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Small Zone Troubles

gn Member Posts: 5
Looking for some advice on how best to deal, short-term and long-term, with a temp limit problem.

I've got a Burnham Series 2 boiler sized (I think) appropriately for my 2500sf 1930 Tudor house in Minneapolis. 100K BTU I believe. Had it put in 3 years ago, has been running fine. One big zone for main floor and second floor combined - supply/return pipes were too intermingled to try to zone it separately.

Added a new zone in the basement over last few weeks as part of a reno. Thought about baseboard but had a large CI radiator available that was leftover from an upstairs reno and wanted to use it. Bought another medium-size radiator and had the guy who put in the boiler zone them up in the 600-ish sf basement. Separate supply/return for each, new zone valve, etc. Really don't need a huge amount of heat in the basement - 90% underground, two small windows.

Problem is that if only the basement zone is calling for heat the boiler is way bigger than the radiators can emit. I knew this would happen and expected the high-limit well-inserted Honeywell L4080D-1234 switch to cut out the burner when it hits the setpoint - but it is not working. Even when set at 140-deg it never trips despite the boiler temp gauge reading 180-200. I've pulled it, tested it in hot water, and it works (multimeter reads open). I've tried re-inserting it with some conductive compound, no go. I've pulled the wire off of it during operation to simulate it going open at the high limit and the boiler correctly stops the burner and continues circulating. I've also stuck a meat thermometer in the well when the boiler temp gauge says 180 deg or so and it reads way lower, like 120-130. Not easy to get a good reading with this of course. I even coughed up $$$ for an identical replacement switch - same result.

Question #1 - Is it worth draining the boiler and pulling the well? Could there be some sort of film or buildup on the well that could account for this? I'm okay with doing this, but if it builds up in only three years of operation I'm not sure I feel safe trusting this alone to keep the basement radiators (and overall system) from driving way too high in temp.

Question #2 - I'm thinking of wiring up a separate, strap-on high-limit switch in series with the main one. Something like a WR 11B02-1. Would it make sense to put this on the combined return (after upstairs and basement returns combine) and trip at something like 120-130 deg, or the basement zone return line, or just put it on the basement zone supply line and trip at a value that corresponds with a boiler temp gauge reading of 180.

Thanks in advance for any help/advice you can give.


  • hot_rod
    hot_rod Member Posts: 21,868
    cap tube controls

    could it be just a slow response time. After shutting off the burner manually does the aquastat ever read correctly after a minute or so. Could be with a tiny load the boiler heats up faster than the control responds. You could test the control in a pot of water on the stove with a thermometer along side.

    You will still will have a problem with short cycles with micro loads like that. A buffer tank can help. Also with a buffer being able to run it up to 180 and draw it down to 140F would extend the cycles. Maybe the basement could be heated with 140- 150F water with a small load.

    tekmar has some boiler controls that may help by adjusting the boiler setpoint based on outdoor temperatures


    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • jp_2
    jp_2 Member Posts: 1,935
    could also

    allow the basement to call for heat only when another zone is calling.  use the zone that is most frequently on.   then the operation will appear seamless.
  • gn
    gn Member Posts: 5
    could also how to

    Ahh Interesting idea. Is that a pretty straightforward wiring setup? I have only one other zone.
  • gn
    gn Member Posts: 5

    It never opens the circuit even after 4-5 minutes when in the well, set at 140deg, with the circ off, burner off, and the boiler temp gauge reading 200+.

    I did test it in a pan of near-boiling water and it opened after 2-3 seconds when set at 190 - so the switch itself seems fine.

    Keeps pointing me back to the well being the problem. Probably makes sense to drain and pull it. Curiousity and all that.

    Thanks for the comment.
  • jp_2
    jp_2 Member Posts: 1,935
    might be real easy

    but since i do not know how your stuff is wired, its only a guess.

    got a wiring diagram for your set up?
  • Tim McElwain
    Tim McElwain Member Posts: 4,608
    Sounds like

    the hi limit is not wired into the new circuits for the new zones. Do you have a Honeywell control center on this boiler? It will have terminals R-W-G-Y-C on a wafer board on the transformer inside the vestibule of the boiler?
  • gn
    gn Member Posts: 5
    never trips

    I may be wrong but I think the switch is wired fine, it just never trips open at the high limit. I can pull the wire off the switch during operation and it correctly stops firing the burner, but continues the circ pump, as long as either zone is calling for heat. So I think the wiring is fine.

