Tankless Hot Water Garage Heater

Discussion in 'Boiler Forum' started by James Glenn, Nov 28, 2020.

  1. James Glenn

    James Glenn New Member

    Joined:
    Oct 29, 2020
    Location:
    Utah
    I've designed a hot water heater in my garage that works well, but I'm wondering if the pump is under-powered for the application and if what I've got set up is safe.

    I have 23 feet of 3/4 in copper pipe (inclusive of elbows using a conversion chart). I have 3 feet of galvanized pipe (all junctions between copper and galvanized have dielectric couplings on them). The connectors to the tankless HWH are 1/2in. I was able to bring the system to a balance by adding two 15 fin cast iron radiators in series (bought them from a guy who was tearing out the old hot water heating system in an old home). The system will run at 150 degrees at the HW outflow and 65 degrees on the inflow. I have a watts 500800 pum[p on it right now and it seems to do fine for moving the water (I can't find the GPM or head rating for this pump).

    The tankless HWH has a mechanical temp adjustment on it. If I keep the temp and gas adjustments on the lowest settings after 25 minutes the system stabilizes at 155 degrees outputting a lot of heat from the ceiling mount blower and both floor mount radiators. My concern is one of safety. Is this safe? It's all either galvanized pipe or copper. I've been advised on another forum to ditch the tankless heater and get a boiler. Honestly, I can't justify the cost when I'm currently getting excellent heat out of my setup.

    Here are a few questions:

    1) The Takagi T-H3M-DV-N High Efficiency Indoor Tankless Condensing Water Heater (NG) is rated for recirculating systems. At least that's what the documentation says. At 672$ it appears to be the best option for replacing my current hot water supply if that's needed. I'm not ready to replace the hot water heater in my home and quite frankly, I can't justify much more cost than that for a garage heater (3 car). It has PVC exhaust which is a plus. I'm going straight through the exterior wall for venting.

    2) Currently the system is set up with the taco air scoop and expansion tank on the hot water side right after it exits the heater and before the watts pump. I've heard that it would be better on the cold water side. Is that the best design? Is it worth changing the position? I'm assuming the expansion tank and air scoop still should be placed before the pump regardless.

    2) With 21 feet of 3/4 inch pipe and 5 feet of 1/2 in. galvanized pipe, a 50,000 BTU rated ceiling radiator with fan and the two fifteen fin cast iron radiators on the floor, what size pump would work best? In looking at the charts it looks like the Taco 007 0r 008 would work. Is the Taco 008 the best option? I'm not sure what the BTU rating is for each of the cast iron radiators or what they add in terms of additional pipe length for the GPM or head loss calculations.

    3) If the system is producing lots of heat with the current Watts pump, which as I mentioned, seems to run nicely at around 155 degrees at the hot side outflow and 65-70 on the inflow, is it a good idea to redesign? Is what I have potentially unsafe? Can it work more efficiently with a more powerful Taco pump or should I just start over with a better design (e.g., new Takagi unit and a new Taco pump to support it.

    Thanks for your input. I live in Utah and the winters are cold.
     
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