Power Vent Boilers

First thing is to make

sure to drill your test hole 18 to 24 inches away from the combustion air blower.

Combustion Air Blower functions to overcome the resistance of the heat exchanger and insure atmospheric air is brought into the burner area. The vent should have typical negative draft readings. Combustion readings should be normal as though it was a straight atmospheric burner with out fan assistance.

If a device is a power vent then it will have positive pressure in the vent. It's purpose is to create venting to over come excessive resistance created by tight sections or restrictions. The literature for the equipment should explain how to adjust those that are adjustable.

I should learn to

not answer questions when I am in a hurry, which I am also this AM. I will get back when I have some time and address this subject hopefully with some clarity.

The problem is often correct use of terms.

This is a difficult

subject to try to explain without a volume of words. First of all and most important is to identify what Category the equipment is Category I, II, III or IV as this identifies what pressure should be in the vent and also vent temperatures.

Fan Assisted (Category I)Annual efficiency below 84%

An Appliance equipped with an integral mechanical means to either draw or force products of combustion through the combustion chamber and or heat exchangers. It uses air from within the dwelling and the burners are atmospheric. The flue in all cases is negative pressure (not pressurized) and does not have to be a sealed system. It can use standard galvanized single wall or Type “B” double wall vent pipe. It can according to NFPA 54 be vented with a natural draft appliance. EX. A boiler with a storage type water heater flued together. There are however some issues with this type of dual venting, which I will not discuss here. The vent gas temperature must be above 275 degrees (F)

Forced Draft – Blower is ahead of the combustion chamber; oil burner and gas conversion burner are examples.

Induced Draft – blower is after the chamber.

Both Forced and Induced Draft can be Category I

Category II annual efficiency above 84% there are very few appliances that meet the criteria for this which is flue gas temp below 275 degrees (F) and negative pressure in the vent.

Category III Positive pressure in the vent flue gas temp above 275 degrees (F)these are you mid efficiency boilers and furnaces. Annual efficiency below 84%.

Category IV Positive pressure in the vent flue gas temp below 275 degrees (F) these are your high efficiency condensing boilers and furnaces. Annual efficiency above 84%.

Power Venter – mechanical draft forced is a venting system in which the fan or other mechanical device is used to cause the flow of the flue or other vent gases under positive vent pressure.

The code lists Venting System - Mechanical Draft this way: A venting system designed to remove flue or vent gases by mechanical means, which may consist of an induced draft portion under nonpositive static pressure or a forced draft portion under positive static pressure.

Based on all of this when doing a combustion analysis you must know what category you are dealing with. Hopefully the manufacturer has parameters listed in the I and O manual to help you know when you have dialed in the correct combustion analysis for their equipment. When they do not then it is difficult.

I try to use these figures as a guide line:

O2 between 5 to 7 %

CO2 between 9.5% and 10%

Excess air 25 to 50%

CO under 100PPM air free unless specs state other wise.

Flue gas temps will vary as to which category

Draft readings with fan assisted should still be negative
-01, -02, -03

Do not take readings directly at the exit point of boiler and furnace with blowers involved go 18 to 24 inches up the flue pipe or PVC vent to drill your hole for testing.

If the manufacturer uses baffles adjustable or other wise or restrictors then size and length of the flue have to be factored in. As you increase length then resistance increases and draft decreases if adjustments are there use them to try to over come this problem. Or increase the size of the vent pipe. Again I would hope there are specs in the I and O manual for how long the vent should be.

An example of how tricky this can be is an Weil-McLain
CGi-6 boiler gas fired with induced draft. The vent termination is 3" it can be side wall vented using AL29-4C up to 55 feet with one elbow. It can also be vented into a chimney by increasing directly at the termination on the power venter to 5" vent. When side wall vented it is Category III when chimney vented it is Category I.

Nobody said this was easy.

I hope this makes it a little clearer for you. It helps when I teach this to be able to have actual examples of equipment to show this all in operation.