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Thread: Which type of gas water heater tank makes the most sense (venting related question)

  1. #16
    DIY Senior Member houptee's Avatar
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    All that is true what Dana said.

    What size gas pipe comes into the house directly from the meter now?

    I only suggested the tankless since it can vent thru the wall (in the proper location from the ground, any windows, doors, intake openings etc per code) so his chimney issue is solved, and the cost can be very reasonable if ordered online and he installs it himself with PEX tubing.

    The indirect systems require a boiler running all year. That is a lot of thermal mass giving off heat in the summer inside the thermal envelope of the house, and heat rises. So your central a/c has to work harder to remove the heat coming off that chunk of cast iron or steel heat exchanger that has no insulation just a sheet metal skin on it.

    There are pros and cons to everything, so many variables, and no perfect solutions in this world.

  2. #17
    Retired Defense Industry Engineer jadnashua's Avatar
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    Quote Originally Posted by houptee View Post
    The indirect systems require a boiler running all year. That is a lot of thermal mass giving off heat in the summer inside the thermal envelope of the house, and heat rises.
    A modern boiler doesn't stay hot between calls for heat, and with a smart controller on an older one, you can park it at its safest low temp setting, so the heat load isn't huge...It's MUCH bigger with an in boiler heat exchanger...there, to get adequate hot water, the temp must be MUCH higher.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #18
    In the trades Dana's Avatar
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    Even old school cast iron gas boilers don't generally need to be kept at temp unless the through-bolts have stretched. Most can be safely cold-started, and as Jim correctly points out, even if this one leaks if allowed to cool to 70F over the summer, setting the low-limit to 125-130F with a heat-purge controller the standby loss will be a fraction of that of a boiler that has to be kept a 150F+ to serve up hot water. Unless it's a known leaker when cool, setting it up for cold start is the right way to go.

    Running an indirect isn't the same as running an embedded tankless coil- there is no point to keeping it hot with an indirect. When the indirect calls for heat it doesn't take much time for the boiler to self-heat sufficiently to where it is hotter than the indirect, and actively heating. How long it take varies with the thermal mass of the boiler and it's water content, and if in practice you get a temporary dip in domestic hot water temp due to that lag you can just bump the aquastat on the indirect up by a few degrees so that the output never drops below 110F at the taps (tub-fill temp) during the initial firing.

    Cold-starting oil boilers that weren't designed for it is much more problematic than cold-starting old gas boilers, due to the much greater acidity of oil exhaust condensate relative to that of natural gas exhaust. Many gas fired boilers of the 1970s were already set up for cold start except when they were fitted with embedded hot water heating coils, (I'm not sure if that was common in the 1960s).

  4. #19
    DIY Senior Member houptee's Avatar
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    Doesn't the indirect tank have to maintain a minimum temp all summer even when no calls for domestic hot water?
    If you go away on vacation for 2 weeks its going to run the boiler every time the indirect tank cools below set-point correct?

  5. #20
    In the trades Dana's Avatar
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    Yes, if you leave the system on when you go on vacation, it'll keep the indirect at temp. The standby loss of an indirect is still VERY low compared to a standalone tank. The standalone tank has a center-flue heat exchanger that convects heat into the room constantly, whereas the indirect has only plumbing connections, and is more completely covered by insulation, since it has no burner. The boiler will cool off to near room temp between firings, but those firings will be every other day or so if there is no draw on the hot water heater, unless you've really screwed it up. Insulating all near-tank plumbing to at least R4 (including the cold feed and the boiler loop) ensures a very low standby loss.

    I turn off my system when I leave for more than a couple of days. If it's only 2 days it's still hot enough to be useful when I get back, but it doesn't take an extremely long time to bring it fully up to temp even if I've been gone for 10 days and it has cooled to near room temp.

  6. #21
    DIY Member Belmondo's Avatar
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    What an awesome thread! I'm gonna look into the Economizer control. Another control I installed was a programmable thermostat for the tank to reduce the standby firing of the boiler. I was getting annoyed hearing my CGi cold starting boiler firing up all day in summer even after I installed heat trap loops and nipples. If you set the on-off far enough apart you can keep the boiler from firing all day between morning showers and dinner cleanup.

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