Musings on the "Design Day"

"There is a fundamental asymmetry between heating and cooling with a heat pump."

Note that there is an asymmetry in hot weather vs cold weather too. Above I posted the hours-by-degree chart for Washington, DC. On the heating side it goes to zero hours at 5F, 17F below the 99th percentile temperature. On the cooling side it goes to zero hours at 101F, 9F above the 99th percentile. The all-time low is -15, which is 37F below the 99th percentile and 20F below the zero-hours temperature. The all-time high is 106F, which is 14F above the 99th percentile and 5F above the zero-hours.

So the cooling end of the curve is a lot steeper than the heating end of the curve.

You see this regionally as well, that the cold in the coldest parts of the country is a lot more severe than the heat in the hottest parts of the country.

OK, this is what I'm thinking for modeling short cycling. Stipulate that:

1. At the start of each cycle there is 2 minutes of "wasted" power use while the compressor gets up to speed.
2. The compressor has short-cycle protection that limits the minimum on time or off time to any cycle to no less than ten minutes.

So if the duty cycle is less than 50%, the on time for each cycle will be ten minutes, the off time will be whatever is needed to achieve the duty cycle. With two minutes wasted from each ten minute cycle, the efficiency hit is 20%.

If the duty cycle is more than 50%, the off time for each cycle will be ten minutes, the on time will be longer, again whatever is needed to achieve the duty cycle. So if the duty cycle were 66% you'd have 20 minutes on, ten minutes off. With two minutes wasted out of 20, that's a 10% hit. If your represent the duty cycle with DC, the on time would be:

OT=DC*10/(1-DC)

The efficiency hit is 2/OT, which simplifies to 2/(DC*10)/(1-DC) or 2*(1-DC)/(DC*10) or 2/10*(1/DC-1), which goes to 20% as DC approaches 0.5 and to zero as DC approaches one.

How does that sound?

I think on my unit the circulator never shuts off. It modulates from 3- 6.4 gom the outdoor fan runs unless the call for heat ends. So its mainly the compressor modulating. My control display shows fan rpm and compressor Hz

So in my case I only see on/ off of the fan and compressor when the buffer tank is satisfied

So I would say they are engineered well as to cycle efficiency.

I know that was a complaint with early A2whp, with fixed speed compressors and probably a sizable amp draw when they kick on? Fan and compressor

 The old Solar gas turbine genset I had at a waste treatment plant, about 40 years ago, could pick up the full plant load in 15 seconds, cold start.

That's hard to do today. The clearances in both the compressor and turbine are so tight that they need a certain time at idle power to warm the cases. If you tried to use a modern commercial aircraft turbine engine to takeoff in 20 seconds……………..it would most likely seize on the case ID.

Check out the flight of Pinnacle 3701. These jokers nearly stalled the aircraft at 41000 ft. and both engines shutdown due to poor airflow at a very high angle of attack. They could NOT restart them!! They didn't live to tell about it.

OK, new chart. This is the same format as before, but I added in a correction for short-cycling:

The Y-axis is efficiency, expressed as a percentage of the most efficient size. The x-axis is sizing, expressed as the rated capacity of the heat pump divided by the design heat load. Left is undersized, right is oversized.

This is more the shape I would expect, although I will admit that's a contrived finding because I'm just guessing at the effect of short-cycling, so of course I'm going to see what I expected to see.

Anyway, the most efficient size is slightly oversized, there's not much difference between 100% and 200% sizing, get outside of that window and efficiency starts to fall off.

All Systems need 10 - 15 minutes run time yo reach peek performance. Anything less is not good.
do you step on the gas hard until you hit the speed limit or control the speed.

sounds like a interesting read!

and @Jamie Hall Surely, spoiled is the wrong word, "fortunate" (and in some cases ungrateful) is proper. As you mention, we shouldn't 'try to live the hard way' simply because people before us did. Great correction.

My (failed) point to make was that maybe we lose on the efficiency end of things because we're more concerned about the convenience of things. While the hospital or a business certainly isn't interested in 'being chilly' at the benefit of saving a few pennies, maybe some homeowners would be?

"An important part of this calculation is, what is the penalty for oversizing and what is the penalty for undersizing? " @DCContrarian

Great question, would love to learn more!

Every load calculation software I have ever used has been capable of taking into account all of those things. I can input number of people in each room, individual devices that populate as an estimate, or if I know the devices that are in the room I can manually enter an internal load number. I can do this as a block for the whole building, or room by room. Including solar gain etc, even whether or not there are blinds on the windows and what type of coverage those blinds will have

When you're sizing for cooling you add all those things, assuming the house is fully occupied and the occupants will be bathing and cooking. When sizing for heating you assume it's night and there's no solar, no equipment is running and the occupants aren't doing anything but sleeping.

