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Baxi 40ga duo tec cycling a lot

So I improved the performance big time of the boiler by fixing the primary loop like ironman told me.

But when a zone, maybe a small zone at 1gom or may be a medium 2 zone of two 1gmp flow valves is on. The boiler fires, and instead of reaching and staying at my set water temp, it overshoots it up to 176(my mah CH tenp setting, options are 113 and 176)

Then pump overrun runs for 3 minutes, and after 3 min firing delay time I have set, it fires back uo and does it all over again.

It is still scavenging heat into the return from supply at close fitting T’s but not as bad when were connected twice in secondary loop. There is a grundfos pump in baci pulling on return, and I have 1 alpha 2 grundfos pump in basement before EP zone manifolds pulling on the supply.

I need to check the expansion vessel insise to see if idiot that installed it set the presseure correctly, but under full pressure boiler says 22.5-23psi, and is 15-17psi static when cold. Could the little expansion tank be effecting it? Or is the gas valve somehow not set uo right? I am running on propane, piped from outside wirh 3/4 black pipe and set on baci for propane.


  • betweentheframebetweentheframe Member Posts: 97
    Also, there is a coear tube coming from the too of the heat exchanger (black composite rectangle to left of firing cyclinder, i have never seen anywhere this is suppose to go?
  • betweentheframebetweentheframe Member Posts: 97
    Anyone know what these do on flu/air vent to outside?
  • betweentheframebetweentheframe Member Posts: 97
    I don’t think this boiler is designed for primary loop, it is killing it causeing it to cycle constantly and not causing it to condensate as much. It has built in air venting and air vessel.
  • betweentheframebetweentheframe Member Posts: 97
    Manual says it has a primary circuit internally
  • betweentheframebetweentheframe Member Posts: 97
    Says has automatic bypass and if radiators are all on TRV, then needs an additional, heating bypss is synonym for primary loop correct?
  • DZoroDZoro Member Posts: 1,035
    Does that internal pump have 3 speeds? If so turn it to speed 1.
  • betweentheframebetweentheframe Member Posts: 97
    No it modulates from min to max in a tight curve. Will attach pic, but I forgot my brother in law’s dad is a an HVAC enginner, mostly very big commercial stuff in SF, but he said yhere is a bypass and a primary curcuit inside this boiler’s schematic and he thinks the primary loop below is a total waste because cannot control circulation of it and not needed especially since it is so far away fron the other pump and manifold(16 ft away in basement) i bet I could run the system on just that pump)
  • betweentheframebetweentheframe Member Posts: 97

    As it stands now, had another set of close fitting T’s I deleter already, i think this thing has air seperator, bypass, primary circuit, air vessel and pump inside to be a one box solution.

  • superdavesuperdave Member Posts: 155
    I just posted to you But I can find it on web site if you are better then me on computer can you post back on line
  • betweentheframebetweentheframe Member Posts: 97
    Just responded to message back, let me know if you can find it
  • superdavesuperdave Member Posts: 155
    I cant find it on web site?? But like any boiler you need to make sure you get the right amount of flow the heat exchanger other wise the water will break down and turn all kinds of ugly. what is your heat load?? the boiler goes down to high teens BTUs what is the DT between the supply and return ?? the pump will change speed in side Baxi.

    I will be MIA for a few sorry for the delay.
  • superdavesuperdave Member Posts: 155
    Where are you located??
  • betweentheframebetweentheframe Member Posts: 97
    Hudson valley.

    What are these pipes called? And shouldn’t theg be bigger like 3:4 the whole way, i only see british listing of boiler tail piece.

    House is small 1600sq ft with two rooms with 24x48 oversized buderus radiators, one 250sq ft staple up under old wide plank with no subfloor with reflective foil and then fiberglass behind that(i want to spray foam that area beloe the foil bubble stuff) and about 250 sq ft of staple up upstairs, and 250sq ft of cement slab on soan decking radiant) run the temp at 140 degrees.

