Heating Degree Days NOT useful for sizing

[buffalobillpatrick]

Heating Degree Days are NOT useful for sizing a heating system at Design Temperature??? (which is how it's done)

I took the total January HDD for my location, say it was 930 / 31 days = 30*
60* indoor - 30* = 30*

I can't size a heating system for a Design Temp. of 30* I must use 0*


HDD are useful for estimating about how much heat might be needed for a typical January.

Weather Data Depot: free downloads of heating & cooling degree days

"
What is a degree day?

A degree day is a measure of relative heating and cooling energy required by buildings. It's calculated as the difference between the average daily temperature and the balance point temperature (60 degrees). When the average daily temperature is above the balance point, the result is cooling degree days; when below, the result is heating degree days.

Example 1: Average daily temperature = 80. Balance point = 60. Cooling degree days = 20 CDD. (80-60=20)

Example 2: Average daily temperature = 45. Balance point = 60. Heating degree days = 15 HDD. (60-45=15)

Example 3: Average daily temperature = 60. Balance point = 60. No degree days.

You may ask, "Why not use average temperature instead of degree days?" The problem with average temperature is that highs and lows cancel each other out. A warm day (80 average temp) combined with a cold day (40 average temp) average 60. So do two mild days of 59 and 61. But in the first case there are 20 CDD and 20 HDD while in the second there is 1 CDD and 1 HDD. Using degree days, you can see that the relative amount of energy required for the first set of days is much greater than for the second set of days. But if all you looked at was the average temperature, you would conclude that both sets of days were about the same."

[stevehull]

BBP

Good points on not simply using total seasonal heating (or cooling) degree days for design load - alone.

I use both and expected wind and thermal mass. I look at the heat (or cool) required to offset an environmental load as BTUs per square foot per degree day. Even a super insulated home, with outstanding minimal wind infiltration, will require extra heat when it is bitterly cold AND windy.

Thermal mass also allows a home to absorb or buffer temperature extremes. You can quantify this on a very cold, cloudy and still day by turning off the heating system and looking at the fall in home temperature vs time. A semi-log plot (temps vs time) shows a straight line and this gives a very good estimate of thermal mass.

Years ago, I did and experiment where I added a lot of thermal mass to a home I was living in. I had access to dozens of 42 gallon plastic barrels. I filled them with water and compared the regression lines described above. VERY different. As a consultant to DOD, I suggested that potable water tanks (IBC containers) be kept inside living/conditioned spaces in cold regions. That is working well today and minimizes the size of the heating units needed to keep personnel comfortable.

The vast majority of cold mornings are met with a typical 20 F rise in daytime temp. With a significant thermal mass, the temp does not fall to the outside, but lags. Then the daytime heats things up again. Or to be proper, the daytime lessens the load.

Total degree days are very useful as you can look at yearly variance and plan for a prolonged cold spell that is outside the ability of the home to buffer (thermal mass).

For example, here in central Oklahoma we have about 3200 winter heating degree days. But the variance in this can be large - some 20%. January is our worst month for variance in terms of heating degree days and can be 50%! . In that month, we can also have terrific winds and cloudy days when we see little if any daytime increase in outside temperature.

This tells me that I need to plan for at least the minimum design temp - or have available a supplemental source of heat. In many home, there is a 15 kW electric resistance heater (electric oven) that can be briefly used. Correct, not a code approved heating source, but one that can give me some extra BTUs on a cold morning. I usually cook some biscuits which works well by heating my stomach AND the house.

That is where analyses of serial yearly (or monthly) heating degree days come in - not as a static value, but to estimate extra loads not described as an average.

Most HVAC installers have a simple response - just oversize the heating unit! It is not that simple as extra fuel must be provided to provide peak load (yet another waste). Highly mobile military units hate heavy stuff. But they crave warmth.

So much for my musings at 6 am . . .


Steve

[Mobile Master Tech]

Bill, good point! The most extreme day you are likely to see is the one you have to size for. The further that day falls from the average for it's month, the more shortfall you have if only using degree days to size.

Buffering helps, but how much? Steve, about how much water did you use, and about how much buffering does it give you?

