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Thread: Hi Lo Swith on Aquastat

  1. #1

    Default Hi Lo Swith on Aquastat

    I have a Honeywell aquastat on my oil fired boiler. I saw the hi-lo settings which were on 180-160. Makes for REALLY hot DHW. If I turn these down to 140-120 will it save oil in the summer when I get hot water from the tankless coil? Thanks!

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    It sounds like you have a HW coil in the boiler. What you really need is a tempering valve on the outlet of the boiler for the potable hot. Because water in the coil will get to the boiler temp when it sits there for awhile, the initial shot will be that temp. You need the tank to be hot so that water passing through it can warm up if you run more water through than is already hot. Lowering the boiler static temp decreases how much heat can be released to the water passing through it. You could try lowering it, but may find when using more than a few gallons, the temp changes and cools off too much.

    This type of setup should always be installed with a tempering valve to moderate or temper the output to a safe level - it basically mixes cold in when required to keep the max temp within safe bounds.

    If you have one, it could either be set too high, or it has failed and needs to be replaced. If you don't have one, install one for safety reasons.

    Where I live, these are required even for tank type WH. They normally come preset to 119-degrees, but are adjustable higher or lower.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3

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    Yes what you say is true. The hot water is way too hot. Since the boiler is 40 feet away from the shower, it tends to be cooled somewhat by the time it gets there. Recently I insulated the half inch that pipe brings the hw to the shower and its much hotter now! But its not a problem since I just turn on the cold water a known ammount along with the hot water and everything is pretty managable. There are no kids in the house. Just the wife which could be a problem but hasnt been so far. But really what I wanted to know....will lowering the lo switch on the aquastat from 160 to 120 save any money heating summer hw? I think its good when set at 160-180 in the winter because that brings more heat to heat the house.
    Actually now that I think about it. My boiler only has about 5 gallons of water in it. The oil gun comes on very soon after turning the hw on. So it works very much like a tankless system. Summer hw use typically usues 10 gallon of oil per month. Which isnt bad but maybe it could be better?

  4. #4

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    Hmm just tried the hw with the limit set on 120. Now you can turn just the hw on and the temp is OK! You dont need to turn the cw on about 1/8 turn at all. After about a minute, the boiler fires up. About 2 mins after that, the hw is getting hotter as it comes out the shower head. (sort of like a tankless flash heater would do) Another minute and you have to turn the cw on a little. It seems it really doesnt matter hw wise what the limit is set on except at first. Once the boiler kicks in the water is getting pretty hot. Right now its very comfortable using just the hw spigot to take a shower...at least for the first couple of minutes.
    Im thinking you could very easily make your own tankless hw system out of an old gas or oil 100,000 btu boiler.(with a hw coil in it) Just reduce the water around the coil so theres like a gallon of it. (put bricks in the boiler to take up space for the water)Turn on the hw, then fire up the boiler. Take your 15 minute shower. Leave the water running then turn off the boiler. Then the water. You wouldnt need any specialized controls or sensors in the boiler. (which play hell when they go bad) My 100k btu boiler burns .75 gallon of oil per hour. So a 15 minute shouls uses .17 gallon of oil @ $2.25 a gallon is like 60 cents for a shower.

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    For safety and proper operation, install a tempering valve. There will be little temperature variation in the delivery HW temperature if you do, then the occasional guest won't fry themselves and you won't have to tweak the balance over time and through the seasons.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6
    Master Plumber nhmaster's Avatar
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    save yourself a crap load of oil (and money) and get rid of the tankless coil. Have an indirect installed and whil you are at it have them put in a boiler temp reset control at the same time. Should cut your bill a good 20% maybe more.

    Turning down the low limit aquastat can make a marginal difference but don't expect to see a whole lot of savings.

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