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  1. #1
    Join Date
    Jan 2010
    Posts
    8

    Domestic water too slow to heat up when in use

    Hi all. I have an oil-fired tankless hot water setup (Kerr Comet) with my aquastat set at 180-160-10. When I run a bath for my daughter, the water starts to run cold long before the furnace kicks in to heat the domestic water back up. I've tried setting the aquastat up a bit on a trial basis (190-170-10) and it doesn't seem to be much better. The house heats well, but we consistently have to run the water to luke warm, turn it all off and wait 5 minutes to finish putting water in the tub. Mind you, there's only 6 or 8 inches of water in the tub by the time I'm finished, and it's only hot enough for a 2 year old; not exactly scalding (P.S. The "anti-scald" feature of the faucet is set at it's lowest for most hot). I've tried pushing the mixing valve to send less cold, and it seems to make the hot hotter, but no real chance when it starts to get chilly again. I live in Atlantic Canada and winter can get pretty chilly, but summer or winter I have the same problem. My furnace cleaner says that he thinks it's the mixing valve, but I'm not so sure; the mixing valve seems to be doing what it's supposed to in response to me changing it's position. Does anyone have any insight? Thanks so much in advance.

  2. #2
    Join Date
    Feb 2008
    Location
    Massachusetts
    Posts
    701
    The tankless coil in your boiler could require cleaning or replacement, if your tech can't figure that out, you might have to try someone else.

  3. #3
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,837
    Actually, tankless water heaters are basically simple but a mystery to most technicians for some strange reason. There's a relatively simple way for you to determine where the problem lays. Go to the boiler and determine which is the hot pipe between the tankless outlet and the tempering valve. Have someone run the tub and as the water begins to cool at the faucet, feel the temperature of the "hot" pipe and see if it's really hot, cool or downright cold. If the pipe is hot, then move your hand from that hot pipe to the pipe coming out of the tempering valve. If the "hot" pipe stays hot but the pipe from the tempering valve is cool/tepid/cold, then the problem is with the tempering valve. If on the other hand the "hot" pipe runs cool/tepid/cold, then the problem is internal to the tankless coil. It's just that simple, provided the boiler temperatures are proper. I'd say you're running on the low end of the boiler temperature based on what you've posted. I'd run a boiler with a tankless at 200° on the high limit, 180° for the low limit and leave the differential at the lowest setting, not that I've EVER been able to see any difference in the actual differential no matter where that is set.

    Another very common issue with tankless coils is using the improper tempering valve. Most plumbers will install a Watt 70A tempering valve, which is not the correct valve to use. The reason it's not the correct valve is that is can only meter the water temperature at flow rates of 3.0-gpm and higher. That means that when you run anything less than a tub/shower/laundry at full hot water, the tempering valve doesn't temper. So if the valve is set to try and throttle the hot water at a lav faucet so nobody gets burned, it won't produce anywhere near the temps required for a tub/shower.

    Now after I've said all of that above, if the shower works great but the tub is the only problem, then the issue is most likely with the tankless. Again, the "hot" water pipe will tell that tale but if it is the tankless, then the coil needs to be flushed. It takes the installation of some valves and some knowledge of how to perform the task, as well as a pump to circulate the fluid used through the coil. The problem is that the minerals that 'cook' out of water when it's heated get attached to the inside of the coil and forming an insulation. So when the water sits unmoving inside the coil, it gets heated and you get some hot water at first. When the water moves at maximum rate, it moves too quickly through the coil to get heated. This is not a DIY site so I can't give you anymore than I've already given but what I gave should help you and/or your service tech determine where lies the problem. It's not rocket scienice but for some reason, many seem to believe it is.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  4. #4
    Join Date
    Feb 2008
    Location
    Massachusetts
    Posts
    701
    Skip, you just taught him more than his supposed tech knows.

  5. #5
    Join Date
    Jan 2010
    Posts
    8
    Thanks for the great advice! I'm trying to locate my tempering valve. Is this the same as a mixing valve? On the front of the unit I have a hot pipe that mixes with cold water at my "Honeywell Sparcomix AM Series", then goes off to the rest of the house. I can only assume that this is the tempering valve you're talking about, since I don't see any other pipes going to my domestic hot water supply for the house. Thanks again so much.

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