[stevehull]

Thermal buffering is the sum of many thermal masses. The smallest thermal mass is the air mass. The most substantial are heavy dense things like stone counter tops and such. The resultant thermal mass sum is the aggregate of all the masses acting independently as the environmental temperature changes.

Turns out that just one or two IBC totes of potable water make a huge difference in a 100 meter square space. The problem DoD has (Afghanistan) is that nights are bitterly cold as the air is so dry.

The way temporary field structures are built allows totes to be placed first and then the structure dropped (or built) into place.

My initial experiment was with about 200, one gallon water jugs. The next experiment was with steel drums (42 gal). My wife was frankly not very pleased with 30 steel drums in the house and was VERY glad when the experiment was over.

Those notes are about 30+ years old, but I did some calculations and was amazed at the potential buffering. Turns out the reality was very close. The specific DoD stuff is, believe or not, classified.

Michigan winters have a typical low in the low teens (F). When Kathy was on call, I would put house temp to 70F, run the blowers to fully saturate all the thermal masses. This took about 12 hours. Then I would turn off the heat and record the temperature with an old style recorder (circular device with pen writing thermometer on graph paper). The house would drop rapidly in an exponential fashion to the mid 40's the next am. Then turn on heat so wife had a warm home to come back to!

Didn't take rocket science to do a semi log plot.

Then brought in barrels and filled with water. One problem was that ground water in Michigan is COLD so it took a while to heat up all this water to 70 F(a few days). This time the house did not drop much below the mid 50s.

The house was crappy in terms of air infiltration so I had to choose my nights carefully.

The DoD has huge (hanger size) environmental chambers (Natick, MA) where these concepts were done in a more elegant way.

When we built a later home, here in Oklahoma, I put in a lot of thermal mass. That allowed a smaller geothermal heat pump to work very efficiently as I was really just heating (or cooling) against the average outside temp and not the full extremes. Used lots of concrete . . . and built a large basement with insulation on the outside walls. Could keep that huge house cool with a 12 kBTU GT heat pump in summer in Oklahoma.

Back in the 1980's, I recall some people putting water jugs in walls, sealing them up. At first this is tempting - until you realize that the polyethylene is time unstable and tends to start cracking after a few years . . . .

There is another thread where a solar greenhouse with aquaponics uses water filled steel barrels to buffer air temperature changes. Same concept.

Bottom line is that thermal mass, constrained with the envelope, can be a huge help to minimize energy bills.


Steve

[gtojohn]

I agree, manual j can be off quite a bit. Our design temp here is 96F however in the past ten years we have long stints of 100f+ days. One year we had over 60 days straight of triple digits. HVACs sized too close to manual J couldn't keep up. Lots of unhappy customers. Heating obviously you need to plan for those extremes as well, just because you average 40f for a low doesn't mean you won't have a freak 20f week. Perhaps a good argument for multistage equipment that could ramp up when you need it.

[MN Renovator]

Originally Posted by gtojohn

I agree, manual j can be off quite a bit. Our design temp here is 96F however in the past ten years we have long stints of 100f+ days. One year we had over 60 days straight of triple digits. HVACs sized too close to manual J couldn't keep up. Lots of unhappy customers. Heating obviously you need to plan for those extremes as well, just because you average 40f for a low doesn't mean you won't have a freak 20f week. Perhaps a good argument for multistage equipment that could ramp up when you need it.

I can't wrap my head around this post. ..at all.

I live in a house built in 1985. 2100 square feet. Before starting my insulation and air sealing retrofit efforts with about R30 tops cellulose in the attic, R13 in the walls with a 3/4" wrap of non-foil(plastic instead) polyiso wrap(R18, but not that much once considering thermal bridges with 2x4s 16" on center. No slab insulation. Not a pile of windows but enough on the northwest summer side to heat the place in the summer but not provide much benefit in the winter, low quality windows, most with failed seals that condense between the panes. 1500cfm50, roughly 5 ach50.

This house manual Js at about 30,000BTUhr at -11f design load. I haven't bothered calculating the manual J for air conitioning but I'd bet the 2 ton air conditioner was sized under manual J. The furnace has a 57,000BTUhr output.

