one register much hotter than others
I have a 60k BTU gas furnace. noticed recently that it's heating the living daylights out of my bedroom. using a klein probe, i read that register to be about 136F after the heat has been running for a while. most of the other registers are around 105-115, which honestly seems more normal. this was while it was about 70 in the house, meaning a DT of about 35-45 for the other registers (Which seems right on the money) , and that 1 bedroom register is giving DT of about 66 which seems far too high (the plate on the furnace says it should be 30-60)
i'm aware that too high of a DT could indicate not enough airflow and can cause HX cracking which is big problem, this is my first & foremost concern. i'm just trying to understand why this one is so much higher? any thoughts? it's no closer to the furnace than the other registers, and it might be a little better insulated, but really enough for that big of difference?
Comments
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How close to the furnace is the hot register and how close are the other registers?
You may have a poor duct design where the first dict off the furnace is getting a large push of air while all the others are getting the proper flow.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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they're all about the same distance roughly. if anything the hot one is a little farther
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Any dampers either in the registers or the ducts partly closed?
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
Then you are getting a lot of air velocity going down the main trunk duct and that branch duct is installed incorrectly allowing the velocity of the air to just push strait on thru. Check to see how the hot duct is connected to the main trunk.
The branch duct at the far end does not need the air flow to turn before going to the register All the other branches are off of the side or top of the trunk duct, so each branch needs the air flow to take a 90° turn before going to the register. I remember when that is the only way to install a branch that way, I would put a baffle in the end of the trunk to give the air something to stop it and build static pressure so more air goes to the branches on the sides.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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You need to check the heat rise at the furnace, not the registers, and go from there. If the heat rise is over 60° at steady state, the first thing to check is lockup and manifold gas pressures. If they're in spec, you could try increasing the fan speed. But that's just a Band-aid for poor design.
Is velocity different on the hotter branch duct? Are there volume dampers on the branch ducts? Does the hot duct run parallel to, or come in contact with the flue? It's extremely odd. Electric heat strip in that branch duct only? To quote Dr. Frankenstein, "Such strange goings on."
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it works something like this
Now If you place an opening at the end cap so the air can just blow on thru, then the static pressure that is supposed to push the air out the side branches will be reduced. Less air flow out the branches and more air flow to the duct that was incorrectly installed.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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couple things. i have an octopus / spider web style duct system. so there is no trunk and branch. my thought is, the DT at the furnace is probably even higher at the furnace than at the registers no? now granted in my case i've got the 1 much hotter duct and a few other more normal ones, so maybe it's more like the average of those or something …. and no, definitely not an in-line electric heater. velocity is about the same
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i have an octopus / spider web style duct system, not trunk and branch
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it's hard to say sadly. there are dampers in a few of the ducts off the plenum but they're poorly installed so it's not even clear if they are opened or closed. they obviously are not fully closed because air comes out of all registers, but they could be impeding more air flow on those
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Can you take pictures form far away from the furnace?
And maybe ever farther back so we can see the duct the is the problem one. You may need to add a damper to slow that air down. Perhaps the damper for that duct has fallen off the holder in the old thing.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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will take a pic tomorrow, but the problem isn't necessarily velocity / airflow. it's just that as mentioned that 1 duct / register is much hotter. i'm concerned that restricting airflow any further may increase the overall DT / temp rise more
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Is there a fan or blower involved with this octopus duct system? Is this a new furnace connected to an old duct system ? (never a good idea). Or do you still operate by gravity warm air rising, and cold air dropping to the basement?
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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the same amount of heat can be transferred in many ways. Steam, water and hot air. Hot air can be convected is several was also, Slow gravity flow thru large ducts, forced with a blower thru standard size ducts, high velocity thru small ducts. You can change the air flow speed and decrease the temperature or you can increase the temperature and slow the air speed down.
You say your room is too hot, so you want to reduce the temperature or reduce the air speed/volume. Think of it this way. you can move 100 pounds of 100° air into your room or 50 pounds of 135° air into your room. both will get you the same amount of heat. right now you are sending 100 pounds of 135° air into your room. You got to change something or live with the hot room.
And the measured ∆T is for the furnace overall temperature rise, not for the individual duct run. Those temperatures are taken ear the furnace in the ductwork with no line of site from the probe to the heat exchanger. The heat exchanger radiant heat can influence the temperature, and we don't want that.
