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Aquastat Agony!

lmanc2222 Member Posts: 13

I'm not understanding the aquastat set points.

I just installed a switching relay for an additional zone on a boiler. I fire the boiler and set the hi for 190* and the lo for 170* and set a 20* diff. The boiler is cold. So...it fires but it goes past the hi limit and continues to fire as the call for heat kicks in. I panicked at 210* and shot the hi limit down to 160* and it turned off. In trying to understand this, I'm guessing the boiler has to reach idle lo temp first and turn off at 180* but not go past 190*. If there's a call for heat I understand the burner kicks on BUT ONLY IF THE HI LIMIT IS NOT REACHED AT WHICH POINT IT RUNS UNTIL 190*. Am I wrong with this? WHAT DON'T I GET WITH THIS? 210* is 20* above the hi limit. The 20* diff setting is the operating limit for the lo limit control and has nothing to do with the hi limit. Sooo....this should play out as an operating range of 160*-180* (including factory fixed limits) and fail open at 190* due to the hi limit setting which is supposed to be a safety. Do you get this?

I'm heading towards steam with this. I left it set for 180* HI, 160* LO and a 20* DIF.

At what temperature does 12 PSI water flash to steam? Is there a typical lead time for the aquastat bulb to heat up before registering the correct temperatures to the aquastat? Is the bulb liquid filled?


  • Big Ed_4
    Big Ed_4 Member Posts: 2,246
    edited January 2018
    Loosen and pull the aquastat off its well , from the back pull out more line for the bulb .. Push the bulb fully in the well with an screw driver until it dead ends . Now reattach the aquastat ... Extended wells you need to do this ... I believe the diff works like this .. There is always an 5*above setting , say 190* is your set point . If there is an call for heat..it will rise to 195* , with an 20* diff the burner will come back on when it drops to 175* .. No call for heat the boiler will maintain the low setting ... It will turn off the circulator if boiler temperature falls below the Low ... Used if a hot water coil is installed ...
    I have enough experience to know , that I dont know it all
    rick in Alaskalmanc2222
    HVACNUT Member Posts: 5,034
    Was all ok before you installed the zone relay?
    Check the new wiring. Did you bypass the limit with the new relay?
    What boiler, relay and aquastat? Pics of the controls and wiring would help a lot.
    I've seen some boilers, like the WM 68 series overshoot the set limit by as much as 30 degrees on the down cycle.
  • Steamhead
    Steamhead Member Posts: 15,998
    Also make sure there is plenty of heat-transfer grease in the well that the bulb can contact. If you can't find any, try a computer store- same stuff is used between processors and coolers.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • rick in Alaska
    rick in Alaska Member Posts: 1,420
    I don't use heat transfer grease on my probes because it has a tendency to turn to cement after being in for awhile. I find if you seat them all the way, they work just fine. Also, if it is a Buderus deep well aquastat, you must push the probe all the way down and make sure it gets past the sharp step they mill in to the immersion well. It will feel like it is in when it is in only half way, but you need to wiggle it to make sure it drops down all the way. I had a heck of a time troubleshooting one that kept over and under shooting temps until I found this out.
    Also, I believe water boils somewhere around 240 degrees at 12 psi.
  • lmanc2222
    lmanc2222 Member Posts: 13
    Hello Everyone and thank you for your input. I found it to be most helpful and reassuring. When I install an aquastat I set the bulb at full straight 'erection' and it works every time. The bulb will always bottom out first regardless of the well length and all I need to do is slightly add pressure to the aquastat to curl the capillary tube enough to mount the aquastat to the well mount. When I ask for dielectric grease people look at me with that deer in the headlights expression. They've never heard of it. Frankly, I think mayonnaise would do the trick. LOL. Thank you again Gentlemen for your insightful help. It was comforting.
  • NY_Rob
    NY_Rob Member Posts: 1,370
    ^ it's actually "Heat Conductive Compound"....


    Wear gloves when working with that stuff, it's nasty and hard to wash off skin!
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,133
    edited February 2018
    There used to be a small packet supplied with every aquastat.