Oil-burning Tankless Hot Water Heater Temperature-recovery Problems.
December 3, 2014 6:45 AM   Subscribe

We have an issue with the hot water supply from a 2004 Peerless oil furnace/boiler with tankless hot water. The shower starts at full temp, but quickly goes tepid (too cool for a comfortable bath/shower, but not ice-cold) and returns to full heat after 5-10 minutes. This is most apparent during the first two showers of the day or bath in the evening - subsequent showers or baths don't seem to have the problem to the same extent. This makes troubleshooting difficult, as we can't tell until an hour or so after running all that hot water to see if the problem returns. It's most pronounced in winter, during the summer the issue seems to go away. (Details inside)

- It only seems to affect the shower or tub.

- It was working without a problem the first year-and-a-half we owned the house

- It's not the mixer valve in the bathroom, as the hot water tap on the bathroom and kitchen sink also produces tepid water once it starts happening in the shower.

- Furnace technician diagnosed it as lime scale in the heater core. He recommended we replace it completely, along with a few pressure release valves and a hot/cold anti-scald mixing valve. This didn't help.

- The technician then came back and replaced the aquastat with a new, state of the art unit. This didn't help.

- The technician then came back and throttled down the intake valve to the heater core, theorizing the hot water was being flushed from the heater core too quickly. This didn't help, made things worse the next day, and had weird side effects - the heat was constantly on at full-blast as the furnace was running all the time and hot water started running through the home heating circuit for whatever reason. Opening the valve up again all the way fixed the heating issue.

- After doing some research, this is a "temperature recovery" problem, and I've adjusted the anti-scald valve to a lower temperature. It's working, as I initially set too low a temp, but set as low as we can stand it for a hot bath still gets us the same "recovery" problem.

We're going to be using yet another servicing company going forward (obviously), but I'd like an idea of what they can or should be doing to diagnose this. We've sunk a lot of money into useless repairs.
posted by Slap*Happy to Home & Garden (5 answers total) 1 user marked this as a favorite
Can you clarify the contradiction between your number one and number three bullet points? Do you mean that, after a period of non-use, if you turn on the kitchen faucet or the bathroom sink faucet full blast, you NEVER experience this tepid cycle (ie., nice hot water there, consistently, every time), but if you start with the bath/shower, it ALWAYS happens, and it always then affects all the other faucets?

In any case, my money would be on the mixing valve or the anti-scald feature. What happens if you turn the anti-scald all the way up (ie., just eliminating any anti-scald protection for the moment and relying on the mixing valve)?
posted by beagle at 7:29 AM on December 3, 2014

Response by poster: The symptoms don't occur when using the sinks, only when using the shower or tub - BUT, once the tub or shower goes tepid, the water is tepid at all hot water taps in the house. You can feel the water run cool through the pipe from the heater coil to the anti-scald valve, and then the furnace kicks on, and returns to full heat after a few minutes. The aquastat is brand new.

I'll try opening up the anti-scald valve all the way to see if the problem recurs.
posted by Slap*Happy at 7:54 AM on December 3, 2014

I think that means the mixing valve or antiscald at the tub/shower are the problem. If you can get continuous hot water with no problems at any other faucet, it means the whole boiler/tankless end of things is just fine. I would start by replacing the anti-scald (assuming it is separate from the mixing valve). My guess is that for some reason it temporarily over-compensates by cutting down too much on the hot water flow. This then is causing imbalances elsewhere in the system.
posted by beagle at 8:30 AM on December 3, 2014

Response by poster: Boiler --1--> Anti-Scald Valve --2--> House

When it happens, you can feel the pipe at (1) go cold, as the boiler takes a while to bring the temp of water in the coil back up to temp. I'm not certain how the mixing valve at the shower can cause that. I thought the difference was the volume of water the shower/tub moved vs. the volume a sink could move - hence adjusting the anti-scald valve to throttle back the water leaving the coil.

I'm concerned it may be an issue with the boiler's control electronics - which is why we're only having trouble in cold weather when the house thermostats are on. I just don't know where to begin to look for a conflict between the control box and the aquastat.
posted by Slap*Happy at 9:51 AM on December 3, 2014

It turns out there are no new questions. Previously. That case also involved a tankless water heater. The timing is a little different but you might check out some of the theories offered there. See JackFlash's answer, particularly. See also this, which is along the same lines. And more via this search.
posted by beagle at 11:02 AM on December 3, 2014 [1 favorite]

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