Ask Me Help Desk - Protect condensate drain from freezing?

This is my second question relating to a forced air furnace in my attic. In tracking down a replacement circuit board on the internet I found out that Goodman is offering a recall for all furnaces installed with Ultra Vent or Plexvent plastic vent piping, and one option is a replacement 93% efficiency furnace for only a few hundred dollars, they pay for installation. This is for vent pipe that Goodman didn't even sell. So instead of replacing the circuit board, I am planning to take them up on the offer.

My question is about how the condensate should be routed and about how to keep the condensate drain from freezing during winter nights when the attic can get down to -15 deg F. I want to understand what the industry standard practice is in this regard, as I will not be selecting the installation contractor and would like to be able to verify the quality of the work. I will be getting a building permit and inspections, but would rather rely on my own observations than a hurried building inspector.

There are no drains in the attic, but the furnace is located about 20 feet from a plumbing vent stack. Is is acceptable to "T" into the vent pipe and discharge the condensate there? If so, what is the standard way to make that connection (for example, should the pipe be cut and a "T" should be glued into it)? If it is not acceptable to drain into the vent stack, then where should the drain be run?

I don't think that there's enough elevation between the furnace and the attic floor to allow for sufficient direct pitch of the drain to the vent pipe. The furnace is oriented horizontally and is suspended from the roof rafters about 3 inches above the top of the attic floor joists. I calculate that I would need a minimum of 5 inches of pitch to the drain. So that would seem to call for a condensate pump. Correct?

Finally, I've done a fair amount of looking, and can't find information about freeze protection for the condensate system? The line and trap from the furnace to the condensate sump, the condensate sump itself, and the line from the condensate pump to the drain. The one reference I found discussed heat tracing, but it provided no specifics. Should that whole run be heat traced? Will it be thermostatically controlled so that it doesn't heat when the temperature is above freezing? What else should I look for to judge the quality of the freeze protection?

Thanks in advance for your help.

Around here you can only place a condensing furnace (90+) in an attic if, 1) It is in an insulated room area. 2) the condensate drain stays in that area and the confines of the home 3) there is a safety drain pan under the entire length of the furnace and coil 4) There is a safety float switch, or visible alternate drain from the safety pan.

Quote:

Originally Posted by NorthernHeat

Around here you can only place a condensing furnace (90+) in an attic if, 1) It is in an insulated room area. 2) the condensate drain stays in that area and the confines of the home 3) there is a safety drain pan under the entire length of the furnace and coil 4) There is a safety float switch, or visible alternate drain from the safety pan.

Well... that complicates things. Maybe I'm not going to replace the current furnace. I'll have to check the code requirements for my area (central New Jersey).

Quote:

Originally Posted by NorthernHeat

Around here you can only place a condensing furnace (90+) in an attic if, 1) It is in an insulated room area. 2) the condensate drain stays in that area and the confines of the home 3) there is a safety drain pan under the entire length of the furnace and coil 4) There is a safety float switch, or visible alternate drain from the safety pan.

I checked with our local plumbing inspector and he recommended that I heat trace the condensate trap and drain line. I told him that I would connect the condensate drain into the plumbing stack and re-vent it into the stack just below the roof (there are no other re-vents into that stack). He was OK with that pending receipt of the drawings to confirm my description.

I'll be using the self-regulating type of heating tape, and my plan will be to run 2 separate strips. With 1/2" insulation they will together provide protection to about -40 F, and independently (in case one fails) will provide protection to below 0 F. That'll give me safety margin if one fails for some reason.

That only leaves the backup safety for the drain pan. The float switch seems like a better option for me than a visible drain. I'm not sure that I'll look frequently enough.

I am presuming that it should interlock with the furnace to terminate operation if the switch activates, and not just sound an alarm (what if no one is home). Is that correct? If so, is it acceptable to place it in series with the 24 V thermostat heating circuit (I have no A/C on this unit)? That way I can control the furnace using only low voltage circuitry instead of intalling a relay in the main power line. The switch will be normally closed, but if the switch trips it will open the circuit and the furnace will think that the thermostat is no longer calling for heat or has been turned off.

Alternatively, are there float switches for this purpose rated for the high current draw of the main power line? The only high power switches that I see online are for pumps and require a much larger activation distance than I'll have in this drain pan.

Thanks again for your initial guidance, and in advance for your thoughts on these issues.

Just use the float switch to open the low voltage red wire. That will cut the unit off if it fills up. The inspector had no problem running the drain into a plumbing vent? What about sewer gas? I really don't want to burst your bubble, but if the unit fails to heat for long enough, the secondary heat exchanger, draft inducer blower motor and trap could freeze and burst. I know this is an expensive proposition for you and wouldn't like to see you have to do it again in 6 years, unless I'm the salesman, lol.

