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Stupid question here but can't seem to figure this out.

DanDan Posts: 15Member
Went to look at a nuisance call today. Former customer but I did not install or work on the system. Old(1972) weil Mclein gas fired hot water boiler. Did not see an old tankless on it. Has a high/low aquastat controlling boiler and zone 1. since then zones 2 and 3 have been added. zones 2 and 3 are single zon relays for each. all zones are baseboard. Complaint was that 2nd floor is heating all the time. Customer has shut off all t-stats as well as all switches on the boiler. no chance that pumps are running. Zone 1 is 1st floor, zone 2 is 2nd floor and zone 3 is basement. Upon arrival noticed that all tstats are off. second floor heat is migrating and all baseboard is hot. Figure a bad flow check. go to boiler room and see all switches in the off position, yet all supply pipes too hot to touch. second floor return pipe too hot to touch. basement and first floor returns are cold. figure bad flow checks. look at hi/low aqaustat and realize it is set for 180 hi and boiler is firing to 220. It is a millivolt system.

Am I dealing with a bad aquastat and three bad flow checks all at the same time? or am I missing something?


  • SteamheadSteamhead Posts: 9,386Member ✭✭✭✭
    No such thing as a stupid question

    First thing I'd do is see if there's heat transfer grease in the aquastat well. There's no reason it should be getting that hot.

    Do the circ relays all have dry contacts to start the burners?
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  • EastmanEastman Posts: 618Member ✭✭
    One possiblity...

    Let's assume the aquastat is malfunctioning for some reason.  Is it possible the check valve for Zone 2 never closed.  It is the tallest zone, creating the greatest potential for a gravity induced pressure delta due to temperature differences.  The other zones' supply lines could be hot purely from heat conducting though the copper across the check valves.
  • MikeJFMikeJF Posts: 1Member
    Bad flow check for 2nd floor

    You most likely have a bad flow check for the 2nd floor. If it is over heating there, then the t-stat is satisfied and the circulator never comes on. And if the other circulators come on you would get reverse flow through the second floor zone. That is why the second floor return is hot. Bad flow checks are very common, but not three bad at the same time. Hope this helps. Mike.
  • unclejohnunclejohn Posts: 658Member ✭✭✭
    It's to old to worry about

    First of all it’s a 40 + year old boiler is any time spent repairing and not replacing is a waste. But if the customer insists then I would first make sure he’s gonna pay regardless of results. Then I would start by disconnecting aqua stat and cooling down boiler and starting top floor zone only. Let boiler fire up to maybe 160* and see what happens in the other 2 zones. If other zones are fine most likely aqua stat if not you might be able to check by feel whats up with the other zones.
  • smklinsmklin Posts: 41Member
    return line or supply line

    i would let the system cool of and than check what side of the check gets hot first
  • smklinsmklin Posts: 41Member
    return line or supply line

    i would let the system cool of and than check what side of the check gets hot first
  • Steve WhitbeckSteve Whitbeck Posts: 669Member

    First of all YOU have to see if the aquastat is bad. Powerpile or 24 volt can still have a problem. When the burner is firing remove one wire off of the aquastat to see if the burner shuts down ( it may take a few seconds to respond to open or close) If it doesn't then something is wrong with the wiring, gas valve or aquastat relay controller.

    You will have to figure out where the circuit is closed.

    As for the second floor getting too hot. The flow checks may be working just fine BUT normal operating temperature is 180* - 220* is possibly over powering the 2nd floor flow check. Get the temps back down to normal and see what happens.
  • R ManninoR Mannino Posts: 362Member ✭✭
    Happened to Me

    "And if the other circulators come on you would get reverse flow through the second floor zone."

    In my own home, scratching my head until I figured it out.
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