No 24vac at thermostat on 25 year old Carrier HP... sometimes
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No 24vac at thermostat on 25 year old Carrier HP... sometimes
Sorry for the long post. Lots of detail, though!
I have a 25 year old (I know... External Serial No. 3094E24778 so true 1994 model) Carrier (External Model 38AY024300) Heat pump (I am not sure of the model numbers/serial numbers of the crawlspace portion, just yet).
This is the downstairs unit in a 2400 square foot house in North Carolina. It is original to the house. I've replaced motor capacitors once or twice in this outdoor unit. I'm no stranger to electrical work and have had to deal with a few electrical hijinks in this house since we bought it in 2003, mostly related to kitchen and bathroom remodels. I offer this info as evidence that I have some skill in electrical work...
When I left for work this morning, the heat pump was functioning normally. It was about 38 degrees this morning and drizzling rain, temp hovered around 40-45 degrees all day.
The wife calls me later this morning indicating the downstairs heat is not working. To be more specific, no compressor noise, no outside fan noise, and no inside blower noise (I've trained her well on the details collection).
Now, once or twice in winters past, usually during a rainy or icy period, this has happened, but self-corrects after a day or two, when I'm able to get into deeper troubleshooting; I've always noticed that there is no 24vac present at the thermostat during these failure events. In one instance last year I was able to apply 24vac myself at the appropriate contacts at the thermostat and "command" the heat on successfully.
I get home today and started at the thermostat and discovered the same thing - no 24vac at the thermostat. I went to the outside unit, checked breakers which were not tripped and seemed to have snappy throw (I did once replace them due to lightning which caused one phase to not work). I switched them to Off and back to On, just to make sure. I switched them off again and pulled the side panel on the outdoor heat exchanger unit, switched the breakers back on, and was able to measure 240VAC at the contactor contacts, as well as 120 VAC from each one to ground, so I know the breakers must be functional.
My initial thought way back when was that the 24vac control transformer might have failed, or maybe a control board. I bought a replacement transformer in preparation to have it when climbing under the house to start troubleshooting back then, but before I could get under there the system started working again.
I have done some research on the low-voltage control circuitry of these systems in general, and have learned there are a number of safety cutouts, limit switches, etc for the 24vac control circuit.
A last note or two. The wife reported that after noting the temp drop to about 60 in the house, she thought to try emergency heat, and it kicked on. It brought the house up to temp (we keep it at about 64) and turned off, and did not turn back on. When I came home the thermostat was commanding the heat pump on, but nothing was running, and no 24vac at the thermostat terminals.
Second last note. In troubleshooting starting at the outside unit, I used one of those laser thermometers to see if I could find anything "warm" or "hot" - Everything measured 42 degrees or so (expected), except the evaporator/condenser fan motor which read about 51 degrees F. That tells me that there's probably a bad motor capacitor for that fan, and it got very hot much earlier in the day.
Third last note: The thermostats are LUX PT2110, which are self-powered units (run off 2xAA batteries instead of 24vac control source).
So, at this point, I'm thinking to ask some experts their thoughts or resources - a schematic diagram would be lovely, or some basic description of the circuit theory about how the various components of the 24vac system (outside, inside air handler, inside thermostat) interconnect along with cutout switches and whatnot, and that kind of thing. Otherwise, I guess I'll need to start drawing one myself and tracing out the circuit, one component at a time.
I know it's an old unit, but it's been functioning well; and I hope to squeeze another couple of years out of it so I don't have to pick which kid can't go to college in a few years. Any help at all would be very appreciated.
I have a 25 year old (I know... External Serial No. 3094E24778 so true 1994 model) Carrier (External Model 38AY024300) Heat pump (I am not sure of the model numbers/serial numbers of the crawlspace portion, just yet).
This is the downstairs unit in a 2400 square foot house in North Carolina. It is original to the house. I've replaced motor capacitors once or twice in this outdoor unit. I'm no stranger to electrical work and have had to deal with a few electrical hijinks in this house since we bought it in 2003, mostly related to kitchen and bathroom remodels. I offer this info as evidence that I have some skill in electrical work...