    Again, switch seems to work when in a pan of hot water, but never trips when in well in boiler.. I bet I find a 1/4-inch coating of gunk or something on the well when I pull it (waiting for a warm day).

    It seems reasonable to me to wire in a separate strap-on high limit switch in series on the small zone to give me more control without limiting how hot the boiler can get to serve the rest of the house. I just dont like the fact that the main high-limit switch just doesn't seem to operate properly when in the well. If the circ pump goes...

    Thanks for your comments.
  • icesailor
    icesailor Member Posts: 7,265
    Small zone troubles:

    You don't need a buffer tank with radiators. Radiators could be considered buffer tanks because of all the water they hold compared to 3/4" baseboard.

    I'd change the well before I started in on something else.  Do the radiators get hot in the cellar zone? I've never heard of anything like this. Something stupid is going on. There's a wiring issue with the new zone. Something is over riding the high limit. It's a wiring issue in the end. Or a piping issue. Have any pictures of the boiler piping?
  • Tim McElwain
    Tim McElwain Member Posts: 4,608
    This is typical

    of adding zones and forgetting to wire the activation circuit for the new zones through the limit. It was the number one problem I ran into when servicing systems that electricians and others did not wire the added zone through the boiler limit. It can be tricky because it will allow the system to run but will not shut off.

    Eventually the relief valve starts blowing due to excessive pressure. As temperature goes up so does pressure.
  • icesailor
    icesailor Member Posts: 7,265
    Right on Tim,

    Most electricians are clueless on the functions of the ZC/ZR terminals.

    I once found a six zone system with circulators that was forty years old and had only one circulator activating the high limit switch and the rest running on the operating control. The 5 zones never told the burner to start. The house wouldn't heat when it was really cold so they had the operating control set to 180 degrees.

    The owner didn't believe me at first when I moved a few wires and set the high at 160 degrees and the operating at 135. I told the owner that if when the outdoor was really cold, and the house didn't seem as warm as they wanted, "move this to 170 degrees and try it". They never did. And their oil bill dropped a lot.
  • AFred
    AFred Member Posts: 81
    Can you post pics?

    The series 2 does take some time for the aquastat to kick in.

    Could you could take some pictures of how the zone valves are wired?

    Did they add a transformer? Are they stealing 24V somewhere?

    I'm dying to see some pics, I could use a laugh.

    Did you have a "hanyman" or "plumber" put the zones in?

    Have you called the person who installed the zones?

    If you need I can recommend plenty of good boiler shops in south minneapolis.
  • Randy Baerg
    Randy Baerg Member Posts: 26
    wiring, debris, flow ?

    It sounds like the temp switch on the operator works fine. Here's a few ideas.

    Check to see if the boiler shuts of properly (at a low aquastat

    140°Fsetting) when only the upstairs zones are calling. If it does, then it

    sounds like the new zones wiring is bypassing the aquastat and cutting

    out only on the high limit. Check the wiring.

    If the boiler stays on past the aquastat setting when the other zones

    call, then it sounds like the installation of your old leftover radiator

    may have thrown some debris at the aquastat well.  Maybe some wood

    chips/debris (I saw a dead mouse cause this!) are blocking flow around

    the well or in the zone itself. Either way the temperature at the well

    may not be accurate. Check the well.

    Check the flow in the zone. Is there an air lock? Does the new piping allow for proper

    pressure difference between supply and return to generate flow? etc.

    You also mentioned that the new zone is small compared to other zones.

    Even once you get everything running, a call from this zone will probably

    lead to short boiler cycles. There is a feature called 'Zone

    Synchronization' on tekmar Thermostats / controls that  can synchronize

    all your zones together. This can really help reduce the chance of that

    one small zone calling by itself. E.g. tekmar 400 House control with a couple tN2 thermostats. You would get Zone Synchronization and Outdoor Reset.
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