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Vaillant GA92-100 w/ Monoflo System

PeterPeter Member Posts: 30
Hello All,

Haven't been on here for a few years, but I still like to read different posts every now and then.

We sold our previous home and and have since moved into a ranch home. The home has 1162 sq. ft, a basement with approximately 1200 sq ft and a garage with approximately 568 sq ft. All three spaces are heated with copper fin baseboard. I am going to work on tightening the place up, since nothing was really updated, other than the windows (double insulated) and the front door.

The first problem I encountered was that there was a constant drop of water from the bottom of the expansion tank. When the system started up all you would here was a constant gurgling, waterfall, dripping water in the baseboard. I knew there was air in the system after dealing with the heating system in my previous house. Apparently, after the previous owner neglected the heating system for what I assume was years I decided to get to the bottom of the dripping to correct the air problem. I found that the Taco air scoop was so corroded from the inside, from all the air in the system, who knows for how long, that it had a pinhole causing water to leak onto the expansion tank and off the bottom. I replaced the expansion tank and the scoop, but not ever dealing with a diaphragm one, neglected to make sure the pressure was equal to the supply pressure. After many attempts to purge all the air from the system, I finally researched enough to get all the air out and have the system working totally air free. This was all done about 2 weeks ago, so mind you, I just needed it to be working as to not be left without heating for a prolonged period. I believe the pressure is too high, since it is a ranch and I think the pressure should just be around 12 psi. I think the electronic tire pressure gauge I used to set the expansion tank pressure was not too accurate, but it's all I had. Plus, I want to avoid draining the system again since it has no isolation valves to help with servicing the system. I also changed all the only manual air valves to auto air vent ones.

I know that the ideal way to install the air scoop was to have it behind the pump, but the timing to get this done is not the best right now, so I will get this squared away after this winter season. I would like to get all the work done at one time and avoid having to drain the system and have something major go wrong and be left with no heat.

A few question:

1. I really like the way the monoflo system delivers heated water to virtually every baseboard at the same time, so what can I do to make the system more efficient? I was thinking of buying a Delta T pump to get the most out of every heating cycle.

2. I would like to create a 3 zone system with the existing monoflo one in place. By this I mean making the 3 heated spaces described above into their own zones. Can this be done?

3. i would like to keep the Vaillant for now because it seems to be working without any problems, but would it be wise to change to a High Efficiency Boiler even a Combi one? I ask about the Combi because the gas water heater was installed back in 2005 at this house and I know it might be on it's way out soon.

All input and advice is greatly appreciated!


  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 3,283
    we would need to know are the basement and garage heated now?? I suspect not. It looks like you only have one zone. If not you could keep the monoflow for the main house and pipe the basement and garage as separate zones which is how it should be.

    I would not alter the monoflow system.

    12-15 psi with a cold boiler is the correct pressure. You must remove the expansion tank from the system to check it's pressure.

    If you remove and reinstall the tank install an isolation valve between the tank and the system. Between the tank and the new ball valve install a tee with a plug, or a manual air vent or a boiler drain valve so you can isolate the tank and bleed off the water pressure to check the air (bladder side) wihout removing the tank
  • PeterPeter Member Posts: 30
    Yes, the basement and garage are heated. The basement has 5 -(approximately) 5' sections of baseboard and the garage has (approximately) one 25' section.

    The tanks's pressure was set to approximately 15 psi. In the haste of worrying about being left without any heat for any extended period of time there was no isolation valve installed between the tank and the system. I will do this with all the other modifications.

    it's interesting because the sections for the basement and garage were branched off of the mains with regular t's not diverters. The radiators in the basement and garage heat up just fine. I would like to zone both the garage and basement, but I do want to keep the monoflo for the living space.

    I took a couple of pictures of the t's used to supply the basement and garage baseboards, but I fail to see how they work withoudiverter t's. There is only one pump (Taco 007) at the bottom of the boiler.

  • IronmanIronman Member Posts: 3,992
    edited November 2017
    The cones that are in monoflo Tees can be purchased separately and installed in standard Tees. That's probably what was done on some of yours.

    The unusual 45 in the 3rd pic is some form of diverter fitting.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • PeterPeter Member Posts: 30
    Interesting, I did not know the cones could be purchased separately. That fitting is where the main 1 1/4" splits to two 1" that supply the house radiators. Looks like a Y fitting with valves to balance the split loops I think.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 3,283
    I never knew about putting separate diverter cones in myself. New one on me. Learn something all the time. That must be what they did as they look like regular tees
  • PeterPeter Member Posts: 30
    I think there's no other explanation that would let the other 2 branches heat up so well without the cones.

    As for zoning the garage and basement it's gonna be tricky since the supply and return for both garage and basement runs are not that close together, so running closely spaced tees is impossible with current layout.
  • PeterPeter Member Posts: 30
    I've noticed that before a call for heat the boiler's temp gauge is around 160 deg., stays at this temp because it has a standing pilot and the flo-check valve, but after startup the temp starts to drop sometimes below 100 deg. The return temps are making the boiler temp drastically low and I am worried about thermal shock.
    Could this be because the flow rate is way too high for the system? This is why I was thinking of changing the pump to a Delta T.
    Would a boiler bypass be beneficial?

