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Setting surface Aquastat on early 1900s hot water furnace

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Howdy folks,

A first post, and a question.. naturally. 

I have an early 1900s Mueller hot water / steam furnace that is set up for gas-fired hot water heating. It's a simple gravity system with no pumps, automatic valves or electrical anything except for a thermopile generator - aka a millivolt system.

Four 3" pipes connect to the boiler - one on each side at the bottom and two straight out the top. The righthand top pipe has a Honeywell Surface Aquastat which acts as the only high-limit switch.

Someone messed with the Aquastat setting and I need to reset it correctly - but at what temp? It's -5 here and the furnace isn't quite keeping up. The radiator in the room above the furnace peaks out at 125F (I have a surface thermometer on the inlet pipe).

How do I determine the correct setpoint for the Aquastat? 

Thanks!

Comments

  • retiredguy
    retiredguy Member Posts: 919
    edited December 2022
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    What is the setting now? On a gravity system that relies on temperature differences for water flow, I would set that aquastat at about 180F, There is no exact maximum setting as long as the water in the boiler does not exceed boiling point of the water. Make sure that the aquastat is in good and tight contact with the discharge (supply) pipe it mounted on. Also make sure that the contact point is relatively clean so there is good heat transfer to the control. You could start by raising the setting by 20F until the boiler again heats the house. If that were my boiler I would buy a second aquastat, mount it on the other supply pipe and set it to the same setting as the original control and wire the 2 controls in series. This would give you a second "safety" control.
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,737
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    I agree with @retiredguy. 2d control is a good idea.

    Raise the temp a little at a time and see if this helps. A strap on aquastat is not as accurate as a control in the boiler water so use caution about raising it too high
  • WimWalther
    WimWalther Member Posts: 7
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    Ok, helpful comments - thanks.

    So what is a good peak temp for the radiators? These are old type cast iron rads built up from sections, about 40" tall for some and maybe 19" for others.

    The Aquastat is fairly secure, and I believe it's an original retrofit part from the 40s (?) when it was converted to oil, probably around WWII. It's been burning gas since 1958 I think. Rating is 180,000 BTU.

    I'll consider putting on a second Astat. NOS Honeywell units show up regularly on eBay.

    If you guys have any more to add, I'll be listening.

    Merry Christmas to everyone! 
  • JUGHNE
    JUGHNE Member Posts: 11,089
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    Is there any type of pressure gauge on the boiler?
    How about an open expansion tank upstairs or in the attic?

    One critical item is your pressure relief valve, should be a 30 PSI type.

    They are flushable, but may not reseat if not done on a regular basis.

    I do not open/test/flush them unless I have a replacement on hand.

    Plus you may have to drain the entire system to change.
  • WimWalther
    WimWalther Member Posts: 7
    edited December 2022
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    Yep, it has the original pressure gage as well as a mercury thermometer that might have direct water contact. Both stick out the top of the outer shell, next to each other.

    When the system is completely full of water at ambient temp, and bleed valve open on the highest / furthest rad, the pressure gage reads about 12 PSI. I maintain it between 12-16 during heating season by bleeding rads, adding water if needed etc.

    QUESTION: What temp should the boiler be at/below so it's safe to add cold water? I've always let it fall below 90F before adding..

    It does have a 30 PSI T&P safety valve that I installed 25 years ago. You're right, it's not very keen to reseat if you test it.. I leave it alone.

    There is an expansion tank on the basement ceiling, it was added during update work 20 years ago. I haven't drained it in several seasons... then again, it ran for almost 100 years without one!
  • WimWalther
    WimWalther Member Posts: 7
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    Thanks for the friendly help, folks.. it's refreshing not to deal with the keyboard warrior types who seem to live just to tell you how dumb and wrong you are about everything. 
  • JUGHNE
    JUGHNE Member Posts: 11,089
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    If your cold water fill connection is down low on the boiler and the boiler is nearly full and you want to just top off for pressure, then your cold water would be added to the existing hot and heat up pretty fast.

    It is the hot empty boiler that could be at risk, IMO.

    Did you ever have an open top expansion tank up high in the house?

    If not, what the dead men could have done is not bleed all the top floor radiators completely of air. Using the air cushion in the top of those rads for system expansion. If your compression tank is water logged, you may be using this system already.

    And we assume you have a 30 PSI relief valve....pressure only.
    I am in the habit of saying P & T also.....water heater valve, some rated for 150 PSI.
    Not good for cast iron boilers.
  • leonz
    leonz Member Posts: 1,165
    edited December 2022
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    If your system is bottom fed with 2 risers and 2 returns your gravity hot water heating system should have an open to air expansion tank in the attic if it has an attic or located in the ceiling of the top floor; NOT a compression tank hung in a basement ceiling joist.

    It would help us a great deal if you could provide pictures of the boiler and all the piping in the basement and the radiators and piping on the radiators.

    I for one would like to more pictures of the compression tank and its piping as it should not be there.

    My thoughts on Christmas Day 2020.
  • JUGHNE
    JUGHNE Member Posts: 11,089
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    My thoughts on eliminating the open top expansion tank is to eliminate absorbing air (O2) into the boiler water, prolonging the life of the system.

