I think you need to look at excess humidity in the space that houses the parts that are condensing water. Where is this system located, in the basement? Is there outside air leaking in nearby?. If you are getting a large amount of air leaking in, you might have an imbalance problem because of leakage in the new ducts. Try some smoke tests with the system running or just check with your hands.

The equipment is in the basement. during the summer, the humidity in the basement is high. I had been running a dehumidifier but it failed a few months ago. The installer of my new furnace and heat pump suggested he could add a return in my basement and that circulation and air turnover would take care of the humidity.

I think you may be on to the root cause of my condensation. I noticed the condensation yesterday and today during the hour or two following the starting of the HP calling for cooling. In an hour or so of running, the basement humidity comes down and the condensation on the supply plenum reduces.

Before I invest in another dehumidifier (and huge electrical cost), I'll continue to work with the installation Tech and add insulation if required.

What I saw today is a small amount of condensation formed on the first foot or so of the ducts coming off the supply plenum. No water going anywhere, just moist to the touch. I can get him back to insulate those short runs, but I'm not sure it's really a problem.

Does this sound reasonable?

I would be reluctant to install a return in the basement if there are no supplies. While it will reduce humidity, it could cause back-drafting of your water heater (if there).

Under some circumstances, it could increase infiltration of outside air.

V

I have a supply register at one end of the basement and a return on the other end which made a huge difference in the humidity without messing with a dehumidifier.

veesubotee,

I have an electric water heater so no concern there. Also, my basement is never isolated from the upstairs. I am prepared to accept any additional air infiltration.

countryboymo,

I have two supply registers in the basement that are always shut. I may consider opening them.

I am also gathering data on the humidity in the basement when the HP is operating and when not...

your contractor increased the fan speed to decrease the temp drop across the coil. The condensation forms based on the temp difference between the two areas and the humidity. The temp difference can ofter be overcome w/ more insulation. Make sure your contractor has check the temp drop at the evap coil.....not the closest supply and return register. If the drop is excessive, there is a different problem.......generally low airflow due to ductwork issues, dirty coil, dirty filter, etc.

If all checks out, your option is to reduce the humidity or add insulation.

Think of a cold glass of iced tea. As long as there's a big temp diff between the cold side and the somewhat humid warm side, condensation will form because the two sides are talking to each other. Good insulation will cut that chatter!
A good insulated glass of iced tea will have no condensation on the outside.

Sure insulation will help, but be sure you are not treating the symptoms and ignoring the illness. Monitor the humidity levels down there and correct if it goes over 50% for any significant time. Basements can be inherently damp and they can be made damp due to introduction of too much humid outside air.

To all,

Thanks for the help!

Basement temps in the mid 60s and basement humidity in the mid 60s.

Consumers Reports rated this dehumidifier a Best Buy and Costco backs their products quite well. I'll get a Condensate Pump to allow getting rid of the condensate in a basement sink. (No floor drain.)

I think I'll get the HVAC installer to add insulation to the first foot or so of ducting coming off the supply plenum.

Here is a link that might be useful: Dehumidifier

In your original post you mentioned the bypass. Is this a zoned system? If it is then when you bypass some air that air will get cooled a second time. This will lead to some really low supply air temps. This isn't really a problem but it may be contributing to your condensation problem.

funnycide,

It's a single zone system. The bypass is to allow using my humidifier during the winter heating season. The humidifier is closed off now during cooling season.

The condensation was forming on the first 12-18" of the bypass ducting off the supply plenum. The flow is shut off at the humidifier at the return plenum end of the bypass.

These replies have been extremely helpful and I'm learning a lot.

Right now my plan is to install a new Dehumidifier with a condensate pump to get rid of the water. I also am going to have the installer again come back and insulate the first 1' - 2' of ducting and bypass off the supply plenum. (They have already insulated the supply plenum itself.)

Now that you guys have made me smarter, I'm looking for and finding lots of air leaks on my 37-year-old ducting. I'll fix those leaks in the accessible areas of my basement. I gotta believe the ducting in the walls are also leaking but I don't see what I can do.

I've seen "smoke guns" on TV to assist in locating air leaks. Tomorrow I'll look on-line as all I can do now is feel with my hand.

I will say this, the house is more comfortable than it has been during our twenty years of occupancy. Wife wants to know why we didn't do this years ago! (The installer told me to tell her that it wouldn't have been as good 10-15 years ago and the advances in HP design explains the current performance.)

I am looking forward to see what happens to heating and cooling costs. Montgomery County, MD, just increased the energy taxes like three-fold!

Buy a few pails of brush on mastic from home depot or wherever and some cheap chip brushes .99 cent jobs and go nuts. The in wall runs might have areas that you can still get to after removing the registsr.

This sealing duct thing is time consuming but will make a huge difference in comfort and how the air feels coming out of the registers. I had a 1200cfm blower and less than 400 cfm was being sucked into the returns. The rest was coming out of the attic and basement.