If you do not have a trunk duct, then your furnace delta tee must be measured in each duct and averaged to get an accurate measurement.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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If the temperature rise through the furnace is excessive, that suggests restricted airflow. The obvious first things to check are that all return grilles are open and unobstructed, and that the filter is clean.
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Bburd0 -
couple things. yes there is a blower, it's a newer forced air gas furnace. old house with old ductwork, probably used to be gravity. i hear you on not a good idea to do that but it's probably pretty rare that homeowners want to pay to redo the ductwork.
anyways yes i hear you about the temp vs velocity (inverse relationship) the thing that's just bugging me is how it's much hotter in that 1 room. if i turn the blower speed up it will have the same effect on the other rooms as well. correct that i don't have a trunk line and if i measure DT inside the plenum, then yes the HX is visible to the temp probe which we don't want since it will be artificially higher. so yeah i guess i will have to average it out over all the ducts. should i measure each duct close to where it comes off the plenum?
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Hi, Where is the return air grille? I just had a situation where the room with the return air grille was much hotter than the rest of the place. It was not great design. Any chance that's part of the problem here, affecting air flow? Also, are the ducts pretty tight, or leaky? That could affect delivered temps.
Yours, Larry
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return is in the living room which is not the room that gets very hot. ducts are older and definitely not the most air tight but generally that applies to all of them. now granted the one for the very hot room might be a little better insulated
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Hi, I'm thinking that as long as you'll be taking photos, do get one of the furnace control, so we can see what sort of adjustment is possible. After checking gas pressures, setting high limit, and then balancing ducts might be the path to not too hot and even heat.
Yours, Larry
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Spider system. So the hot duct is getting a direct blast while the others build up some SP.
The heat rise must be addressed at the furnace. Gas pressures must be checked. How about the flue and chimney? Get a pro to make it safe, then keep up on the air filters.
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no chimney, it's a direct vent w PVC. I last changed the filter end of August (AC goes through the same system) so probably a little overdue but granted the heat & AC ran not that much in Sep, Oct.
Overall I'm thinking do I need to just create a little more resistance in the "hot duct" by partly closing damper on that one? will that only slow down the air for that one but keep it the same temp? or will the added resistance help move more of the heat to other ducts?
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I am quite frankly puzzled by the high temperature — regardless of air flow — in that one register as compared to the others.
What I want to know is what is the air temperature in the bonnet or outlet plenum of the furnace? How does that compare with the temperature at that hot register, and how does it compare with the other registers?
Regardless of air flow velocity, the temperature at a register can't be higher than the temperature in the bonnet. There is no other heat source. Therefore, at least part of the question is why aren't the other registers as hot/ Again, either cool air is mixing into that air flow — which is exceedingly unlikely, as the pressure in the ducting should be slightly higher than the pressure outside — or they are losing a lot of heat through poor insulation on the way to the register.
It also strikes me that the temperature reported at that one register is uncomfortably high — and I'm concerned that the bonnet temperature may be unreasonably high. That does point to restricted air flow overall.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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you hit the nail on the head. last night i measured the temperature at the top of the plenum to be around 140. this was after the unit had been running for about 10-15 mins. now, that temp could be impacted by radiant heat from the HX, no? but in any event, the data plate on the furnace says that the max temp can't exceed 160 so we seem to be doing fine there. the air coming out of the "hot" duct i read to be about 136 which checks out i guess against the 140 at the top of the plenum.
thoughts?
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Now to find out why the other registers are cool… I have a feeling we were chasing the wrong critter here.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
it could be better insulation on the "hot duct" - or it could be that the "hot duct" is the path of least resistance for the blower air and thus is hogging the hot air. just brainstorming ideas. thoughts?
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True it may be hogging hot air — may well be. But… that doesn't explain the temperature drop in the other ducts from the bonnet to the register, although it's true that if the air in the other ducts isn't moving as fast it will have more time to cool off. Which is what we don't want. How is the insulation on those other ducts? Air leaks?
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
the more i look at it, the hot duct is the best insulated one.
i guess the truest way to determine if it's heat loss through the duct, or simply the hottest air in the plenum going towards the hot duct, is to measure the temp of each duct line a few feet after it leaves the plenum. if they're all roughly the same temp, then it's heat loss, presumably via lack of insulation. if the other duct lines are still cooler than the hot duct immediately off the plenum, then the hot duct is hogging the hottest air just due to the way the air likes to move in my system. agreed?
