Boiler Draft Woes...

Cody350

Completed a major overhaul for a friend last fall, see pictures below.

Considering his natural draft water heater was long in the tooth, we decided that an indirect would be a great option. To this point, I had never added an indirect to a cast iron, natural draft boiler as it was usually an add on for higher end, mod con systems. Boiler wasn't terribly old at 12 years and the majority of the system is high temp so going high efficiency wasn't a huge priority.

Boiler worked fabulously all winter.

Fast forward to this summer. Boiler has tripped the spill switch at the draft hood a few times. It happens only on the hotter / more humid days and I'm curious what the collective here thinks about solutions?

Here are some facts.
- Chimney is clay lined, center of house
- Chimney is a guesstimated 6x6, maybe larger
- Chimney has cap
- Hole in chimney from water heater was capped and mortared
- Chimney damper set to open to test for potential issues
- Boiler is intermittent pilot w/ ribbon burners
- Boiler is 96k btu/h
- Boiler aquastat set to 170F
- Indirect set to 140F with mix valve

I haven't been back to check draft at the chimney or boiler (Covid) but I can assume it's a poor draft issue and not just a blockage in the chimney. I have no way of adjusting draft currently.

How would you propose this draft problem is dealt with?

I appreciate any of your thoughts / comments!

MikeL_2

Cody,
          Is there enough combustion air? Could there be mechanical venting devices requiring make up air that compete with the boilers need for combustion air - kitchen exhaust hood / fan, bathroom exhaust fans, clothes dryer, or whole house fan, etc.?

Zman

What is the height of the chimney cap compared to the highest point of the house?

EBEBRATT-Ed

Check the gas pressure, it's probably supposed to be 3.5"wc. You can probably back it down to 3" and recheck with a combustion test.

JUGHNE

Actually a lot of things have changed in the room.
You no longer have the pilot of the WH keeping the chimney warm.
You are now using the chimney in the summer for the boiler which probably never happened before.

Are you sure you have enough pipe and pump to move the btu's thru the boiler for the indirect. If not it is possible the boiler runs hotter than usual, raising the flue temp and getting the limit warmer than usual.

Is there a window that could be cracked to allow more combustion air?
Could you lower the water temp in the boiler?

Cody350

MikeL_2 said:

Cody,
          Is there enough combustion air? Could there be mechanical venting devices requiring make up air that compete with the boilers need for combustion air - kitchen exhaust hood / fan, bathroom exhaust fans, clothes dryer, or whole house fan, etc.?

This is a house from the 70's, I believe. It's a tri-level home and the basement is completely open / unfinished. There is certainly bath fans and a kitchen exhaust but no whole house fan. Definitely considered a lack of make up air / negative on the house but it never happened in the first 6 months of operation. Thanks!

Zman said:

What is the height of the chimney cap compared to the highest point of the house?

I'll have to check chimney size and this when I'm able to get back there. Still odd that it's only happening now / when it's hot. Thanks!

EBEBRATT-Ed said:

Check the gas pressure, it's probably supposed to be 3.5"wc. You can probably back it down to 3" and recheck with a combustion test.

I was hoping to get in there and do a maintenance on the boiler, I'll check then. Thanks!

JUGHNE said:

Actually a lot of things have changed in the room.
You no longer have the pilot of the WH keeping the chimney warm.
You are now using the chimney in the summer for the boiler which probably never happened before.

Are you sure you have enough pipe and pump to move the btu's thru the boiler for the indirect. If not it is possible the boiler runs hotter than usual, raising the flue temp and getting the limit warmer than usual.

Is there a window that could be cracked to allow more combustion air?
Could you lower the water temp in the boiler?

True on the WH pilot and the boiler use in summer.

1" piping from boiler to indirect coil with an Alpha. More than enough flow.

Lowering the water temp in the boiler would likely result in more draft issues since it would take longer to heat up the chimney, no?

Thanks!

Bob Harper

The natural draft tends to reverse in summer. You removed the std. pilot of the WH that may have been just enough to overcome this force. 96MBH ain't much to start with. The vent damper is attempting to block the flow of heat loss and dilution air up the stack. Instead, at standby, warm moist outdoor air cools and condenses. The high Rh condensing sucks further BTUs from the air causing it to accelerate to dewpoint. With the vent damper closed, you have a flue full of a slug of cool, dense, soggy air. When the unit fires, it slams into this obstruction. If there is any negative pressure in the CAZ, flow will be downwards. Since there is no pre-purge available and no stack losses from a std. pilot or from the boiler thx to the vent damper, it is not a "self-starting" chimney. If the attic is depressurized by ventilation combined with penetrations in the top of the thermal envelope, you will have a LOT to overcome to in order to generate draft pressure and positive flow.
The room volume presents plenty of combustion air. The pressure gradient is of question from outside the CAZ to inside.
Before you mess with temp settings and such, I'd start with a simple means of priming the flue. Get a thingy called a "draw collar", install it right above the vent damper and wire it into the circuit with a relay so whenever there is a call for heat, it cycles for about 6 minutes then cycles off using a simple delay on make switch. It will prime the flue with dry heat from an electrical resistance coil. That should do it.
As for the chimney and flue, they should have a level II inspection. If it has defects it should be relined with a properly sized thin walled 316ss liner by code. You have improved your vent rise, which helps. Get a low level CO monitor.

Cody350

So after a phone call with the homeowner I believe that I have the culprit.

There's an attic fan that is controlled by a thermostat in the first floor attic. This is a tri level home. There's pretty much a straight shot opening from the attic down to the basement around the chimney as it is not sealed. There is fiberglass insulation around the chimney at the top but this is not an air seal. This space has been used as a chase for piping and wiring.

What I need now is to test draft with and without the attic fan running. Hopefully I can do this on a hot day next summer. Since things have gotten cooler, no more tripped spill switch.

I'll update when I get back in to test draft.

STEVEusaPA

Looks like @MikeL_2 nailed it on the first post.

MikeL_2

Steve,
       I've learned to recognize opportunities for learning these past 50 plus years in the trades. Experience, & this fantastic forum are great teachers. I learned the difference between combustion & make - up air right here from Sensei and Mentor Tim McElwain......