HVAC Setup and Modifications

November 20, 2022

Over the last couple weeks, I’ve been working on getting the HVAC system online.

This 30ft heating cable uses up to 90 watts to heat 15 ft of pipe, but is constructed in a way that is only uses the energy required to bring it to 50ºF.

To kick things off, Adam came by again and helped me get the exterior drain installed. Since it will be exposed to cold temperatures, the pipe is wrapped in a heating coil and then insulated.

The new line-set cover was not only a perfect color-match but also large enough to fit the line set, communication wire inside a conduit and an insulated drain line.

We then ran it along side the line-set and covered it with the new metal line-set cover.

I used liquid sandpaper and some actual sandpaper to ensure the paint would bond well to the powder coated finish.

I then directed my attention at painting the custom-made filter return grilles.

The pump itself has an integrated float switch, but I also wired the system to include a float switch at the heat pump’s p-trap and a water sensor on the bottom of the main drain pan.

While those were drying, I added an alarm to the HVAC control system which blinks and makes a loud beeping sound when the system is disabled due to drainage issues.

The entire color-coded wiring diagram is also printed out and stored in a maintenance checklist clipboard that also stores the manuals.

I then finished wiring the HVAC control system which uses a clever DIN rail system and includes junctions, distribution blocks, relays and a 24VAC transformer.

Because the ERV vents inside air into the attic and then exhausts attic air to the outside the attic should stay fairly warm in winter, ensuring the drain lines don’t freeze.

Next, I connected the remaining ducts for the dehumidifier and ERV, as well as their drain lines to the condensate pump.

The heat pump drain pan also has a backup drain set at a higher level, but I haven’t connected this to anything yet.

I also installed the heat pump’s drain line, including a p-trap with an integrated float switch and clean-out.

The water diverter is made of solid rubber, making it heavy enough to stay put. However the natural stone-look as convincing as I had hoped.

Then I added a short extension to the drain line, which is also heated and insulated and drops water onto a special water diverter to avoid erosion.

Vents from left to right; 2 warm/cool air from the heat pump, 2 fresh air from the ERV, 2 dry air from the dehumidifier, 4 warm/cool air from the heat pump.

Adam came back out and helped me go through the final steps to turn on the heat pump for the first time. I cut the protective covers off the vents to let air flow and we fired it up.

The compressor and fan are incredibly quite compared to traditional cooling systems at only 55db.

The compressor was not starting, which led us to realized we had to open the line-set ports. Once we did that, it kicked on and worked. However, it was much louder than I had hoped, and the culprit was turbulence.

It turns out that I made some calculation mistakes and undersized the ductwork in the ceiling. I also biased too far toward directing the air around corners to reduce sound transmission and causing excessive turbulence in the return plenum. I was pretty stressed about this, but I came up with a plan.

After some re-calculation, the undersized ductwork had 1/3 the capacity required, but adding this duct will result in having 1/3 more capacity than required.

The wall just behind the air handler in the attic is more or less empty. It’s got a wire for the smoke detector in the entry and is now filled with open-cell insulation and covered with drywall. However, it’s a great location for a very direct path for additional airflow from the supply plenum.

Extending the supply plenum down and out in this way adds over 900 CFM of capacity to the supply duct system.

Meanwhile in the attic, the return plenum could be made much shorter, making space for an extension to the supply plenum that would be in exactly the right spot.

Thankfully there was enough room for the elbow without having to redo the drainage p-trap.

The next day I tore down the return plenum to make space for the extra supply duct.

This shorter and simpler return plenum should help to reduce friction.

I then simplified the air path in the return plenum by using only one elbow to enter the plenum more directly.

I stocked up on these special 3-wire in-wall junctions which are legal in NH, and it really came in handy today.

Next, I cut a hole through the attic wall into the live room and extended the electrical wire that was in the way using some in-wall junctions.

This location worked out incredibly well, with just enough space to put a 16 in. x 16 in. duct without running into the ceiling.

The hole on the other side is up high enough that a register will be well out of the way.

I made the takeoff myself using some left-over sheet metal, a metal break, tin-snips and rivets.

I constructed some new ductwork out of duct-board and sheet metal, which would fit snugly into place to extend the supply plenum, venting a massive amount of air directly into the live room.

To seal the back of the duct I taped some foil duct-board backing to the back and sides of the supply plenum and then taped that backing to the duct as it was being positioned.

And then I attached it into a hole I cut in the bottom of the supply plenum.

The foam will be covered in duct-board material to make the exit area smoother and further reduce friction.

I used some spray foam to bridge the gap between the ductwork and the drywall and seal up any openings between the live room and the attic.

I think I’m going to respray these flat-black, as I really don’t like the shine of the semi-gloss paint.

And then finally I installed the filter return grilles which have very low-friction filters designed to catch dust, thus keeping the ductwork clean.

I turned the system on again and it was about 10db quieter. However, there’s still a lot of low frequency rumbling due to the air bouncing around in the corners of the square duct-board plenums. My plan is to add turning vanes in six locations where air is changing direction around a square corner. This is likely to make a major impact in efficiency and noise level. However, it’s less critical, so I’ll be putting that off for a bit to focus on some other more pressing tasks.