Dave Yates: Heat pump retrofit

Twenty-nine years ago, when we built our forever home, my central air conditioning systems consisted of two 2-ton condensers with two air-handlers in the attic and ductwork that utilized fiberglass ductboard with short runs of flex ducts terminating at commercial-grade ceiling registers. The condensers and air handlers were used that we salvaged from one of our job sites. I kept them with the notion of using them and figured we would upgrade from 10-SEER to higher efficiency units once they died.

About 16-years ago, the one serving our living areas (kitchen, dining and living rooms) kicked the bucket. Now empty nesters, we figured its replacement could wait because, after all, we could retreat to the master suite for cool conditioned air for the few weeks of hot and humid air in August.

Meanwhile, our sales of inverter mini-split heat pumps were soaring to new heights. If you’re not familiar with inverter technology, it simply means the equipment varies its fan speeds (both indoor evaporator and outdoor condenser) and meters refrigerant as needed — and all of this is coordinated to meet whatever existing demand for comfort is requested from the thermostat and reacts to outdoor and indoor ambient air/humidity present at that moment in time. We were in the process of retrofitting a very large church with inverter mini-splits and had obtained an outstanding discount because of the dozens of systems we would be purchasing.

Me, thinking I was clever, ordered two wall-mounted indoor heads and condensers to replace our deceased central AC system along with the church order to take advantage of the discounted pricing. Unfortunately for me, my bride was also the company bean counter and confronted me with “Why are there two extra systems for the church job?” Before I could even answer, she added: “They better not be for my house, I don’t want to see wall-hung units. Isn’t that why we installed hydronic radiant heating, so we wouldn’t see anything?” Busted! No worries, however, because both units were sold the next day.

What to do, what to do? I did not want another summer to pass without air conditioning and let’s not forget the still-working condenser and air handler were used equipment the same age as the ones that died more than a year prior. The prospect of no AC for this mechanical contractor was a no-way no-how hard-stop.

The mini split inverter heat pump manufacturer we were using had just come out with concealed miniature evaporator air handlers. Out-of-sight, out-of-mind, and something I could get the boss-lady to grant approval for, and this time, I made sure to keep my bride/office manager in-the-loop. “Sure, as long as I don’t have to see any indoor equipment.”

Given the big push to electrify everything and claim it decarbonizes and is good for the planet and reduces global warming, we all are going to be faced with retrofitting heat pumps into existing homes and businesses with a host of varying challenges. It’s not our fault that those who are pushing this upon the public fail to acknowledge, or reveal to the public, that that’s a bunch of garbage because a heat pump, just like an EV, simply pollutes elsewhere.

The fact is, installing an inverter mini split system to replace our A/C meant heating was coming along for the ride. I had to decide if the inverter mini-split systems would be sized to meet the lower Btu/h A/C load or the larger Btu/h heating load. If you’re going to be installing inverter units as the primary heating/cooling source of comfort, you had best size for the larger Btu/h load. If not, that will come back to haunt you at the most inopportune time of the heating or cooling season when you will be running around like a chicken with its head cut off in an attempt to keep up with customers’ demands! I decided to size for the larger heating load and based that upon the Manual-J heat gain/loss calculations I had done 16 years prior.

Ductwork and friction losses! Unlike today’s concealed inverter air handlers, that first generation was not designed for the application intended. Their blower motors couldn’t handle hardly any resistance to airflow. We always used .03 for supply ducts and .05 for return ducts because it is easier to push than pull air. We used .08 for flex ducts because of the turbulent airflow resulting from the rippled interior. You can use an online duct calculator: https://ductulator.com or the ACCA Manual-D program to properly size your ductwork, which results in balanced flow, whisper-quiet delivery, and longer equipment life.

What to do? Simple: convert all of the return ducts — and every room (except bathrooms and kitchen) had them — to supply ducts and the friction losses were within the range specified by the manufacturer. We cut in new return air filter grills placed just a short distance from each of the four indoor inverter mini split air handlers.

What to do? Simple: convert all of the return ducts — and every room (except bathrooms and kitchen) had them — to supply ducts and the friction losses were within the range specified by the manufacturer. We cut in new return air filter grills placed just a short distance from each of the four indoor inverter mini split air handlers. The manufacturer had a conniption: “You can’t do that with our equipment!” However, their national trainer listened as I went over my calculations and agreed we had done everything right. We broke new ground, and within a few years, all of the manufacturers recognized the huge market potential and produced concealed mini split inverter air handlers with more robust blower motors that could accommodate utilizing existing ductwork (providing it is sized correctly).

To fully test the newly installed inverter mini split heat pumps, I disabled our hydronic radiant system and we relied solely upon the inverter heat pumps for that winter. I was pleasantly surprised to find the heat distribution was even and consistent, even in the living room with its vaulted ceiling. That was the one room where I thought the heat being delivered by ceiling registers 14 feet above the floor would not reach the floor, but it did and our feet/toes were quite comfortable. While we were not as comfortable as we were with our hydronic radiant heating, it was more than acceptable. Operation costs certainly rivaled or natural gas 95% efficient modulating condensing boiler and now we had a second source for heating. Instead of just two zones of cooling, we now had four because we micro-zoned while modifying our ductwork.

Since that time, we retrofitted numerous central unitary-equipment A/C and heat pump systems with much higher efficiency inverter heart pump systems and reduced customers’ energy usage by 30% to more than 50%. So long as our government is going to promote electrification and offer incentives and rebates, we should up our game, take full advantage, and reap windfall profits. Make hay while the sun shines ladies and gents toiling in this wonderful PHVAC world.