Hi, I have a perhaps unusual situation, and it's kind of a long story, which I apologize for: We have a house, built in 1965, that was later added on to. They didn't up-size the main trunk lines of the hvac. It's an 1800sf house, with a 104k btu high efficiency Rheem installed 2012. We owned the house 4 years before I realized the situation. The original trunk lines are 18x8 at the end where the furnace is, and taper down to 14x8 in the center area, before entering a soffit I've not opened up. The house has 19 registers, 3 of them in bathrooms, and 1 in a regular bi-fold closet (they moved a wall at some point). The bathrooms are all 'core' rooms with no exterior walls. So, 15 registers that are doing 'real work', 3 that are doing interior bathrooms, and 1 in a closet that's probably wasted.
I realized there was an issue when I noticed a huge grill in the bonnet above the furnace, just venting hot air into the furnace room. Our master bedroom being cold (it's toward the end of the main line), I covered up that vent to try to force that air to the rest of the house. The furnace promptly started short cycling, and that's when, through internet searching, I realized the main trunks were very undersized: I don't have a 'duct-ulator', but 'rule of thumb' internet searching led me to believe that an 8x18 duct can only supply about 8 registers. 3 of the 19 registers vent directly from the bonnet, leaving 16 coming off the trunks, so the ducts are about half the size needed. And indeed, the vertical drop of the return, which was probably installed with this furnace, was 25x10 - if it were a supply that'd serve 20 registers or so?
Using my trusty grill thermometer poked into pre-existing plugged holes in the supply and return, I was measuring approximately the maximum temperature rise the manufacturer gave for that furnace. We'd never noticed a problem with the overall house temperature before, so apparently the furnace was still managing to satisfy the thermostat, right about the time it was perhaps going out on limit. I had a local hvac guy come in an immediately patch some extra return directly into that 25x10 duct, which lowered the max temperature about 10 degrees (and also check to make sure the exchanger hadn't cracked). But that was a stopgap. I still don't want to be just wasting air into the furnace room, when our bedroom gets down into the lower 50s on the bad windy winter nights (northwest corner room of course). And that's where I stand now.
We have what I thought an interesting opportunity - there's a closet space directly under the center of the main duct trunks, about 4'x8'. I thought it might solve the problem to move the furnace itself to the center, thus splitting half the air down each direction of the ducts, which I think they're approximately the right size for. 8 supplies are on one side of the room, 7 are on the other, and 4 are above the closet space. So depending where exactly the furnace sits those 4 may be distributed on one side or the other, but in general it's about an even split of supplies on each side. There's also 3 smaller bedroom return grills on one side, and 1 larger living room return on the other. So the returns have some even distribution as well. Nobody seemed to have any particular concerns about having to re-route the freon, gas, and furnace supply/exhaust pvc, to move the furnace (it's basically just extending them in a straight line 15 feet or so).
I've had a few hvac guys in, because I wasn't sure if it's cheaper to replace an entire 32' or so of duct run (2/3 of which is already exposed) or to move the furnace. But I've not had great success with them getting back to me. I've had one guy that gave me a price for moving the furnace, and another that quoted me about 50% more to replace the duct runs. The furnace-move guy is the only one that seems on board with moving the furnace. The other 4 or so guys have all hemmed and hawed when I presented the move-the-furnace possiblity, complained the furnace is oversized for the house, and didn't really seem that interested in moving the furnace. And only one of them actually gave me a price to replace the ducts.
So my question is, is moving the furnace and splitting the air 2 ways a crazy and/or bad idea? I thought it seemed like the perfect way to solve the problem without having to rip out another 12 feet of soffit, and I'd love to have more space in the current furnace room for storage. And the two prices that I got seem to bear out my suspicion that replacing the ducts is more expensive. But I had 4 guys that didn't bite on the moving the furnace idea, and only 1 that did. So I wanted to come here and get a broader opinion. I didn't press the people too hard on it because I didn't want to bias them too much. And if this one guy is just kind of crazy or doing what he thinks I want, well, I'd just like to get some additional thoughts on the matter, before I make such a significant decision.
