I am renting a house with two HVAC units - one upstairs and one downstairs. There were two condensate drain lines (one per unit). The units were older so when they quit working efficiently the landlord replaced the units.

Shortly after the new units were installed we started having problems with the condensate line for the downstairs unit. The technician's solution was to divert the downstairs condensate line into the upstairs condensate line. So now we have two HVAC units draining into one line. We are still having problems with the downstairs unit tripping the safe-t-switch valve because the condensate line backups.

The HVAC company came back out and is blaming us for not keeping the line clean with vinegar every month (which we do).

  1. Does it make sense to drain two units into one condensate drain line? We live in Florida where we produce a TON of condensation and
  2. Shouldn't there be a back-up drain line since the units are inside the house and the leaking could cause structural damage? Just doesn't seem logical to have two units on one drain line especially since we are having a back-up issue.
  • This isn't an answer, but as a fellow Florida resident I've had occasional troubles with clogged drain lines. It happened in my current house just a month or two after I moved in. While I have often heard the advice of using vinegar/bleach to keep the line clear, I've never actually done this (with the setup of the drain line in my current house, it's just about impossible). Over the course of ~8 years of living in houses with cloggable drain lines, I've only had 2 or 3 clogs. My experience leads me to believe this is clearly not your fault, even if you weren't cleaning regularly.
    – conman
    May 21, 2019 at 3:23
  • The safe-t-switch should prevent danger to the house itself. This is Florida, so even a backup drain line will eventually clog if you don't notice the primary line is clogged (which you won't), so I doubt a backup would really help. However, that doesn't mean there isn't a problem. There are lots of ways for drain lines to go wrong. Also In case you don't know, you can open the safe-t-switch and allow the pipe to drain into a large bucket (there is usually somewhere to make this happen safely). That allows the AC to run while you wait for the HVAC guy - just don't let your bucket overflow.
    – conman
    May 21, 2019 at 3:27
  • conman - that's exactly what I did. I opened the safe-t-switch value and let the water drain into a bucket like an HVAC professional told me I could do. This company is still blaming the clog on me and saying he is going to void my landlords warranty.
    – Rebecca
    May 21, 2019 at 3:30
  • That's unfortunately a much more of a legal situation. It sounds like the company is trying to get themselves out of liability and throwing you under the bus in the process. It may actually help to ask about this over on law.stackexchange, especially if the crux of the dispute is that they don't think you are treating the line while you insist that you are. I'm not an expert, but it seems clear to me that this is a problem for the HVAC company to fix.
    – conman
    May 21, 2019 at 11:08

1 Answer 1


I would never use the same line for units on separate levels for this very reason, it sounds like the line is undersized since it was having trouble and now the load is doubled. What brand are the units? I believe it would violate the install instructions to T 2 lines with standard size drain lines and may even void the warranty, this should be mentioned to the land lord.

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