There's no way to calculate accurately because every boiler/furnace is a bit different.
It has to be measured.
Really I only need a ballpark figure of the expected flue gas temperature in order to decide if lowering the minimum boiler water temperature is something worth pursuing. If it is, I will ask a pro to come in and test the idea, take measurements etc. to determine if everything is operating OK.
I don't think the stack temperature should be below 350F or so. It's not just a question of condensation in the boiler, the exhaust needs to be hot enough to rise up the stack and not condense.
It's actually condensation in the flue that I am concerned about. The boiler can operate at 105F return water temperature without condensation on the heat exchanger being an issue according to the boiler documentation.
But I take your point. Is there a way to determine the minimum exhaust temperature needed to allow the exhaust to rise up the stack?
Call a pro out to do a combustion analysis which includes measuring oxygen, flue temp, monoxide and more.
Without testing, I wouldn't go below 150F supply water temp. The return water temp will depend on the design of the loop - rads installed, etc, efficiencies vary, etc.
The boiler uses an outdoor temperature reset controller (ORC). What I am considering is dropping the minimum supply water temperature to 125 F. Since ORC is used, the controller would only drop the supply water temperature to the minimum during warm outdoor temperatures and the return temperature should be about 115-120 F I figure, which is safely above the boiler return minimum of 105 F.
Keep in mind, if the heating system is cycled by a thermostat and heat is controlled properly, the savings of using reducing the water temp from like 180 to 150 are marginal.
The burner has to cycle either way and the efficiency gain is just from improved heat transfer.
I understand the efficiency gain is about 1% for every 3F drop in water temperature. Currently the boiler is operating at 155 F minimum supply. So dropping it to 125 F supply is a 10% efficiency gain.
Now, if the boiler is supplying heat continuously and an outdoor reset is being used to regulate heat output, you can see huge energy savings.
ORC is being used already as explained above.
The problem is, at the minimum safe return water temperature, it will still supply too much heat most of the time.
Yes, this is also an issue as some parts of the building are too warm when the outdoor temperature is warm.
The solution is to get mixing valves installed so the boiler return temp can be kept at a safe level or have a secondary loop with a heat exchanger.
The boiler is kept at 160 to 180f or whatever while the outdoor reset adjusts the supply temperature.
I don't know exactly how it would be setup - a pro should be consulted and do any retrofit.
This would be the ideal solution. I've looked into it a bit. We could probably operated even lower, like down to 115 F if we installed a mixing valve.
However, due to the installation cost, I want to determine how low the supply temp can be without needing a mixing valve. One HVAC pro advised me that the boiler minimum supply temp in the ORC controller can be set at 20 degrees above the boiler minimum return temperature, and hence we could use 125 F minimum supply temp.