So You Want to Decarbonize Your HVAC? Part II
So what did Ric need, exactly? In this article we'll get darn close, but not quite all the way there. This cliffhanger matters, I promise. I need to show the whole thing for the conclusion to make any sense.
(Psst. If you missed part 1, read it here)
THE GOAL & THE HURDLE
Ric wanted to decarbonize his life, and we took care of the "easy" things pretty quickly. (I say we but really it was him. I was just his coach.) New roof. Water heater. Solar. Check, check, check.
This goal came face-to-face with something neither of us really knew how to do: size heat pumps. Couldn't we just call on an HVAC salesperson to do this?
Well, we tried that. Unfortunately it wasn't something that his HVAC salespeople really spent much time doing, either. Why not? Don't they typically do sizing for furnaces and ACs?
Actually, no. They typically just replace whatever you've already got. This is the "like for like" replacement strategy. If you've got a 100,000 BTU furnace they'll just recommend a new one of those. Here's another possible sizing method (note this is only kind of a joke):
THE MANUAL J
Ok, so jokes aside... there really is a method for this, it's called the Manual J calculation. This is a standard and a method for producing HVAC equipment sizing loads for residences. It's published by ACCA, the Air Conditioning Contractors of America. ACCA is a nationwide non-profit organization.
It creates a mathematical picture of all the sources of heat in your home, and the speed at which they're likely to slip out of the house. So you'll add up all the insulation, windows, internal loads (like warm human bodies), and the direction faced by the house. From this information, the Manual J will spit out a heating load and a cooling load; how much equipment you need. It requires measuring. A lot. Of things.
Sounds amazing, right? Well, there are two problems. 1. This is very rarely done by HVAC salespeople. It's too time intensive and they'd rather just sell you something kinda big so you don't ever call and complain. None of their competitors are doing Manual J load calculations so they don't either. 2. The Manual J is known to oversize equipment. So even if you could find someone who would run a Manual J, you might still end up with equipment that's too big. (Closer to right sized, but still too big.)
Actually there's a third problem. The smallest furnace is typically around is 60,000 BTUs, and 80,000 BTUs is way more common. So even if you need something below 50,000 BTUs, you're probably getting an 80,000 BTU or 100,000 BTU furnace; 60%-100% too big. In other words, your home would have to be 60% to 100% leakier than it is to justify those pieces of equipment. An even more dramatic example that really happens: what if you need about three tons, or 36,000 BTUs and you get a 100,000 BTU furnace? Your home would need to be almost three times leakier to justify that unit!
OVERSIZED EQUIPMENT
What's wrong with oversized equipment? Wouldn't you want to have the biggest possible unit? Well, no.
With a furnace, oversizing leads to:
Short cycling. Quick, hot blasts of air that satisfy the thermostat and turn off in a few minutes.
Temperature imbalances. Because the thermostat gets satisfied quickly, peripheral rooms stay cold.
Wasted money. Repeated cycles through startup mode uses more gas than is really needed.
Excess wear and tear. Like driving a "city car" with lots of starts and stops. Again, it comes back to short cycling.
Noisiness. Because your ducts probably aren't big enough for a huge furnace, you've got lots of static pressure, creating noise.
With an AC, oversizing leads to:
Short cycling. As above, but with an AC we've got a compressor, blower fan motor, and other components that get worn down.
Ineffective dehumidification. When the AC just runs for a brief time, it does barely any dehumidification. This causes a cold-but-clammy feeling in the house.
Inconsistent cooling. Aka temperature imbalances. Like the furnace above, blasting the thermostat quickly will cause the AC to turn off, while rooms far away from the thermostat will not get properly cooled.
With a heat pump, oversizing could lead to:
Pretty much all the problems above, plus
In particular, we are worried about damaging your fancy new ECM motor, if you get a very nice piece of modulating (aka "variable speed") equipment. High performing "cold climate heat pumps" come with ECM motors. This is a heat pump that ramps up and down to better match heating and cooling needs. These things don't like long periods of high resistance caused by high static pressure. High static pressure is often the result of oversized equipment, since there isn't enough ductwork to spread all the air throughout your house (hence "oversized"). ECM motors are very fancy, but they'll wear out more quickly if they're oversized for the home.
THE CLIFFHANGER
So here's the situation:
Ric made lots of progress but HVAC was still in the balance.
HVAC salespeople don't do much sizing work. Proper sizing is very important for a litany of reasons.
Manual J calculations aren't even that great.
We needed a new plan.
One designed for this exact purpose.
Ideally created and tested by a renegade band of HVAC mavericks who have been trying to answer Ric's exact question for a decade.
We needed HVAC 2.0.