When I left for work this morning, the heat pump was functioning normally. It was about 38 degrees this morning and drizzling rain, temp hovered around 40-45 degrees all day.
The wife calls me later this morning indicating the downstairs heat is not working. To be more specific, no compressor noise, no outside fan noise, and no inside blower noise (I've trained her well on the details collection).
Now, once or twice in winters past, usually during a rainy or icy period, this has happened, but self-corrects after a day or two, when I'm able to get into deeper troubleshooting; I've always noticed that there is no 24vac present at the thermostat during these failure events. In one instance last year I was able to apply 24vac myself at the appropriate contacts at the thermostat and "command" the heat on successfully.
I get home today and started at the thermostat and discovered the same thing - no 24vac at the thermostat. I went to the outside unit, checked breakers which were not tripped and seemed to have snappy throw (I did once replace them due to lightning which caused one phase to not work). I switched them to Off and back to On, just to make sure. I switched them off again and pulled the side panel on the outdoor heat exchanger unit, switched the breakers back on, and was able to measure 240VAC at the contactor contacts, as well as 120 VAC from each one to ground, so I know the breakers must be functional.
My initial thought way back when was that the 24vac control transformer might have failed, or maybe a control board. I bought a replacement transformer in preparation to have it when climbing under the house to start troubleshooting back then, but before I could get under there the system started working again.
I have done some research on the low-voltage control circuitry of these systems in general, and have learned there are a number of safety cutouts, limit switches, etc for the 24vac control circuit.
A last note or two. The wife reported that after noting the temp drop to about 60 in the house, she thought to try emergency heat, and it kicked on. It brought the house up to temp (we keep it at about 64) and turned off, and did not turn back on. When I came home the thermostat was commanding the heat pump on, but nothing was running, and no 24vac at the thermostat terminals.
Second last note. In troubleshooting starting at the outside unit, I used one of those laser thermometers to see if I could find anything "warm" or "hot" - Everything measured 42 degrees or so (expected), except the evaporator/condenser fan motor which read about 51 degrees F. That tells me that there's probably a bad motor capacitor for that fan, and it got very hot much earlier in the day.
Third last note: The thermostats are LUX PT2110, which are self-powered units (run off 2xAA batteries instead of 24vac control source).
So, at this point, I'm thinking to ask some experts their thoughts or resources - a schematic diagram would be lovely, or some basic description of the circuit theory about how the various components of the 24vac system (outside, inside air handler, inside thermostat) interconnect along with cutout switches and whatnot, and that kind of thing. Otherwise, I guess I'll need to start drawing one myself and tracing out the circuit, one component at a time.
I know it's an old unit, but it's been functioning well; and I hope to squeeze another couple of years out of it so I don't have to pick which kid can't go to college in a few years. Any help at all would be very appreciated.
#2
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Sounds like typical issue with low pressure - refrigerant loss. Find the low pressure sensor in the outside unit. Probably measure 24vac on one side and 0 on the other.
How are you measuring 24vac? R to Y or something else?
You stated "thermostats" - is this a zoned system? Zone controller issue?
How are you measuring 24vac? R to Y or something else?
You stated "thermostats" - is this a zoned system? Zone controller issue?
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Hi - there are two separate systems, one for upstairs (working just fine) and one for downstairs (the problematic system). Each has a separate thermostat, but they're identical thermostats.
I may have confused things with using 'thermostats' plural. Apologies for that!
Where is the low-pressure sensor usually located? Is it accessible from outside the unit? I'll track it down.
Thanks,
Josh
I may have confused things with using 'thermostats' plural. Apologies for that!
Where is the low-pressure sensor usually located? Is it accessible from outside the unit? I'll track it down.
Thanks,
Josh
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I measured every terminal on the t'stat against every other terminal. In other words, I put one probe on the left-most terminal, then tested voltage on ever other terminal with the other probe; then moved the first probe to the second terminal and tested every remaining terminal with the other probe; until I had tested voltage across all terminal combinations. All of this at the t'stat.
#5
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Do you have a C connection at the thermostat? Check next for 24vac across R and C at the air handler - sometimes instead of terminals these are loose red and blue wires wire-nutted to the thermostat wires.