Prior to doing any retrofit work, this standard code minimum 1985 construction had an actual measured heat load based on run time of 25kBTUhr during the middle of the night while the temperature was a steady -20f or lower for an 8 hour period. In the summer(after air sealing) under design conditions of 88f with the 2 ton air conditioner it more than keeps up. With 72f inside(I don't normally keep it this low but for measurements sake I did) and 95f outside I've extrapolated that the actual cooling load for 75f inside and 88f outside to be around 15kBTUhr. Since I'm plenty comfortable with the temperature at 75f in my house, with the smallest standard split system in my house I could have the system keep my house at 75f while there is full sun and 91f inside with a 1.5 ton system.

With that being said, if I kept the windows the same size and orientation, based on how the surface area of a space gets larger linearly at the perimeter in relation to the heat load, I could grow my house to 4624 sq keep and keep the same equipment that I have now. ..and if you don't buy the linear concept and multiple by square feet, I'd be at 3150 square feet before I actually run the furnace constantly at -20f temperatures and the air conditioner would then be properly sized for a load that I'm very comfortable living with.

After I got done with a basic level of only air sealing and resolving thermal bypasses into the attic for a cost of two $20 sheets of XPS and about 15 cans $60 of Great Stuff. I've brought a 25k BTUhr heat load at -20f down to 19kBTUhr and 17kBTUhr at a -11f outside, 70f inside design load.

I seriously think that if someone can't maintain heat with a 40k BTUhr furnace, that person either has a very large house, they have too much glass(or inefficient glass), and/or there is a need for additional insulation and air sealing.

Air conditioning is a little bit different, it's more dependent on window size, orientation, whether the glass rejects heat, air sealing, cooking loads(are you really running the oven for hours in the middle of a blazing sun day, and internal humidity loads). Even with there being different configurations, I think that a 1.5 ton and 2 ton system could manage in a well designed house where the cooling loads were in mind during its construction in my climate. You'd go up a little bit with temperature but solar and moisture loads on the hottest days are a larger factor. Windows, air leaks, and humidity loads are the largest contributors.

..with that being said, I'm thinking that if the central air even breaks down it's getting replaced with a 40k 90+% condensing furnace and a 1.5ton air conditioner. ..although the reality is that I'll probably toss in a super efficient inverter 12k mini-split heat pump upstairs instead to handle most cooling days and only use the 8.5SEER system I already have on days where I feel it needs a boost. I live in a higher cost electricity state where natural gas and geothermal has too high of an initial cost and the electric costs wash out the long term savings. An air source inverter mini-split has a lower initial cost way to get high performance heating but the cooling savings is where it's at in a moderately hot and lake humid state.

In short, especially for heating loads: I want to know the furnace size, square footage, outdoor temperature, insulation level, and air leakage of any home that a natural gas furnace heated home is experiencing a situation where it can't hold a steady state temperature. I'm having a really hard time, based on my own experience, that a home air sealed reasonably well and has at least R13 in the walls can't maintain its heat with the furnace that is installed to manual J spec. ...also keep in mind oversizing to some degree typically ends up being built into the incremental sizing of the equipment unless the manual J is sitting right on the numbers.

[Elcam84]

Originally Posted by gtojohn

I agree, manual j can be off quite a bit. Our design temp here is 96F however in the past ten years we have long stints of 100f+ days. One year we had over 60 days straight of triple digits. HVACs sized too close to manual J couldn't keep up. Lots of unhappy customers. Heating obviously you need to plan for those extremes as well, just because you average 40f for a low doesn't mean you won't have a freak 20f week. Perhaps a good argument for multistage equipment that could ramp up when you need it.

Man j is seen as the best way to size and it is good but... It also has the GI-GO problem. (Garbage in garbage out).

When using the recommended numbers in our area it doesn't work for cooling. The design temp here is 99* however the last 5 years we averaged over 100 days with the temps over 100* with many of those days being 5+ hours over that temp and it will be upwards of 110* for a few hours. We had nearly a month where it was 115* to 117* every day.

