by someone onsite. It may be one simple change that you made setting up your new boiler but that one item is not that discernible via a few posts here.

    Let us know how you make out.

  11. j chesnut | | #11

    I'll plan to do that. The system was started up during the hottest part of the summer. We got the burner going for the indirect DHW and everything checked out. Even the space heating is working it is just noticeable the temperature differential between the two branches. Something just needs to be tweaked.

  12. aj builder | | #12

    Do you have manual valves on each zone?... and do use a standard wet floor water pump in a pail to make sure you have all air out as per Chandler's advice.

    Here's link to lots of better air eliminators... sometimes good to have crap trap style one if your pipes are spewing lots of junk since they are centinarians.

  13. aj builder | | #13

    Do you have manual valves on each zone?... and do use a standard wet floor water pump in a pail to make sure you have all air out as per Chandler's advice.

    Here's link to lots of better air eliminators... sometimes good to have crap trap style one if your pipes are spewing lots of junk since they are centinarians.

  14. J Chesnut | | #14

    I have the Honeywell PV Series Residential SuperVent which is listed on your link.
    I was surprised at first when Michael said the problem might lie with the eliminator because my impression was the Honeywell Air eliminator looked like a decent piece of hardware.
    The house is not zoned so there is one valve wired to the thermostat.

    As Roy points out I have larger diameter piping seen in the basement, ~2" cast iron. At the radiators it looks like 1.5" some of which has obviously been replaced over the years.

    Over the last couple of days I've set my thermostat at one temp and as the weather is getting colder the boiler is able to maintain the temp. The temperature differential has balanced a bit between the boilers at the front of the house and at the back.

    My sense is still that the system isn't running optimally because most of the water if following the path of least resistance and if I added pressure valves to balance the amount of water heading towards the back of the house with that running to the front. The digital display on the Buderus GB142 reads that it is operating normally but I do plan to talk to the local product rep just in case.

  15. Riversong | | #15


    If you could post here or link to a schematic and/or pictures of your distribution system, it would be easier to diagnose the problem.

    With large diameter pipes, the water velocity is going to be quite low and if the 5 back radiators are much farther from the boiler than the front 3 and the thermostat is near the front 3, then the boiler will shut down before much heat can get to the rear.

  16. J Chesnut | | #16

    Robert I think this is exactly the case. Not only are there 5 boilers on one side of the tee but those 5 boilers are further away then the front 3. My house footprint is 24' by 24' and is two stories with a basement. The boiler is nearly in the center of the house towards the front. The back boilers are at the back corners of the house while the front boilers are located closer to the front center. A schematic probably wouldn't add much info beyond this.

    I'm assuming the fact that I'm using a variable speed pump (Taco Variable Speed Delta T (00-VDT) that the water velocity (and water temp) is lower and more constant than with the previous system so this exacerbates the different quantities of heat reaching each radiator compare to the old boiler setup.

    The temperature differential between the front and back radiators is much more pronounced when I'm trying to raise the temperature.

    My home is getting heat. Do you think the system will work better if I add 'pressure valves' (like what you see on manifolds) so I can balance the quantities of water heading in each direction of that initial tee? I'm also wondering if I can make the system work with setting back the temps during the workday and at night. Currently if I set the temp back 5 degrees it takes over a day to bring the temperature up those same 5 degrees. (This aspect I believe depends on the outdoor temperature and the settings of the outdoor reset. The warmer it is outside the lower the boiler fires which I think is the reason it takes so much longer to raise the temperature as opposed to maintaining a temperature.)

  17. Riversong | | #17

    You've got a high volume, high mass system which apparently has too slow a response time for a setback strategy to work. If the basement is only semi-conditioned and those iron pipes aren't insulated, then you're also losing heat through that extensive cast iron heat sink in the basement, which is absorbing a lot of the heat before it can reach the rear radiators.

    It sounds like your heating contractor didn't know what he was doing. I would suggest getting someone in there who does.

  18. Roy Harmon | | #18

    I've actually been where you seem to be at this point, went through a similar school of " soft knocks"~ pun intended~ and the solution ended up being the replacement of 2" cast iron pipe with smaller pipe. It was actually pretty simple and the system balanced very well. I would definately do a system Flush, as Michael Chandler suggested as well, before the pipe replacement.( If it turns out that's what's needed).

  19. Michael Chandler | | #19

    The issue with the air bubble is in how the lines are commissioned after they have been open for service, the air vent by itself will not get all the bubbles out of the lines even if you have a good one rather than those cheap ones we see from Graingers. The flow in your system is not fast enough to dislodge the air bubbles clinging to the sides of the pipes so you want to isolate one line at a time and flush the bubbles out with fast moving water to loosen the air bubbles and move them to a point where they can be removed such as by pumping through an open topped five gallon bucket. The "gravity flush" you describe won't clear these bubbles. All you need is a sump pump and a five gallon bucket and a couple of washing machine hoses.

    I recently picked up a 1941 copy of my great-grandfathers book "Flow of Water in Pipes and Pipe Fittings" originally written in 1892. It's amazing how much impact turbulence from clinging air bubbles had on their equipment and consequently the effort they put into purging air from lines back then. The work was largely having to do with municipal water distribution to fire sprinklers but is still referenced in the codes to some extent.

  20. aj builder | | #20

    Any competent HVAC pro that knows your particular modcon could figure out your needs. Get on the horn.

    You mentioned a tee many times and now you say you have two variable speed pumps. If you have a pump on each loop , each loop should work well, that is if all is set in one of many correct manners. Not sure why or how you are using the variable speed? You also never said what btus you had and what btus you now have from your old and new boilers. You also can run your boiler without the set back and you can change temp settings. What are the outgoing and incoming temps and do you have primary and secondary loops.