Good luck

Quote:

Originally Posted by NorthernHeat

Just use the float switch to open the low voltage red wire. That will cut the unit off if it fills up. The inspector had no problem running the drain into a plumbing vent? What about sewer gas? I really don't want to burst your bubble, but if the unit fails to heat for long enough, the secondary heat exchanger, draft inducer blower motor and trap could freeze and burst. I know this is an expensive proposition for you and wouldn't like to see you have to do it again in 6 years, unless I'm the salesman, lol.

Good luck

Thanks for the input. After I posted my note I realized that the easiest thing to do was to cut the hot line from the secondary side of the transformer and wire the float switch into that circuit. That way a trip will cut all all 24V power to the circuit board.

The condensate drain line has a trap in it like the other fixtures in the house. Won't that handle the sewer gas? The condensate drain will be sealed into the vent stack, not inserted into a hole. That's why I have to vent the condensate drain to the stack using a re-vent line. The house has a septic system, so there is no issue of draining into a sanitary sewer.

The only trap I saw in the installation instructions is external to the furnace and part of the condensate drain line. I was planning to heat trace it to the same area watt density as the piping, and insulate it somewhat more heavily. That seemed to be OK with the inspector. Is there another trap (or something else that will accumulate water) somewhere inside of a GMS9 furnace.

I realized that the HX and blower housing, as well as the vent piping, could have a small layer of water on them after the furnace turns off. Most of the water would, I thought, drain into the condensate trap, and there would be no accumulation of water elsewhere. Then, if the furnace is off for long enough, that thin layer of water on the surface of the HX, blower housing and vent pipe could freeze. But I figured that it wouldn't cause any damage because the water layer would be free to expand as needed as it froze, since it was only there in the form of a thin layer in a large open space. Then the next time the furnace turned on any frozen water on the HX, blower and vent would melt and the cycle would continue.

If new water deposited on the frozen layer without melting the ice, then a problem could result, I thought. Over many cycles it could build up multiple layers until something got clogged or damaged. But that's not going to happen in the HX or ID blower. They are going to get good and hot every cycle. The only danger spot, I thought, was the vent line. If the vent is long enough and cycles are short, I thought that the flue gas could cool enough that it might not thaw ice deposited near the end of the vent line. To address that issue, I will be insulating the vent pipe over its whole length from the furnace to the outlet using 1/2" closed cell foam sleeve. That should, I thought, keep the flue gas hot coming out of the vent and thaw any ice deposits every cycle.

Again, I appreciate your input. I'm just a homeowner and I recognize that I may be missing the point of the issues you raised. Have I overlooked something?

Quote:

Originally Posted by NorthernHeat

Just use the float switch to open the low voltage red wire. That will cut the unit off if it fills up. The inspector had no problem running the drain into a plumbing vent? What about sewer gas? I really don't want to burst your bubble, but if the unit fails to heat for long enough, the secondary heat exchanger, draft inducer blower motor and trap could freeze and burst. I know this is an expensive proposition for you and wouldn't like to see you have to do it again in 6 years, unless I'm the salesman, lol.

Good luck

NorthernHeat, did I write something offensive? I apologize if something sounded improper. I have found your insight helpful in planning this project and would appreciate your thoughts to the points in my last posting? I felt that I had thought through the issues, but when you raised the points above, it seemed like a good idea to go through them point by point and get your input on each one.

Again, I apologize if there was something offensive. I did not mean any insult, and I appreciate the help you've already provided.

Quote:

Originally Posted by bkspero

NorthernHeat, did I write something offensive? I apologize if something sounded improper. I have found your insight helpful in planning this project and would appreciate your thoughts to the points in my last posting? I felt that I had thought through the issues, but when you raised the points above, it seemed like a good idea to go through them point by point and get your input on each one.

Again, I apologize if there was something offensive. I did not mean any insult, and I appreciate the help you've already provided.

No! I'm not offended at all. Local codes won't let us use a plumbing vent unless you use a condensate pump with a check valve, which about all of them have built in. I personally would be very worried having a 90+ in my attic. Every year somebody leaves their garage door open the furnace freezes ond destroys it. Most of the time the ice does so much damage replacement is as cheap as repair, and faster to get them heat too. It can take 3-4 days to get a secondary heat exchanger delivered, and when it is -10 degrees outside who can wait that long?

Quote:

Originally Posted by NorthernHeat

No! I'm not offended at all. Local codes wont let us use a plumbing vent unless you use a condensate pump with a check valve, which about all of them have built in. I personally would be very worried having a 90+ in my attic. Every year somebody leaves their garage door open the furnace freezes ond destroys it. Most of the time the ice does so much damage replacement is as cheap as repair, and faster to get them heat too. It can take 3-4 days to get a secondary heat exchanger delivered, and when it is -10 degrees outside who can wait that long?

I'm glad that I haven't offended. Thanks again for your input. We'll see how the plumbing inspector responds regarding the drain connection in my permit application. It is clearly spelled out on the drawings. So if his initial verbal approval was based on my giving him incomplete information, he will have another chance to catch any issues and kick it back to me.