  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 3,283
    Boiler shock. Maybe. How long before the return temp comes up? If it's on a cold start I wouldn't be too concerned but if you are keeping the garage at a low temp that might be a cause for concern. Every boiler has low temp returns....for a while but extended run time with low temps is very bad. A bypass or three way valve....theres many ways to fix it
  • PeterPeter Member Posts: 30
    edited November 2017
    I will have to monitor how long it takes the return temp to bounce back. Only one zone system, not controlling the temp in the garage.

    I like the Caleffi Thermomix. I will have to put something in when spring comes around and work on the zoning.

    The boiler just fired up an was on for only about 15 minutes, but it hadn’t been for about approximately 2 hours. Temp at boiler was only at 130 when it shut off. Not good!
  • PeterPeter Member Posts: 30
    I haven't had a chance to check the return temp timing rise on cold start, but I did get a chance to see what time it would take to satisfy a heat call for a 1° rise. I timed at 11 minutes. I didn't check the boiler temp gauge, which I should've done. Will get it this weekend along with the other check.

    I'm still contemplating if I should invest into a Mod Con or just keep the old Vaillant and try to get it to work more efficiently, if there is such a thing. One thing I am sure of is that I don't want to change the monoflo system.

    I will measure the radiation, get a heat loss estimate and get all the other measurements this weekend and post back with the info.

  • PeterPeter Member Posts: 30
    I timed the boiler this morning and after being off for approximately 2 hours a call for heat started with a boiler temp of 140 deg and quickly over about a minute dropped to 90 deg. The run time of approximately 10 min had the boiler reaching only about 130 deg. I am worried about thermal shock and it's obviously a problem.

    The other question I have is after performing a heatloss calc for the home, how do I get one for the garage and account for the 16' garage door?


  • PeterPeter Member Posts: 30
    We have recently closed on the ranch home that we were renting while we were waiting for title issues to be remedied. The home has a Nest thermostat installed and it shows that with the cold spell we have been having , the longest the system has run has been for 14 1/4 hrs. and this was yesterday. I know a lot of you don't like the Nest thermostats and unfortunately it's what came with the house and I'm trying to make the best of it. I am also trying to make the best of the boiler that came with the home.

    Although the boiler is, from my calculations, about 32 yrs old I want to keep it running until it dies. I'm guessing that the second and third number in the serial number is the year the boiler was made. If I am incorrect please let me know.

    It runs quite well and heats up nicely. Now, I am sure it's over sized after seeing that it doesn't run for 24 hours even at below zero temps. So, with this in mind, how do I deal with an over sized boiler and try to keep the short cycling down to a minimum? How can I make the system more efficient? I know one of the first things would be to work on tightening up the house by minimizing air infiltration and adding more insulation to attic and it's on the list for spring. What else should I keep in mind?

    I know there are high efficient boilers on the market now, but I don't think there is a need for replacing a good working boiler. I do have to see how much my gas bill will be after this winter and analyze if it's in my best interest, financially, to replace or not.

    Thanks in advance to any advice on this matter,
  • hot rodhot rod Member Posts: 7,531
    That is one of the top 3 or 4 boiler manufacturers in Europe, it is a fine boiler as long as it is piped and operated properly, like any.

    Return temperature is one key to a long healthy life. The return should be above 130F within 10 minutes of a start and should run at least 10 minutes or so to warm and dry the boiler and flue piping. I'd keep the boiler and spend $$ on upgrading the structure.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • PeterPeter Member Posts: 30
    Thanks hot rod! I’ve looked around and I can’t find any info on this boiler, anywhere!
    As noted before I am concerned with the return temp being low for too long and I’m looking into getting a thermostatic mixing valve an repiping in the near future.
    Does it matter if it’s oversized and would a buffer tank be a wise option to add to the system?
    What is interesting is that this boiler has been in this house for about 20+ years I would say and it has not had any upgrades, so how has it lasted this long with below 130 return temps? I was worried about that in the recent below zero temps we’ve had recently and I checked around the boiler for condensation or checking if any dropped on the burners, but nothing at all. I know this doesn’t mean it’s not happening, but I nevertheless checked.
  • jerseyfitterjerseyfitter Member Posts: 11
    vaillant at one time had an office in Cinnimonson N.J. If you replace existing circulator with a spool piece you can place it after exspansion tank
  • PeterPeter Member Posts: 30
    I will find info and email the support dpt and see if they can provide any info.

    As for the circulator, I will have to re-pipe accordingly because the circulator is on a fixed position at the bottom of the boiler. I can either try to make a straight pipe adapter to replace the original circulator and move it so it can pump away or I can move the scoop and the tank. Trying to make this as simple as possible, but I don't think I can get around that.
  • PeterPeter Member Posts: 30
    Well, as expected, I went through and performed a heat loss calculation with the Slantfin Heatloss Calculator and it's 49.9K. So the boiler in here now is almost 2x what it should be. When the house is tightened up it would be even less than that. What would my options be at this point?
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