    Piping from that primary loop is 3/4 hepex down to basement feeding 2 supply and 2 return uponor EP manifolds.

    Should these bends be 3/4? Also the baxi 40ga seems to have an internal bypass, primary circuit, air seperator at pump and air vessel(should the air vessel be set to cold or hot system pressure)

    Right now the system runs at about 22psi, it has an autofill on the primary loop with a 15psi reducer and backflow preventer.

    This primary loop sucks, because my return temp from return manifolds is 110 but boiler is shutting down eafly because scavenging the supply temp.

    I have tried the grundfos alpha 15-55f on highest constant pressure and highest fixed speed and that seems to work best but I think the whole system could probably work on just the baxi’s internal pump?

    Here are the pump graphs of baxi internal and grundfos and a pic og f those bends that seem too small.

    Prob about 1200 feet of 1/2 HEPEX for radiant and two radiators about 25 and 35 feet away each. 8 total zones, mostly on about .6gpm flow rates to keep delta 10-20 across supply and return, a little higher for concrete and a little lower than .6 for radiators
  • betweentheframebetweentheframe Member Posts: 97
    Grundfos pump in basement
  • betweentheframebetweentheframe Member Posts: 97
    Tailpiece I speak of. Is it only baxi? Uncommon threads?
  • superdavesuperdave Member Posts: 155
    What was the DT before you had problems at both areas Baxi supply and return and the manifolds. Are you running the same temp for all zones? and if so what are you targeting 140F what do you have the Baxi set up for on temp? I like starting PSI when cold so if you are set at about 15 working PSI that is what I charge system at. I am sorry of I missed it what is your BTUs on smallest zone??
  • superdavesuperdave Member Posts: 155
    I found it!! I hate my computer sometimes.

    On Baxi you don't always need Primmer and secondary loop It depends on your heat load and if the house pump can handle the piping. What are you running for temp on Baxi and also what are you running for radiant temp?? Also what is the loop run per in feet and what it the size of Pex??

    When I am installing new system with radiant I always use a hydro Sep. 548 or a 549. What is the Alpha 15/55 set on?? I like it with radiant on PSI 2 if your pump it not at right speed the system will run a lot harder. What is the copper piping off Baxi sizes 3/4" if so this is part of your problem. Nine out of ten times I go to 1" manifold or hydro.
  • superdavesuperdave Member Posts: 155
  • superdavesuperdave Member Posts: 155
  • superdavesuperdave Member Posts: 155
    I know your boiler room is small but I just wanted to show you both way with hydro and with out. The biggest thing is knowing what you need and what you have for BTUs and pumping requirement
  • superdavesuperdave Member Posts: 155
    On the Baxi info 1 will give you the supply temp info 2 is out door temp (do you have one hooked up) Info 5 will tell you the PSI of system info 6 will tell you the return temp, and I like to look at info 9 is the target temp this is good to look at when you at using out door sensor.
  • superdavesuperdave Member Posts: 155
    How is the DHW working?? also are you getting and E codes or mostly you want to stop turning on and off??
  • betweentheframebetweentheframe Member Posts: 97
    I have 15psi at cold, and full temp going strong is at 22psi, i will check the vessel when I drain it to clean it to see what it is set at(should be 15psi right? What the pressure reducer on backflow preventer autofill is set to)

    The DHW comes out a little hot at first but is stable at 120 after. Usually only a little too hot when CH is running and then you call for water.

    On mine info 5 is the psi of system, info 6 is return temp, return temp is way too high even when big load is on system, i an feel the heat from primary loop supply side crossing over and feel the copper hot side of the T for return closest to the supply, whereas the rest of the return line is fairly cool, say I have system humming along at 150 degrees in order to be hitting 140 on my supply manifold in basement, then my return temp is like 138/140 on baxi info, but my return temp on return manifold in basement is 110/120, I think the baxi’s own pump pulling on return and pushing water into the grundfos circulator pulling on supply line in the basement(which feeds all zones) is confusing the circulator downstairs because it never says it is pushing more than 1/2 gallons on any settingg, and the fact the the return on baxi is pulling harder causes the supply water coming out at primary to be sucked into the return on prinary loop.

