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has thermosiphoning snuck up on me?

In the summer of 2010 I had a new Burnham MegaSteam installed with a SuperStor external tank, and we've been very happy with it. But this spring we noticed an issue that we don't *recall* seeing before. It became difficult to fill the bathtub with hot water, even with the faucet barely open. Maybe it's my imagination or poor memory, but...

My theory was that with warm weather, the boiler was now cold most of the time. If we tried to fill the tub when the tank was at or near its coldest (thermostat set at 120), and thus wound up calling for more heat, the boiler would take a long time to fire up and, eventually, heat the water in the tank. We can have all the hot we want, if we let the boiler catch up. Sounded reasonable to me.

Question 1: Why didn't I notice this last spring, or summer? AFAIK nothing mechanical has changed. The system seems simple. Is there something that could have broken/worn out without me noticing that would cause a change in performance?

Question 2: Even with a cold boiler and subsequent long reheat times, shouldn't a properly functioning thermostat on the tank call on the boiler as required to maintain temps? Even if 'as required' is more often than it should be?

Question 3: I've discovered the concept of thermosiphoning. Also sounds reasonable: hot water in tank + cold water in boiler = warm water everywhere. But shouldn't the thermostat fight this (at the cost of increased fuel use, I suppose)? There is a check valve in the line from the boiler to the tank. It's on the from-boiler-to-tank line (I think). Should this be helping, and is it in the right place?

I have not done the math regarding tub fill rate, tank capacity, tank temperature, boiler re-heating rate, etc. to know what performance I ought to be able to expect. The tank is 45 gal (I think) and the boiler is MST-629.

In the meantime I have raised the setting on the tank thermostat, which seems to help, but this doesn't feel like the right answer.


  • Tim_HodgsonTim_Hodgson Posts: 59
    Installing Contractor

    Have you talked to the installing contractor? You are correct that the tank thermostat should keep the tank up to temperature. Your cold boiler will simply keep the recovery time longer. if your tank is too small for the tub, you must rely on the recovery rate to cover the small tank limitations. It sounded like it was not a problem when the boiler was heating in the winter. I usually size the tank so 75% of the tank volume will fill the tub. With blending at the tub this gives a cushion of warm water left in the tank. Some 40 gallon indirect water heaters only have 32 gallons of storage.

    Good Luck,

    Tim Hodgson
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