Fire Tube HX exhaust reverse flow after shutdown

Solid_Fuel_Man

So I installed an ECO70 in my home to test out this design and see if I'll sell more in the future.

The consistent problem with this type of top-burner down fired design is when a call is satisfied the hot burner and refractory cause a chimney effect into the intake pipe directly above the burner. This in turn pulls warm moist air back through the burner through the combustion fan and into the cabinet.

I've tried every configuration and it does it every time. I've reached out to my rep about extending the time for post purge fan settings and he says they are unadjustable and says it's the warm HX which is causing my problem. I have a post purge setting and dump all leftover hot water into slab. The water is <90 after any call is satisfied.

It's piped with 2" polypropylene exhause horizontally and 2" PVC intake.

Any suggestions? I'm about to use a small computer type fan on a timer at the intake connection to perform a post purge to my liking but this seems like a hack!

Zman

Where do the CA and Vent terminate?

Solid_Fuel_Man

Vent is to the left of the picture about 5 feet. Intake follows floor and is 20 feet in length with the 3 45's you see in pic and one sweep downturn outside. Intake is on north wall and vent is on east wall.

Zman

I think the draft is likely caused by the locations. When they travel the same distance and terminate on the same aspect, draft is greatly reduced.
I don't recommend it, but it is possible to install a damper on the CA which is interlocked with the boiler. It would solve the draft issue.

NY_Rob

How about fabricating a one way hinged at the top flap (plastic/rubber, etc..) over the rectangular intake air port opening inside the cabinet? Gravity would hold it closed, any minor pull from the induction fan would open it... reverse flow would be blocked. More reliable than a fan & timer.

Sort of like the "Air Damper" that failed in the HTP thread:
https://forum.heatinghelp.com/discussion/165450/htp-uft-problem#latest

DZoro

In your configuration your in 2 different air pressures. If you have a north wind the air will easily enter the north pipe and very easily want to exit the east pipe. Vise versa for other winds. Can you get both intake and exhaust on the same side of the building?

Solid_Fuel_Man

It was worse with both terminations on the same wall. We have prevailing winds out of the North which was my thought process of the North intake.

@NY_Rob The intake is hard to see in the pic but I can't do the flap thing as that would be my preferred method. It seems to be the heat from the burner which is causing this. I've forced so.e air through the blower for about 3 min to cool things off and there is no measurable airflow after that with the smoking match trick and a window 2 feet away open.

I may try making a snorkel on the exhaust and see if that creates enough draught to keep things dry inside.

flat_twin

You made me curious so I pulled the cover off our Eco boiler. No moisture in the fan snorkel, no signs of any moisture in the cabinet. It's been in the 50's all day so the boiler has been cycling. We have 3 inch PVC, 2 sweep 90s, and a concentric vent exiting the west wall of the basement. PVC piping length is 10'.

kcopp

Definitely do the snorkel outside...

Solid_Fuel_Man

Just wanted to follow up. The snorkel solved the issue, I went up 5 feet which now provides sufficient draught to keep things moving in the right direction after a call is satisfied.

Thanks for the suggestions!