    I see a primary loop internal and there is a bypass internally through plate exchanger in baxi, so I am really confused on why it would beed a second primary loop for which I have no circulation controls.

    The smalles zone I have on baxi is a single buderus 24x48 radiator flowing at 1 gpm on flow adjuster. The other zone id 300 ft of radiant staple up and one 24x48 rad, the other zone I have is 3 sections of cement radiant.

    It has an outdoor reset and is all wired up correctly but even when I set it to active it does not select curves like it should and when I read the outdoor temp on info it reads maybe 10 degrees higher than it is(behind boiler under an eve never in sun)
  • betweentheframebetweentheframe Member Posts: 97
    What type of threads are those flat rimmed copper tailpieces? Them seem way too small, like 1/2 in that flares out to 3/4? Should they be full 3/4 or is that how this boiler works?
  • betweentheframebetweentheframe Member Posts: 97
    This is the basement 15 ft away from boiler and 6 ft lower than boiler where curc and manifolds are(yes I know they are upside down)
  • betweentheframebetweentheframe Member Posts: 97
    I have outdoor reset turned off to not do anything, because, well, itvwas not doing anything, seems to have been all wired up for it with the harness and whatnot.
  • betweentheframebetweentheframe Member Posts: 97
    Also the manual mentions nothing about primary loop, only a bypass if all rads are fitted with TRV to prevent dead heading.
  • betweentheframebetweentheframe Member Posts: 97
    The supply amd return lines are cast in a concrete slab on their way to the basement, I cannot change their size, like not even close to possible! But the tailpiece on baci seems to be like 1/2 bend with 3/4 flare.

    My house it totally remodeled wnd insulated in walls and ceiling, only use about 600 gallons of propane a year for everything, but this boiler is reaching over set return temp after a 3 minute pause from firing before it even ignites and finished turning down and I am not condensating because of so much scavenging from supply when I have good cold return water!
  • superdavesuperdave Member Posts: 155
    I have seen others when we do service on boiler they some time use 3/4" right off boiler but this is not NPT.

    If you are losing 10F in about 20' of piping something does not sound right. Check the out door sensor make sure you are not getting heat from the house bleeding out if it stay about 10F off adjust it to your heating curve.

    Again I am still a fan if I am doing primary and secondary I use a hydro separator but if you are only using close Tees I tell people to go at lest 2X the pipe size 3/4" branches I tell them use at least 1 1/4" (so two 1 1/4" X 3/4" tees). But I would ask others for there thought on it also and to replay back.

    If I am understanding right you are looking for 140F supply at manifolds and you are looking for a 20DT at that manifold your return temp targeting 120F ish. If you are not able to get this close you best bet is to use a hydro or up size the piping to help balance the mix. the Baxi pump will always try to change speeds to help keep a balanced DT but if piping is not right size it will fight it self and work 10X harder shorten life of everything. Did you say your DT was coming back after 24Hr still very large that is also not good that means you are not getting enough flow through heat exchange!!
  • betweentheframebetweentheframe Member Posts: 97
    What are the threads coming off those baxi valves? Is that R32? It is a flat rimmed copper tail piece, i can only find it listed in uk sires

    The problem is the pump is sucking the supply water from close fitting T into return water keeping my damn return temp high!