One good thing about our house. It was originally built with a single furnace in the basement. All the original ducts are still in place and just dampered off for the winter. The new basement furnace is slightly smaller in output than the original spec because we now have the 2nd floor heated with the attic furnace. But if the 2nd floor furnace were to stop functioning we could heat the house using just the basement furnace. Maybe we couldn't hold 75 degrees on a -15F night outside, but we could heat it well enough that there wouldn't be any freeze damage and we could still live here.

Thanks again. I think that I am going to take the plunge and keep my fingers crossed... and have as many redundant safety features installed as possible.

Quote:

Originally Posted by bkspero

I'm glad that I haven't offended. Thanks again for your input. We'll see how the plumbing inspector responds regarding the drain connection in my permit application. It is clearly spelled out on the drawings. So if his initial verbal approval was based on my giving him incomplete information, he will have another chance to catch any issues and kick it back to me.

One good thing about our house. It was originally built with a single furnace in the basement. All the original ducts are still in place and just dampered off for the winter. The new basement furnace is slightly smaller in output than the original spec because we now have the 2nd floor heated with the attic furnace. But if the 2nd floor furnace were to stop functioning we could heat the house using just the basement furnace. Maybe we couldn't hold 75 degrees on a -15F night outside, but we could heat it well enough that there wouldn't be any freeze damage and we could still live here.

Thanks again. I think that I am going to take the plunge and keep my fingers crossed...and have as many redundant safety features installed as possible.

Sounds good. Good luck

For similar problem ,I was told that heat tape on plastic condensate drains may not work , live in Ma

The 90+ furnace already has a safety for drain blockage in the heat mode. If the drain is restricted the pressure switch will not close and in turn the furnace will shut down. 90+ in the attic use heat tape on drain line. Use aux drain pan under furnace with float switch in case drain clogs during cooling season, it will shut the ac off.

Quote:

Originally Posted by T-Top

The 90+ furnace already has a safety for drain blockage in the heat mode. If the drain is restricted the pressure switch will not close and in turn the furnace will shut down. 90+ in the attic use heat tape on drain line. use aux drain pan under furnace with float switch in case drain clogs during cooling season, it will shut the ac off.

Thanks for the responses. That's my plan although I'm having some problems finding a plumber who will do this small of a job while the attic is as cold as it is. So I'm delayed until the spring. The only difference is that I've settled on a wet-switch instead of a float switch in the pan because I want it to respond as quickly as it sees a little water. If it turns out to be too sensitive, I will change to a float switch.

You do realize the heat exchanger, trap and drain lines can also freeze in the attic don't you.

Quote:

Originally Posted by NorthernHeat

You do realize the heat exchanger, trap and drain lines can also freeze in the attic don't you.

I think that we've had this exchange in the past. Heat tracing and insulating the trap and drain line using redundant heat lines in parallel provides them with freeze protection to my satisfaction.

As for the heat exchanger, please help me understand the issue(s)? Specifically, how does water accumulate in the heat exchanger to freeze in a way that can cause damage? I understand that water will be present as a thin layer of condensation in the walls of the heat exchanger while the furnace is operating and way too hot to allow the water to freeze. But won't excess water drain from the warm furnace into the heat-traced drain line and trap before the furnace cools to below freezing? And if some does freeze in a thin layer on the HX, won't it just thaw and drain during the next heating cycle? Is there some spot in the HX I'm overlooking where water will pool in a constricted space and damage the HX if it freezes and expands?

I appreciate the input. I just need help understanding the issues.

I am just concerned because every year somebody leaves there garage door open all night and freezes up their condensing furnace, they realize they have a problem because they wake to a cold house, if the furnace doesn't drain properly it will lock out.

Quote:

Originally Posted by NorthernHeat

I am just concerned because every year somebody leaves there garage door open all night and freezes up thier condensing furnace, they realize they have a problem because they wake to a cold house, if the furnace doesn't drain properly it will lock out.

I think I understand and appreciate the advise. I'll take all the precautions that I can to avoid flooding my 2nd floor ceiling and am prepared to risk losing the heat. The current 80% attic furnace is a retrofit into a house that was built with a single zone of heat supplied by a furnace in the basement. All the original ducts are in place. And the heat output of the current basement furnace is about 85,000 BTU vs. about 90,000 for the original furnace (which produced enough heat, but couldn't keep temperature as uniform as we'd prefer as the weather changed). So worst case, if I am wrong and we lose the upstairs furnace, I open the 2nd floor dampers and registers and heat the house the way it was built originally using a single zone from the basement furnace. Maybe it won't be able to hold 70 degrees everywhere during a -10 deg F windy night outside, but we will be able to keep the house habitable and the pipes from freezing.

Wish me luck.

Luck, lol