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hot water won't stay hot - SOLVED!

TOH readers--

I recently overcame one of the more frustrating & lingering problems in our 50s house and wanted to share what I found in the hopes that it might spare someone else countless hours and $$$. Our hot water is provided by a tankless coil in a oil burning furnace. From this coil, all of the hot water is fed through the house off a single 3/4" line. From the day we moved in we had great hot water everywhere except for the tub in one bathroom. You would turn on the hot water and it would heat up nicely for the 1st 60-90 seconds, but then it would go cold - much to the chagrin of kids & guests. This problem made no sense - the water got hot initially, and the fixtures right before (sink in the same bathroom) and the right after (shower upstairs) in the sequence of the feed line had no issues. We were stumped and so was the plumber we finally called in. After dealing with this for weeks we very randomly stumbled on the issue - we were turning on the hot water TOO much. Essentially, by cranking the hot water valve all the way open we were drawing such a flow rate that after the initial couple of gallons that had been sitting in the coil were exhausted, the water coming through behind it wasn't spending enough time in the coil to heat up. It was brilliantly simple once we found it, but terribly counter intuitive until we did - who turns down the hot water to heat it up? Since finding the source of the issue we've been playing with the behind the wall adjustments and the pressure regulator coming out of the furnace, but that's all icing on the cake once you have hot water.

I racked my brain for hours on this one, hopefully it saves someone in the future.


Re: hot water won't stay hot - SOLVED!

That is a very common problem with tankless coils. It's solved with a flow restrictor at the coil. Tankless coils are a very inefficient of making hot water. You would be far better off if you installed a indirect heater.

Re: hot water won't stay hot - SOLVED!

We had a similar problem when we moved into our 1950 ranch in 2011. What was discovered was that the boiler temperature was set too low. It was set for an unoccupied home, which it was until we bought it. I did not discover it until the boiler was inspected prior to getting a service contract.

Since I never had a home that heated with oil I was not aware. Since 2011 it has worked fine. With a goal to save oil I added an indirect heater. I now know what settings the aqua-stat needs. I installed an MS 40 and left the boiler coil in place. The boiler does not run as much as it did and saves us dollars with ample hot water.

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