I was probably going down the wrong path as even with the outdoor unit tripped you should still get voltage between R and W or R and G. The 24vac transformer is in the air handler which originates the R and C wires.
Diagram shows high pressure, low pressure, and temp switches
https://dms.hvacpartners.com/docs/10...05/38AY-3W.pdf
I was probably going down the wrong path as even with the outdoor unit tripped you should still get voltage between R and W or R and G. The 24vac transformer is in the air handler which originates the R and C wires.
Diagram shows high pressure, low pressure, and temp switches
https://dms.hvacpartners.com/docs/10...05/38AY-3W.pdf
#6
You're losing your R (24v) voltage if the stat is going blank. It would not be an outdoor problem. If the display was blank.... the emergency heat should not have worked either.
Typically on a furnace if you lose R it means there is safety stat opening. There aren't any on your air handler except for the electric strips. You need to determine why you are losing the R voltage. The first thing that comes to mind is do you have a float switch in your condensate pan or do you have a condensate pump ?
A transformer rarely goes intermittent.
When checking for voltage at the stat..... everything is measured to R.
R to Y, R to G, R to W, R to C, R to O/B.
Typically on a furnace if you lose R it means there is safety stat opening. There aren't any on your air handler except for the electric strips. You need to determine why you are losing the R voltage. The first thing that comes to mind is do you have a float switch in your condensate pan or do you have a condensate pump ?
A transformer rarely goes intermittent.
When checking for voltage at the stat..... everything is measured to R.
R to Y, R to G, R to W, R to C, R to O/B.
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This is an awesome diagram, thank you. I am reviewing. Sounds like it may not be the pressure switch(es) at all. Would a pressure switch cause the emergency heat strips to also not work? That seems dumb, if so...
-j
-j
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As PJmax mentioned - more likely the safety switch on the condensate pump at the air handler. Probably some drain backed up.
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I would be open to speculation, but - I have already replaced one breaker in this panel, a few years back, but I cannot remember which. It had gotten hot and part of the housing crumbled when I wiggled it, and left the metal clamp on the bus bar. I had to remove that clamp with a glove and a pair of long needle-nose pliers! I suspected lightning damage at the time, as we'd had a LOT of lightning storms that summer - we had three strikes within 100 feet of this panel in that summer.
I'm pretty sure this is the breaker feeds the under-house portion of the downstairs heat pump - the air handler, with the blower, emergency strips, and... low-voltage control transformer.
I'm open to speculation from you all. This panel is on the north side of the house, so prone to more moisture, etc. I plan to replace the breaker, and monitor for unusual heat generation. Maybe can use this as an excuse to buy a nice clamp-on ammeter (or invest in some kind of wifi-enabled clamp-on for continuous monitoring... I've always wanted to do that).
Thanks!
I'm pretty sure this is the breaker feeds the under-house portion of the downstairs heat pump - the air handler, with the blower, emergency strips, and... low-voltage control transformer.
I'm open to speculation from you all. This panel is on the north side of the house, so prone to more moisture, etc. I plan to replace the breaker, and monitor for unusual heat generation. Maybe can use this as an excuse to buy a nice clamp-on ammeter (or invest in some kind of wifi-enabled clamp-on for continuous monitoring... I've always wanted to do that).
Thanks!
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So one weird thing, I think I should note about this breaker problem.
I measured for AC voltage across the two output terminals, expecting 240vac, but got nothing. But measuring each output leg to neutral/ground (the panel itself) registered 120vac. Aside from the visible damage (which I only saw later after examining more closely), this was the indicator for me of the problem. This breaker also has what looks like 6 awg aluminum wiring connected (which it is marked for).
The voltage thing seems very odd to me - but obviously the breaker is getting replaced. I have a few more pictures, let me post them.
I measured for AC voltage across the two output terminals, expecting 240vac, but got nothing. But measuring each output leg to neutral/ground (the panel itself) registered 120vac. Aside from the visible damage (which I only saw later after examining more closely), this was the indicator for me of the problem. This breaker also has what looks like 6 awg aluminum wiring connected (which it is marked for).