Thanks for reading this very long-winded topic. Any thoughts on the matter appreciated.
I realized there was an issue when I noticed a huge grill in the bonnet above the furnace, just venting hot air into the furnace room. Our master bedroom being cold (it's toward the end of the main line), I covered up that vent to try to force that air to the rest of the house. The furnace promptly started short cycling, and that's when, through internet searching, I realized the main trunks were very undersized: I don't have a 'duct-ulator', but 'rule of thumb' internet searching led me to believe that an 8x18 duct can only supply about 8 registers. 3 of the 19 registers vent directly from the bonnet, leaving 16 coming off the trunks, so the ducts are about half the size needed. And indeed, the vertical drop of the return, which was probably installed with this furnace, was 25x10 - if it were a supply that'd serve 20 registers or so?
Using my trusty grill thermometer poked into pre-existing plugged holes in the supply and return, I was measuring approximately the maximum temperature rise the manufacturer gave for that furnace. We'd never noticed a problem with the overall house temperature before, so apparently the furnace was still managing to satisfy the thermostat, right about the time it was perhaps going out on limit. I had a local hvac guy come in an immediately patch some extra return directly into that 25x10 duct, which lowered the max temperature about 10 degrees (and also check to make sure the exchanger hadn't cracked). But that was a stopgap. I still don't want to be just wasting air into the furnace room, when our bedroom gets down into the lower 50s on the bad windy winter nights (northwest corner room of course). And that's where I stand now.
We have what I thought an interesting opportunity - there's a closet space directly under the center of the main duct trunks, about 4'x8'. I thought it might solve the problem to move the furnace itself to the center, thus splitting half the air down each direction of the ducts, which I think they're approximately the right size for. 8 supplies are on one side of the room, 7 are on the other, and 4 are above the closet space. So depending where exactly the furnace sits those 4 may be distributed on one side or the other, but in general it's about an even split of supplies on each side. There's also 3 smaller bedroom return grills on one side, and 1 larger living room return on the other. So the returns have some even distribution as well. Nobody seemed to have any particular concerns about having to re-route the freon, gas, and furnace supply/exhaust pvc, to move the furnace (it's basically just extending them in a straight line 15 feet or so).
I've had a few hvac guys in, because I wasn't sure if it's cheaper to replace an entire 32' or so of duct run (2/3 of which is already exposed) or to move the furnace. But I've not had great success with them getting back to me. I've had one guy that gave me a price for moving the furnace, and another that quoted me about 50% more to replace the duct runs. The furnace-move guy is the only one that seems on board with moving the furnace. The other 4 or so guys have all hemmed and hawed when I presented the move-the-furnace possiblity, complained the furnace is oversized for the house, and didn't really seem that interested in moving the furnace. And only one of them actually gave me a price to replace the ducts.
So my question is, is moving the furnace and splitting the air 2 ways a crazy and/or bad idea? I thought it seemed like the perfect way to solve the problem without having to rip out another 12 feet of soffit, and I'd love to have more space in the current furnace room for storage. And the two prices that I got seem to bear out my suspicion that replacing the ducts is more expensive. But I had 4 guys that didn't bite on the moving the furnace idea, and only 1 that did. So I wanted to come here and get a broader opinion. I didn't press the people too hard on it because I didn't want to bias them too much. And if this one guy is just kind of crazy or doing what he thinks I want, well, I'd just like to get some additional thoughts on the matter, before I make such a significant decision.
Thanks for reading this very long-winded topic. Any thoughts on the matter appreciated.
source https://hvac-talk.com/vbb/showthread.php?2222103-Under-sized-ducting-any-reason-to-not-move-furnace-to-middle&goto=newpost
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