The issue is that those high temps are just part of the equasion. The issue is that it will be 90* by 9 am then 100* by noon then it will still be mid 90s just before midnight with a low temp of 80-85 at around 5:30 am.
So when a unit is sized by man j much of the summer the ac will be running pretty much non stop all day and it won't cool the house to the desired temp till after midnight when the system finally can catch up.

Like mentioned many homeowners here with houses that have man j sized units were hot in their houses through the summer. Many have been up sizing their equipment because of it.

As to the devil of over sizing that is always brought up it isn't as bad as you may be led to believe. There have been DOE research that has shown that over sizing doesn't have an appreciable impact on operating cost unless it's grossly oversized and that nearly never happens. If you have a fear of over sizing you can go two stage systems or thermostats with adjustable temp settings to change from 2* to or 4* delta t (differential temp)

Allot of the same goes for heating as well and man j is still a good resource but it isn't the end all and be all and it even says that in its manual. It does need some updating for its design temps in certain regions. I researched the temp here for the 30 year period and it was a good chunk higher than the man j numbers ie their numbers aren't being updated...

[stevehull]

Elcam,

You are exactly correct that oversizing an AC unit doesn't cost any more to cool than a correctly sized unit.

But ONLY if you are talking temperature . . . .

The condensation of latent heat requires that the unit actually run longer if the system is to remove humidity. That is the reason AC works - to remove moisture and as an added benefit, the air is cooler.

Steve

[stevehull]

Further discussion re prior post.

There are large differences on heat/cooling depending on environment.

One of the worst places I had to contend with was an area in the Philippines that was subject to high humidity (80% almost all day - for months) and high winds. The design criteria was to maintain 50% humidity to protect sensitive electronics in a small "insulated" building with about 4-5 humans operating (mostly watching) the equipment. The both inside and outside walls were covered with foam sheets to prevent anyone outside listening to the goings on inside. VERY well insulated - perhaps R60 or so.

Two sturdy doors, with weather stripping, separated by about 5 feet comprised the entrance. But again, a lot of air leaks.

The combination of human water off gassing (mostly exhaled water, some perspiration) and the incredible imposed outside humidity load was daunting. The outside temperature was not bad - in the upper 80 - low 90s F. The electronics "heat" load was small as it was almost all solid state (no vacuum tubes). Probably a maximum of a few hundred watts (all CMOS technology).

But each person, at rest, contributes about 100-200 watts and five people is a substantial heat load in a 400 sq ft space. Also, no windows. A concrete block structure that one would think would have no air infiltration. Wrong! In retrospect, there were gaps in the manner to which the ceiling fit on top of the walls and this resulted in a large amount of water vapor infiltration. The significant winds essentially enlarged these small cracks as the windy side was pressurized and the down wind side had a slight vacuum.

Today, a small ERV would be helpful to remove moisture.

The smallest ac "through the wall unit" was found, but it cooled off the area too quickly and the inside became a soupy, wet, clammy and smelly mess. The electronics was literally failing as condensation would occur.

The solution came from a Navy submarine where tiny AC units are used to remove moisture from crew quarters where high density sleeping areas are. I don't recall the specifics, but it ran essentially all the time and removed a constant stream of water from the "shed". The problem I had was converting the power supply to accommodate the unit - subs are even stranger that aviation power supplies.

Frigid cold or blazing heat can be easily contended with, but high humidity at low temperature loads is tough.


Steve

[MN Renovator]

Originally Posted by stevehull

...Frigid cold or blazing heat can be easily contended with, but high humidity at low temperature loads is tough.

I usually figure with R60 that they would have prevented or retroactively resolved the air infiltration issue. Normally using foam involves taping as well as offsetting seams including sealing(gaskets/foam/liquid applied flashing/etc.) any areas that meet other building materials. It sounds like something went wrong with the design.

..but either way with an equipment damaging moisture problem, a dehumidifier would have mitigated that when cooling loads are low. In my opinion, an ERV doesn't make much sense if your primary latent gains are through infiltration and internal loads, if it were to replace a ventilating system that was bringing in the moisture, that would make more sense but that doesn't sound like the issue. Granted it would remove the human factor/perspiration issue of multiple people in a small space whenever ventilation is lacking.