Vent sizing?

The boiler is 5" the water heater is 3" and the gas stove is 4". The gas stove is only used if the electric power goes out, so it doesn’t run if the boiler is on. My understanding is that if the boiler was 5" and you add a water heater you must increase the size of the vent 1". Here it’s 5" through to termination. Is this allowed? Owner stated that a local HVAC company did the install…

4791 Hwy 3 South 099 (Small).jpg

4791 Hwy 3 South 098 (Small).jpg


How many BTU’s was the boiler and how many was the water heater?

Hi Stephen,
The boiler was 105,000 input 88,000 BTU per hr, the water heater was 36,000 btu.


Typically you are correct. The connector should increase in size where more than one appliance is manifolded into the flue. The flue must equal the largest connector plus 50% of the others. The smallest appliance connector must enter at the highest level consistent with available headroom or clearance to combustibles.

Of course your area AHJ may vary. Ultimately you should contact the local AHJ and ask them about this installation, especially if no permits or inspection tags are visible.

Thanks for the info William. I did see that the AGA had charts that did show that a common ( 2 or more appliances ) 5" diameter gravity vent with draft diverter’s within 2ft verticle height, with the horizontal run no higher than 5ft with a total vertical height of 15ft could handle 140,000 btu. But that was alot of work…the AHJ would be the best choice.

Was there any evidence that the existing setup was not functioning or signs of implications resulting from incorrect sizing and connections? i.e. visible staining or corrosion from condensation, back drafting, scorching at the combustion chamber, etc…

Both the boiler and water heater drafted well. With no backdrafting. Normal scale / rust from lack of recent service. No visible issues with condensation, roll out or sooting. It seemed functional, I just question the vent sizing as you said it should be 6 or 7" based upon " The flue must equal the largest connector plus 50% of the others."