The Manual J process is about comfort.

An apples to oranges comparison? Failures of bridges, airplanes, fire suppression systems, and sewage systems cost lives, as can the failure of a heating system. Unless significantly undersized, a heating system that can't keep up on the coldest days causes discomfort.

I've skimmed through some of the thread……..

Regarding having a system that is more efficient most of the time and loses some temp on a rare occasion….

It makes sense, but, what's the operating cost difference between having one that is slightly oversized vs one that's slightly undersized? "More efficient" doesn't really say anything useful.

If you're talking about living with a system that falls behind during extreme cold, and makes it so you can't even hold a door open to bring groceries in etc, to save $10 a season……… that's not practical.

I guess you're not familiar with Stoßlüften?

Few will be unless you're German, but do a quick search on it.

Do they practice Stoßlüften during extreme cold? I have no idea, but I'd love to know.

Being a "bear of very little brain", I am at a loss for words. Unable to follow much of the logic of this thread very far I've come to the conclusion that it has devolved into a discussion about how many angels can sit on the tip of a pin.

At 80 years old I have known privation. Hunkering down in dank muddy fox holes, clattering down the tracks in the far corners of empty box cars wrapped in a thin blanket, spending an unscheduled night in a snow cave next to a dead snowmobile at 11,500 feet, feeding the tiny firebox of a small woodstove in a drafty Montgomery Ward "kit house" - I have known privation.

Is there no tolerance left? No willingness to undergo a little temporary inconvenience? Is there no sweater in the closet to don when the design day calculation turns out to be off for a few hours. Not all of us here in these United States expect to spend the winter indoors at 70 degrees Fahrenheit - actually, probably less than 50% of us do. (That would be about 2% of the population of the planet.

Please don't misconstrue. I like comfort. I never became accustomed to want and strove to overcome it. But having experienced and survived it gives me an appreciation of comfort, not an expectation of it. Some automobiles can go 170 mph yet the speed limit is 75. The prudent drive slower yet.

Remember "Cold 70"? I wonder if we could become accustomed to "Cold 60". Talk about energy savings!

I was just reading about California, which uses a different energy code from the rest of the country, what they call Title 24. For heating equipment, they have an anti-oversizing provision:

"Heating design temperatures shall be no lower than the Heating Winter Median of Extremes values." I assume that's the same as (or similar to) the average annual low reported by NWS. For perspective, where I am 99th percentile temperature is 22F, average annual low is 6F.

They're talking about changing this, the new proposed language is: "Heating design temperatures shall be no lower than the 99.0 percent Heating Dry Bulb or the Heating Winter Median of Extremes values." Which makes no sense, because 99th percentile is always going to be warmer than median extreme. I suspect what they're trying to say is it has to be between those two value.

In addition the localAHJ can add addendums. We had multiple pages of addendums to the UPC and UMC where I had my business.

We had the combustion air requirement cut to 1/4" per thousand due to snow banks in, and freezing boiler rooms.

So glad to see mod cons and sealed combustion equipment for the cold snowy climates.

@GGross : "

 "whether I size for 0 or -10f"

design day temp usually comes before the heat loss calc???

how do you do this—from the heat loss somehow? with straight line calculation from the heat loss per degree??? so many K btu's per degree?? or do you redo the heat loss calculation?

Wind is the one factor that AFAIK is not typically accounted for in design day sizing but can really suck a lot more heat out of the building.

In our 4-unit condo building in the Boston area, during the Feb 2023 polar vortex, our boiler run times represented a 40% increase in heat loss over the design day zero-degree heat loss in still air. Normally our zero-degree heat loss in still air is about 100,000 BTU/hr, but during the polar vortex event with near-zero temps and persistent winds at 30+ mph, we lost 140,000 BTU/hr on average.

That is in a 100-year old building with masonry first floor, wood frame second floor, double-pane replacement windows on one side, and professionally weatherstripped original wood windows (with spring bronze and silicone bulb weatherstripping) plus old storms on the other side, and 10" blown cellulose in the attic.

Our boilers are 3x oversized for a calm, zero-degree day, and they were still 2x oversized for the polar vortex, so they just loafed along. But if they had been sized at the ASHRAE recommended 1.4x of design day, they would have been perfectly sized for the polar vortex with high winds.

So an extra 33% pickup factor or 40% ASHRAE margin can be the difference between coping with high winds on a design day, or not.