    I need to straight pipe it and I bet my problems are all gone. The baxi pump is pumping harder and creating negative pressure allowing the water fron supply to creep into my return, whereas the baxi has a bypass and a orimary loop internally and would sort itself out with a straight pipe config methinks.
  • betweentheframebetweentheframe Member Posts: 97
    It is getting so hot so fast it short cycles, gets hot too fast when boiler refires because it is just sucking the supply water back into the return.
  • betweentheframebetweentheframe Member Posts: 97
    I loose 10 degrees in 20 ft of pipe because hepex is cast in concrete before it comes out into the basement.
  • SuperTechSuperTech Member Posts: 1,383
    It is in your best interest to figure out the issue with the outdoor sensor. Do you own a multimeter? You can easily check resistance of the outdoor sensor and find out if it is an installation problem or a bad sensor. Outdoor reset is crucial for efficient operation.
    Superdave is steering you in the right direction, I think you can benefit from the installation of a hydro separator.
  • betweentheframebetweentheframe Member Posts: 97
    What is the resistance for the sensor across the 2 wires? I looked up 100 if these installed in uk, and no one hydro seperates or primary loops them in any installs I can see. It’s clear that it has an internal bypass and it has a copper pipe that runs from supply to return which is effectively a prinary loop, in fact it mentions the primary circuit(british for a “loop”) and it’s internal bypass in manual, and nowhere does it explain or mention needing a primary loop or a minimum flow rate

    Primary loops seem to be important if you want warmer return water and if you have sutuation wjere there would be a call for heat before a valve is open, neither situation do i have.
  • betweentheframebetweentheframe Member Posts: 97
    Just called baxi, they said emphatically, “NO PRIMARY LOOP”, it has its own internally and it will never run right with one, made to be a one box solution with internal bypass through plate exchanger and a primary circuit inside, as well as air seperator and air vessel!
  • betweentheframebetweentheframe Member Posts: 97
    He also said I have no need based on my system for the external grundfos pump and that baxi’s own pump inside the unit can run it easily.
  • superdavesuperdave Member Posts: 155
    I am just giving you what I have seen over time and the way I install heating systems.

    Depending on you heat needs and pump needs you may or may not need other pump!! Was the boiler installed by a Baxi certified plumber??

    I have been installing Baxi boilers for almost 20 years and I have never been told you cannot use a hydro separator on them.

    I would say at this time you best bet is get the plumber back on site and the tech that told you no hydro separator and have them make a piping print on what size and if you need other pump.

    Also the bypass goes through the DHW heat exchange not the tube!! the tube is used to help fill the boiler by the blue or red valve under the boiler it looks like a plastic allen wrench.
  • betweentheframebetweentheframe Member Posts: 97
    No, it has a primary circuit, and a bypass that goes through the DHW heat exchange, he said that both of those serve to give init hydronic seperation. He said, as manual says, if all rads have TRV’s then you need a differential bypass.

    I have also lived in europe and seem 100 of these in apartments for heat and hot water, they are never primary looped, no room for it, often lountes in the kitchen next to cabinet.

    He said those baxi flat rimmed tail pieces look like that to quiet noise from water flow.

    He eas emphatic that this have no primary loop, said it was just totalky worthless, as did my brother in law’s dad whom I called and gave pics and diagram feom baxi, he engineered stanford, berkely, and many other latge commercial HVAC systems.

    Where I am is rural, and the guy who installed this originally is a “baxi guy”, no thanks kn having anymore “plumbers” come out and give me piping for a conventional boiler on a condensating wall unit. I’ve obviously proven to be smarter than rhey are because I came to the same conclusion the baxi tech just automatically said, “no prmary loop, they have integral hydraulic serpation.”

    Thank you all!
  • betweentheframebetweentheframe Member Posts: 97
    I am a general contractor in NYC, not exactly a dummy lol, I’ve plumber a lot of stuff, but only hvac I have real exoerience with is gas forced air, used to work for friend’s HVAC company.
  • superdavesuperdave Member Posts: 155
    Good luck, Sorry you feel I am staring you wrong. I guess I have been installing boilers wrong and totally worthless is the Hydro separator!!

    I would like to hear back from others on this site and explain what I am doing wrong installing hydro separator.

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