Bypass Damper not balanced? - high/low limit trips often...

I had an existing HVAC system zoned a couple summers ago. The installers put in 3 "zones" with dampers and a barometric bypass damper. Normally the system works great, but I recently noticed that soon after the 2nd or 3rd zone requests air, the high/low limit sensor kicks off and cycles the unit. My guess is that the air being passed through the bypass duct is too hot/cold and triggering the limit. If only 1 zone is requesting air, the limit is never reached and the system provides heat/AC until the desired temp is reached. I contacted the installer and they told me that my filters are dirty (they're not, I replace them every 3 months) or too good and are restricting airflow. (have you heard that one before?)

What can I do to resolve this? I've read that the bypass is there to prevent pressure build up when the other zones are closed, but this happens with 2 of 3 zones open. Can I remove the bypass ducting completely, or will that provide an unsafe pressure in the system? What does that mean anyway? Won't the air just go to the other vents.

Thanks in advance.

 

If you are using pleated filters, some are highly restrictive & can cause limit problems.
In my opinion, you wasted a good bit of money having a warm air system zoned. I've never seen one that worked well, if at all.

 

I agree with Grady, zoning of a residential forced air system is usually a mistake. Far too often a really big mistake.

 

There were very good reasons for zoning the HVAC and since that's in the past, it's a moot point.

As for filters, I had been using good quality 3M filters, but switched to cheapie non-pleated filters. As for restriction, I don't think that's a problem as the high/low limits are reach without filters in place as well.

From all I can tell, the bypass is feeding too much hot or cold air (depending on the season) back into the unit, causing the limit to trip. I'm curious about closing off the bypass completely and wondering what side effects that may have, if any.

 

You can try closing the by-pass damper somewhat. The thing which concerns me is you may still cause the limit to trip because you can't get rid of enough air. For a given capacity heater or air conditioner you need to move a certain amount of air. If the ductwork was designed for the whole house & you block off part, you still need to move the same amount of air.

Some zoned systems use what is sometimes called a dump zone. The purpose of said dump zone is go give the air not needed in the zone(s) calling for conditioning someplace to go.

 

This makes sense, but I'm curious why the air would not just go to the open vents for the zone that's calling for air? I guess this is where the noise factor would come into play as a higher volume of air would be pushed to those vents.

As I mentioned, when only 1 zone is open, the limit doesn't trip. It's only when the 2nd or 3rd zones open that the bypass seems to circulate more hot air back into the system. I think I need to look inside that duct and make sure the damper is working correctly. Seems to me that the damper should be more closed when a 2nd zone opens, and not the other way around.

 

Your thoughts about the damper seemingly operating backward are logical. The more zones open, the the less bypass you should have. The reason the reduced number of ducts won't handle all the air is due to resitance. To move more air thru the duct, you have to have more pressure & the fan will only develop so much pressure.

Think of water thru a pipe. If you have a pipe flowing 10 gallons/minute & you want to get 20gpm out of it, you have to increase the pressure out of the pump. If the pump is already maxed out, you aren't going to get any more than the 10gpm you already have.

Afterthought: Is your furnace multi-stage? If so, it may be going up to it's higher firing rate when a second zone opens.

 

Thanks for the explanation. Makes more sense now.

I don't think it's a multistage as it's about 10+ years old and only hear it go at one speed.

This weekend we're supposed to have temps back in the 60's so I'll have a better chance to take a look at the bypass and see what orientation the actual damper is in. I think my longer term solution is to lengthen and redirect the bypass into another return duct rather than right back into the furnace like it is now. I'd hope that would mix it with the air coming from the rest of the house.

 

Just be careful how far from the furnace you move the by-pass duct. You don't want heated or cooled air essentially back flowing & coming out the return grills. Keep me posted. This is getting intersting.

 

Do they make a "Y" junction duct? If so I can use that to at least point the duct in the right direction. When I get a chance to take it apart I'll have to do some testing to see where I can tie it back in so that the air get's appropriately mixed. I'd guess I probably won't need to go that far back that it would cause a problem with backflow.

I'll post an update when I make the changes.

 

If your by-pass duct is round, relocating should be pretty easy. If it is rectangular or square, connecting to the main return is going to be a bit more difficult.