We have an issue with the hot water from a 2004 Peerless oil furnace with on-demand hot water. The shower starts at full temp, but quickly goes tepid (too cool for a 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 a few hours after running all that hot water to see if the problem returns.
- It only seems to affect the shower or tub.
- It's not the mixer valve in the bathroom, as the hot water tap on the bathroom 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 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. This didn't help, made things worse the next day, and had weird side effects - the heat was constantly on as water started running through the home heating circuit for whatever reason. Opening the valve up again all the way fixed the heating issue.
We're going to be using a new servicing company (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.