The voltage thing seems very odd to me - but obviously the breaker is getting replaced. I have a few more pictures, let me post them.
#16
If the bus bars are not 100% clean and shiny..... don't bother changing the breaker.
Change the whole panel. Once the bus bars/stabs are heat stressed the end is near.
You're dealing with 240v. Two legs that are 120v each measure to ground. If you lose one leg at the breaker..... say leg B ....... then the 120v will go out thru the unit on leg A and come back out on leg B so that when you measure leg B it also shows the 120v from leg A.
Change the whole panel. Once the bus bars/stabs are heat stressed the end is near.
You're dealing with 240v. Two legs that are 120v each measure to ground. If you lose one leg at the breaker..... say leg B ....... then the 120v will go out thru the unit on leg A and come back out on leg B so that when you measure leg B it also shows the 120v from leg A.
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Here is a full-on view of the panel. You can see the meter to the left, and the service entrance coming into the bottom of the meter panel. The two heavy gauge wires after the whole-house breaker run to the "main" breaker panel that services the remaining circuits in the house. The feed coming in from the right is from a 5.6KW grid-tie solar inverter, professionally installed about 3 years ago.
All the images are here: https://imgur.com/a/zCb90UR
All the images are here: https://imgur.com/a/zCb90UR
Last edited by PJmax; 02-14-19 at 06:39 PM. Reason: added second pic from link
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The more I look at this, the more it seems to me that it's been wet inside. There's rust on the lock screws of the 50A breaker, oxidation on some of the bus bars, and rust on the metal insert behind the 50A breaker. Is this common/OK?
#19
I added a second pic from your link. There is no way to kill those bus bars supplying that 50A breaker without pulling the meter. That 50A breaker is ahead of the house main and is basically considered a 2P50A main breaker for the heat.
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That melted breaker is Eaton while the bottom two are Cutler-Hammer so is newer and likely the one you replaced before.
With nothing but a transformer as a load on the two legs you might get those readings if one of the two legs was not working. For entertainment measure from the bar coming into the breaker to the terminal coming out.
With nothing but a transformer as a load on the two legs you might get those readings if one of the two legs was not working. For entertainment measure from the bar coming into the breaker to the terminal coming out.
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I think you're right - I'll have to go back through my photos and notes and see if I can find which breaker I changed, back a few years ago. I'm now 90% sure it was this same breaker.
So, corrosion/pitting on the bus bars, sigh. I guess I'll be researching upgrades for this kind of panel. I wouldn't mind something I could back-feed with a generator or powerwall/inverter one day.
And for entertainment measure, the voltage reading from bar to terminal was 240vac (as predicted .
Thanks,
Josh
So, corrosion/pitting on the bus bars, sigh. I guess I'll be researching upgrades for this kind of panel. I wouldn't mind something I could back-feed with a generator or powerwall/inverter one day.
And for entertainment measure, the voltage reading from bar to terminal was 240vac (as predicted .
Thanks,
Josh
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I found a photo of the original breaker. It was indeed the same breaker, replaced on January 12, 2015.
Here are my notes from that event:
"The symptoms were odd. In very cold weather, when the downstairs thermostat would command the system off, the 24vac control signal would go away, and everything would be inoperable. Cycling the breaker, it would work fine again, until the next time it failed. I noticed the breaker action was very sloppy, and that concerned me, so I decided to replace it (it's only $10, after all). That's when I found this evidence.
Bus bars Shockingly clean.
one of the screw terminals was cold-welded to the aluminum cable."
https://imgur.com/a/hsL3pX8
Here are my notes from that event:
"The symptoms were odd. In very cold weather, when the downstairs thermostat would command the system off, the 24vac control signal would go away, and everything would be inoperable. Cycling the breaker, it would work fine again, until the next time it failed. I noticed the breaker action was very sloppy, and that concerned me, so I decided to replace it (it's only $10, after all). That's when I found this evidence.
Bus bars Shockingly clean.
one of the screw terminals was cold-welded to the aluminum cable."
https://imgur.com/a/hsL3pX8
Last edited by PJmax; 02-15-19 at 05:23 PM. Reason: added pic from link