Heater inNtake into exhaust

[cory nystul]

I was in a 2 story house from 2006. Two thermostats in the house. Am i wrong but arent the intake and exhaust tied together in this furnace?

31761

[Raymond Wand]

Looks to be a by-pass. Note the weighted damper at bottom of the ducting on the left side.

[cory nystul]

Looks to be a by-pass. Note the weighted damper at bottom of the ducting on the left side.

What is a heater by pass used for? I have never seen one.

[Jim Luttrall]

What Kevin said about the flue pipe (although I can't really see enough of the vent to declare it a problem.)31764

The duct connecting the supply side the return duct has a weighted damper down at the bottom.
This is commonly used when you have a system with zone dampers in the duct system. This prevents over pressure to the active zone when one of the zones is inactive. Very common here in two story houses where they will use two thermostats to control one HVAC system. Without the bypass duct the air noise becomes objectionable. The weight on the end of the damper control rod lets you adjust the amount of air flow through the bypass duct with the pressure in the duct counteracting the weight to achieve automatic balancing.

[Jim Robinson]

It doesn't seem like it would bypass the filter. I had one a few years ago. I think it was for a humidifier, but that doesn't seem to the case here. I'm not sure why they would do that. It almost seems like a return loop in a boiler set up, where they are trying to make sure the air temp stays high (or low) at the furnace, but that doesn't make any sense.

Are there any zone dampers or other unusual (to me, at least) on the system, where the furnace may be running but not able to output at 100%.

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I re-read your OP, I see that there are two T stats. I think that is part of the situation. There may be a zone damper, and the bypass is to relieve pressure if one zone is closed and the other open. There are some issues with using a bypass on the cooling side of the equation.

[Dave Borowski]

The bypass ducting is perfectly normal. Without knowing all the facts, it would appear of your 2 T/stats one controls the HVAC while the other simply controls a damper motor to a zone. Better systems employ a ZONED control board so either T/stat will duty cycle the HVAC & the you'll have 2 zoned dampers (or however many zones you have). The bypass damper controls the TTL static pressure the blower is working under. I would suggest - there are better configurations and considerations to dump zones that have less side effects than the bypass system. On the flue pipe, it's unusual to see a increaser in the line, but without knowing the TTL BTU and qty of appliances tied in I can't address it -- see NFPA54 to understand flue configurations.