    Get some help onsite.

  21. Riversong | | #21

    I don't think this is a boiler problem. It's a distribution problem, so you don't need someone familiar with the boiler, but rather someone who actually understands distribution. I suspect you'll have to reduce the pipe sizing, as Roy suggests, but it may also have to be reconfigured and balanced.

  22. aj builder | | #22

    Setback recovery time is dependent only on btus delivered. Knowing what was delivered with old boiler is pertinant.

    Knowing what the modcon is firing when AM call for heat starts is pertinent.

    Knowing his distribution setup is pertinent.

    Knowing the flow rates and outgoing incoming loop temps is pertinent.

    Knowing what additives are added to the loop water is pertinent.

    Properly filling and purging air as per Chandler's advice is pertinent.

    It all matters. I have not had an issue with oversized pipes yet though suppose anything is possible. The pipes worked with his prior boiler so they should still is my logic.

    Lots of great advice posted J....

  23. Riversong | | #23

    Setback recovery time is dependent only on btus delivered. Knowing what was delivered with old boiler is pertinant.

    Setback recovery time is also dependent on the building's heat loss, since it's still losing heat which has to be made up while the boiler is also trying to add additional heat to system.

    And BTUs delivered depends on the boiler output, the efficiency of distribution (including distribution heat losses), and the heat delivery rate of the radiators.

    Knowing the old boiler's output is largely irrelevant, since most old boilers were oversized. Knowing the actual heat loss (and hence heat requirement) of the building is what matters.

  24. aj builder | | #24

    Bull pucky Robert. In this "experiment" only one change was supposedly made on your list, the boiler! That is why I asked about old verses new boiler btu outputs! Ok, two changes...we don't know if the install was commissioned properly and completed competently near the boiler regarding valving, loops and pumps. But the radiator and their piping stayed unchanged Robert so nix that aspect.

    Look, the heat loss didn't change Robert, it's the same house!

    Distribution, same!

    As I have repeatedly asked about btus output both now and prior setup, I am fully aware of the possibility that the old boiler was much larger and oversized.

    You are on one high horse Robert.

    Once in awhile others know what they are posting about besides you.

    Love you... LOL

  25. Riversong | | #25

    AJ Low Horse,

    A boiler does not a heating system make.

    A heating plant and all its associated equipment must match the distribution system, and both together must match the heat load of the house. If the systems are incompatible, then there will be a problem in providing heat. And, as you acknowledge, some of the distribution system - such as circulator, valves, connectors - was undoubtedly changed and they may either not be appropriate for the antique piping and radiators or may have been installed incorrectly.

    I stand by my statement that this sounds like a distribution problem.

  26. Riversong | | #26


    You mentioned that a variable-speed circulator was installed. That unit is controlled by the supply and return temperature differential (which can be set for 5°-50°), so where the sensors were placed and what the differential was set for can make a difference in the heat supply to the two loops.

  27. aj builder | | #27

    Assumptions... by the truckload...most likely temp dependent pumps but..... there are other types. And... used with a modcon that is set up to vary btu output by return temp is not the standard set up of a modcon. Modcon normally are set up with constant speed pumps.

    None of us know what's hanging on J's utility wall by his boiler.

    Low horse rider.... that's me LOL

  28. Riversong | | #28

    No, AJ, you don't seem to know.

    But J Chesnut clearly stated that he has a Taco Variable Speed Delta T (00-VDT), which determines pump speed by Delta-T between supply and return.

  29. J Chesnut | | #30

    I'll definitely purge the distribution system of air bubbles per Michael's instructions.

    Robert I didn't realize the variable speed pump can be set for different temperature tolerances. I'll investigate this further.

    I'm fairly confident that the boiler itself is working as it should. The install with all the valves and other components followed the Buderus schematic exactly. While I made a comment about the person who commissioned it what he did does check out with the Buderus commissioning manual. Also I think if the system wasn't working correctly this would appear on the diagnostic function on the digital display.

    AJ, there is only one variable speed pump the pushes the water through the supply. In addition however the boiler itself came with a prefabbed primary loop that complete with a 3 speed pump. Both the Buderus rep and the city desk person at the plumbing wholesaler described how installing a variable speed pump represented less electric consumption. I have played with the low and high temperature setpoints on the outdoor reset algorithm. I plan to talk to the Buderus rep again about what they recommend for these.

    Of course the system would work better if I re-plumbed the entire distribution system (and exchanging the old radiators for wall hung radiators) but thats a lot of $$$$$ and I have other things to finish on the house before I start tearing up more stuff. Of course luck has it that, it would be relatively easy to re-pipe the distribution to the front 3 radiators. The back 5 have a lot of footage embedded in the walls and floors.

    If anything interesting comes up as I work on this I'll post again. Thanks for all the input.

    Can't find my paperwork right now but the old boiler was rated at 100K BTU (A.T.A. rated at 80K BTU). The new boiler modulates up to 75,200 BTU. I've recently insulated the walls which were completely uninsulated and have taken measures to make the home more airtight.

  30. aj builder | | #31

    J, your post is enlightening.

  31. J Chesnut | | #32

    If anything it is a testament to learning from your mistakes.
    Just think how much I would have missed out on if I hired a heating contractor.

  32. Anonymous | | #33

    An guess at why your recovery from setback temperatures takes so long The slow recovery could be because the outdoor reset and set back are working agianst one another. The outdoor reset lowers the boiler temperature to only what is required to maintain the heat in the house, not the additional loading required to increase from the setback temperature. as the house tries to recover from the lower tempratures the reset is saying that it does not need the higher temerature water and therefore takes longer. Goods luck this is only my theory.

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