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What to set auqastat at?

Chris_92 Member Posts: 6
I'm new to having hydronic heating, and as winter approaches, I was wondering, what should the little knobs on the auquastat be set at? There is a high, low and diff knob. It looks like there were some markings on them before, but they have since been changed apparantly. The boiler is a 30-40 y/o Burnham Oil fired. It has a honeywell triple aquastat that used to control the DHW also, but DHW is now disconnected ( I have to shut it off in the summer, or the boiler trys to keep the water up to temp even though no call for heat, I guess that's why it's called a triple aquastat)

Also there is no automatic fill valve, as least none that I can see, and I had to replace the relief valve and expansion tank earlier this year, so I had the system drained down. I refilled it, and purged all the air I could find, but I can't help but think that it doesn't have enough water in. Pressure is 15-20ish, but I hear a lot of "swishing" noises throughout the house that sounds like trapped air.

Do I need to add more water to the ssytem? I also hear I guess a boiling sound when it is heating up, that stops about 7 seconds after the burner shuts off... is this normal, or a sign of low water?

There are 2 valves, one is between 2 pipes going into the boiler on an H type crossmember, and the other comes from the DW feed.. I turn the DW on first and then the H valve on... am I doing this right?

Kind of clueless about hydronic heating, and hvac in general, and hope someone can help, appreciate it in advance!!



  • Brad White
    Brad White Member Posts: 2,398
    For starters

    if you can move that expansion tank to the suction (inlet) side of the circulator, you will be doing yourself a favor especially if you have been doing the annual "bleeding of the radiators". It sounds like you have air problems and that is where I would start.

    On a fixed-temperature oil fired system, a low limit of 130 to 140 degrees to prevent condensation in the flue passages.

    What I would do is automate that with an outdoor reset control with domestic HW priority. Tekmar is a good place to start.

    This way you do not have to run up and down the stairs every time the outdoor temperature changes. (I mean, you can if you want... and your boiler boosts for domestic water on demand.

    The bottom limit would be set for say, 135 and the upper range limit for 180 or whatever you normally use on the coldest day. (Too often boilers are set at 180F and just left there forever. Arrrgggh.)

    Ultimately and ideally what I would do is this:

    1) De-couple the boiler loop from the heating loop. If a single zone, a 4-way mixing valve is a nice way to go. A Taco iValve is a good place to start. Outdoor reset controls built in to the valve.

    This way you can have no low limit on the water your radiators see. You can put out 80 degree water when it is 60 degrees outside and ramp up to 180 degrees when it is, say, 0 degrees outside or whatever your local design temperature is. Also, when making domestic HW, you will isolate those spikes of hotter water from the heating circuit.

    2) Install an indirect (storage type) hot water heater, directly to the boiler (upstream of the 4-way valve) as a dedicated zone. This storage will allow your boiler to rest between showers and save wear and tear on the system.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Tom Hopkins
    Tom Hopkins Member Posts: 552

    Thanks for your help, The tank was just installed in place of the old one, I didn't think of moving it. I am probably replacing the boiler soon, so I guess I will just let it as is, and try to get all of the air out. I'm not using the DHW portion of the boiler anymore, the coils were clogged and flushed too many times, and they leak now from what I was told from the old homeowner.
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