Viessmann Vitodens 100 B1HE-199: flue gas temp is higher than flow temp

erickyung

I regularly check the boiler information on ViCare app. Sometimes I see flue gas temperature is higher than flow temperature. The flue gas temperature even goes higher than the constant set point temperature at times. Is it normal? Does it mean I have a much bigger boiler than I need?

I've started noticing this condition when the boiler is on call for heat and it has been on for awhile, like 3 hours. I don't know why it would not modulate down because the modulation is still higher than the minimum of 10%.

I've read some where that heat is wasted out of the flue. Is there a way to correct this?

EdTheHeaterMan

The flame temperature may start out at somewhere over 1500°F. (even hotter that 2000°) As the flue gas moves away form the fire, that temperature drops as the heat is transferred into the water on the other side of the boiler's heat exchanger surface. By the time the flue gas reaches the exhaust pipe to vent outside, it may drop to 220°F or lower. the water in the boiler will never get hotter than the amount of heat that is exhausted. It may get close, but never higher. The exhaust will always be hotter. Think of it like this: 150° exhaust can not make 180° water. The water may get hotter in some places inside the boiler, but it will also be colder in other places. After it is all mixed together inside the boiler and comes out the supply pipe. It will only be as hot as the exhaust gas will provide. NEVER HOTTER. Always lower. Sometimes lower that other times.

Your heater is operating normally.

Now your heating appliance is capable of using upwards of 95% of the energy in the gas you are burning under certain conditions. It is at least 90% efficient under the other conditions. That being said, if you don't like the fact that you might be loosing 10 % of the energy you are paying for, you could always just pipe the wasted 10% from the exhaust pipe into your bedroom. You won't be worrying about the energy waste after that first night of operation.

Click on the Spoiler for the Sarcastic part of my answer

erickyung

Thank you @EdTheHeaterMan for your insights!