    Also homeowners may be grateful to not have to top off the tank.
    Although their compression tank will probably get water logged and need service.
  • WimWalther
    WimWalther Member Posts: 7
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    You're right, the "T&P" is really just a 30 PSI pop-off valve. New one was installed 20 yes ago.

    No, there was NO expansion tank anywhere in this system when I moved here. I'd gone back & forth over this point with several incredulous folks - pros & amateurs - who just refused to accept this fact. Some got kinda worked up.. but it is the truth.

    The basement ceiling exp. tank was added 20 yrs ago during update work. Job was done by a local heating Co, but the county / gas utility paid the bill. This was the result of a home energy audit that I'd requested.

    I'm disabled and can't go down the basement stairs anymore. Sucks. I'll try to have the wife shoot some pics - but it's really a dirt simple system. As for draining the exp tank, not sure if I trust her with that.. ;-)
  • JUGHNE
    JUGHNE Member Posts: 11,089
    edited December 2022
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    Leonz, you have to tear some pages off of your calendar, it is almost 2023. ;)
  • leonz
    leonz Member Posts: 1,165
    edited December 2022
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    You're right, the "T&P" is really just a 30 PSI pop-off valve. New one was installed 20 yes ago.

    No, there was NO expansion tank anywhere in this system when I moved here. I'd gone back & forth over this point with several incredulous folks - pros & amateurs - who just refused to accept this fact. Some got kinda worked up.. but it is the truth.

    The basement ceiling exp. tank was added 20 yrs ago during update work. Job was done by a local heating Co, but the county / gas utility paid the bill. This was the result of a home energy audit that I'd requested.

    I'm disabled and can't go down the basement stairs anymore. Sucks. I'll try to have the wife shoot some pics - but it's really a dirt simple system. As for draining the exp tank, not sure if I trust her with that.. ;-)

    =================================================================

    Some basic detail is missing with your system and it is not your fault. Do you have an aquaintence that could take the detailed pictures for you?

    Whenever you can upload the pictures of the boiler, ALL the near boiler plumbing, the steel compression tank and ALL its plumbing and every radiator including the top floor and any attic plumbing as someone missed something 20 years ago and was not paying attention.

    If there is a pipe leading to a floor drain or laundry sink that is a clue that tells me the open to air expansion tank was at the highest point in the house and the overflow from filling the tank spilled into
    the drain dropping to the floor drain or laundry sink.

    The only other clue would be a pipe with a downside up U passing through the roof that would spill overflow water on to the roof and there would be a very large rust spot on the roof under the U.
  • WimWalther
    WimWalther Member Posts: 7
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    Leonz, 

    I'm not surprised that you say this, several others have come before you absolutely insisting that there just MUST be an original tank somewhere.

    I can assure you, there is not. When I moved here I was a healthy young man, and have been through every crack & crevice in this place. It's a small house, about 1100sf, 1.5 story.

    And vented tanks etc can definitely be ruled out as the system has built & held 30psi.. briefly, once many years ago. If it were vented, it could never hold pressure greater than the weight of the water column - 12-16psi maybe.
    mattmia2
  • mattmia2
    mattmia2 Member Posts: 9,838
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    Leonz, 

    I'm not surprised that you say this, several others have come before you absolutely insisting that there just MUST be an original tank somewhere.

    I can assure you, there is not. When I moved here I was a healthy young man, and have been through every crack & crevice in this place. It's a small house, about 1100sf, 1.5 story.

    And vented tanks etc can definitely be ruled out as the system has built & held 30psi.. briefly, once many years ago. If it were vented, it could never hold pressure greater than the weight of the water column - 12-16psi maybe.

    As @JUGHNE said, your radiators weren't blead completely and were used for expansion.

    Is the burner shutting off on the aquastat? If the thermopile is worn it may only be generating enough current to open the gas valve some of the time ans could be your source of insufficient heat. they need to be replaced every couple decades.
  • WimWalther
    WimWalther Member Posts: 7
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    I like your unbled radiator idea, it's the first I've heard it and it makes good sense. I do know that when the basement ceiling tank was installed, the system pressure varied much less as it cycled. It used to regularly make 10-15 PSI excursions, now only 3-5 PSI max.

    To the best of my knowledge, yes - the A-stat is doing its job correctly.

    I've been here 30 years, and haven't yet replaced the thermopile. It may well be original to the gas re-fit.. which seems to have been in 1956! I have a spare on-hand, of course, as I expect it to die any moment.

    Question: What's the short-circuit current of a good thermopile? What about the  open-circuit voltage?
  • WMno57
    WMno57 Member Posts: 1,386
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    I'm disabled and can't go down the basement stairs anymore. Sucks.

    Sorry to hear that. Can we help you find a contractor to help you keep your heat on? We have a find a contractor tool here at HeatingHelp.

    https://heatinghelp.com/find-a-contractor/

    I have a system somewhat like yours. I drain my compression tank every year. If you already have a contractor, we can write up some instructions for them on how to do this.
    I DIY.