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When an old octopus gravity furnace is removed and the condensing furnace was installed, there was a planum that fits the new furnace fabricated. From that plenum has 6" holes cut into it that make it look like a single die with 4 holes on one side and 3 holes on the other and so on. each hole is then connected to the ols 8" or 9" original duct with a round duct increaser.
Are any of the holes coming off the top of the plenum? If so, that is your hot duct. If possible move that to a new side opening and block off the top opening. Probably easier said than done. A damper inside the duct will accomplish the same thing.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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all ducts come off the side of the plenum. they do vary a little bit in height but not by much. there are no reducers by the way. they come off the plenum at 6" , and they stay at 6" all the way to the boots
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THIS ^^^ Lead me to believe you had an old system that was converted from gravity.
Well that was a mistake by the installer of the new furnace. Individual 6 " runs to all of the register boots is not the best design. A round trunk with short 6" runs off the 12" or 14" round main would have solved this issue. Are any of your 6" runs 20 ft or more?
Adding a damper the the room that is too hot is still the answer to your problem.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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well it probably did used to be gravity back when the house was built (1935). not anymore of course. honestly, i probably wouldn't have been able to justify the cost of re-doing all the ductwork even if the installer of the new furnace proposed that anyways. none of the 6" runs are more than 20' in length - it's a small house and small basement
wouldn't the damper be more to control velocity / volume of air, as opposed to temp? but maybe the added resistance to that hot duct will push more of the hottest air to other ducts as well?
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Well… nice idea to suppose that the air is stratified into hotter and cooler in the plenum/bonnet.
I don't buy it. There is enough turbulence in there that stratification will be minimal. I think that in chasing flow you are chasing the wrong critter — but I've said that. If you want to chase flow, though, go ahead, but get a wind speed measuring gadget and actually measure the air velocity coming out of each register. Don't guess.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
so then you think it's a heat loss issue through the other ducts?
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@camiarrobino you seem to be overthinking this. Never mind the temperature at the register, it's probably a measurement error from the thermometer "seeing" radiant heat from the furnace heat exchanger, or a difference in duct insulation.
You have a room that is too hot. Reduce the airflow to that room. Install a balancing damper in the branch duct near the furnace if there isn't one already. If you close the damper in the register, you will probably get excessive airflow noise.
The excess warm air will distribute to the other registers in the system. Problem solved.
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Bburd0 -
as long as the static pressure is within range that’s doable. On the edge and it could create other issues.
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Something weird is going on. 160° at the register on a high efficiency condensing furnace is not safe. Someone with the correct tools and testing equipment needs to inspect the furnace and ductwork.
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well anecdotally the temperature of this "hot register" is most definitely hotter than the others, you can just feel it. maybe it's radiant or maybe it's better insulation, not sure.
regarding adding a damper, i don't have a trunk and branch system, each duct is it's own line coming off the plenum, so i'll just have to add the damper to the duct as it's coming off the plenum. even if the air is still hotter, less of the hot air should help to not over-heat that room, i don't disagree.
the only hesitation that i have, as Pecsmg alludes to, is that now i'm decreasing overall airflow thru the system / adding static pressure. my system is already under-ducted and probably over-pressured. i think maybe add another duct line (for a different room, not the overheated one) where it's most needed , and then close up the over-heated duct a little bit to slow it down, and overall i think that will only help the system in multiple ways (more balanced heat and also overall more airflow )
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i never said it was 160. did i mis-state something? it's about 136 at the hot register, and then about 105-115 at the other registers. does this seem more reasonable albeit still high for that 1 register?
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So to be perfectly clear about your concern. Not really caring about what some thermometer reading says.
- Does the room in question feel hotter? (Yes or No question)
- YES = Close the damper (or install a damper then) Close the damper!!!!
- NO = Stop looking at the thermometer
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
1 - Does the room in question feel hotter? (Yes or No question)
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Sorry. What's 24° between friends? Still something funky going on.
What's the actual heat rise?
Whats the lockup and manifold gas pressures?
Even if there wasn't any issues, these things should be known.
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24F is still quite a big difference. regarding measuring "actual heat rise", what would be your suggested way of measuring this? at the supply and return plenums? regarding lockup and manifold gas pressures, my abilities end there. I understand the concept of gas pressure and believe that the outgoing gas pressure should be 3.5" WC for my unit, but i'm just a homeowner and won't mess with the gas valve.
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