What does Design Temperature mean in practice?

biosopher · July 11

Let's assume an HVAC system has been installed in a house with Design Temperature of 94F and this this house experiences 15 days above 100F annually with 5 days above 105F with a peak temperature of 110F next year.

Let's also assume the HVAC system meets this definition of Design Temperature (DT) as the temperature that a heating or cooling unit must be able to maintain when outdoor conditions place the greatest demand on the system.

What would be the target behavior of such an HVAC system when the outside temperature of the house was greater than the DT?

PC7060 · July 11

HVAC System design should be based on the manual J report for the home. The manual J report lists the heat loss and loads at the cold/hot design day based on house location, orientation, window types/locations, insulation, wall thickness and material. .Assuming a secondary heat source for winter, a “perfectly match” cooling system would bring the house to target temperature (assume 75F) while running continuously with the outside temp at the design day temp. For every increase above the design day temp, you would expect the band to shift up linearly by same amount. Would for outside temp of 5 degree over design day, the HVAC would cool the house to 80f.

No system is perfectly matched due to a variety of conditions so actual temperatures would vary accordingly.

Just my 2 cents (and worth less)

DCContrarian · July 11

There are a couple of underlying assumptions about design temperature. The first is that there will be brief periods when it is exceeded — these are called "excursions." Since the design temperature is chosen as the temperature that is exceeded one percent of the time, on an hourly basis, and there are about 8800 hours in a year, the assumption is that the design temperature will be exceeded about 88 hours a year. But those hours won't be all in a row, three and a half days straight. More likely the design temperature will be exceeded for an hour or two at a time, maybe 30 times over the year. Another assumption is that excursions will mostly be small — half the time just one degree, another quarter of the time two degrees.

The other group of assumptions is that houses have heat capacity, so that when the amount of heat flowing into a house exceeds the amount being removed, it takes time for the temperature of the house to change. So if an excursion is small and doesn't last very long the house doesn't change temperature very much.

Allow me to throw around some numbers. Imagine a four bedroom house in a place where the summer design temperature is 94F. The cooling load to maintain 94F with 75F interior temp is five tons, 60,000 BTU/hr. The house has a heat capacity of 50,000 BTU per degree F. Let's say the thermostat is at 75F, and you get a day where from noon to 4pm it's 95, 96, 97,95. Then from 4 to 8 it's 94, 93, 91, 91.

At noon, it's 95. The heating load is 20/19 of the design load, or 63,158 BTU/hr. Since the cooling system is only good for 60,000, the house gains 3,158 BTU. That warms the house by about 0.06 F. The next hour it's 2 degrees over and the house gains about 0.12, then 3 degrees and 0.18, then 1 and another 0.06. So by 4:00 the house has gained 0.42 degrees and is at 75.42F. Then at 4:00 the temperature drops back down to design temp and the house breaks even, and starting at 5:00 the temperature is below the design temp and the system has some extra capacity. That goes into pulling the heat out that was gained during the day, and after a couple of hours the house is back to 75F and the thermostat can shut off.

So long as the average temperature for the day stays below the design temperature and the house has decent heat capacity relative to the cooling load the excursions will be short and mild.

hot_rod · July 11

Simply put if the heat load increases above what the system can manage, expect the indoor temperature to rise as the system is running non stop at full output capacity

This could be couple degree , it could be 10 degrees for a short period

Assuming the system is designed and installed to allow full capacity to be moved into the space

Load calculation are a moving target it can and will change minute by minute

Both internal and external loads or gains constantly change. That is why the load calcs use “assumption “

biosopher · July 11

Thanks guys. The weather will be in the 100's over the next couple days. I'll track the inside/outside temps and post back then.

PC7060 · July 11

@DCContrarian explains it well. Before we had air conditioning, my parent used to use fans at night to ”bank” the cooler temps inside the house. This was in the Midwest where the temp would drop to low 70’s at night. When sun came up, windows were closed and shade pulled to minimize heat from outside. Would typically keep it cool(ish) until mid/late afternoon. Once started to cool off in evening, the process would repeat.
Of course, this did nothing to manage humidity 😑

You can do same thing using AC by dropping the the thermostat to 70” in evening.

Jamie Hall · July 11

In some ways it would be kind of nice if we could all agree to rename "design temperature" to be "design delta T", wouldn't it? Because that's what it really is, and that's what we are really doing.

But then we still talk about "pickup factor"…

jumper · July 11

There is a technology called adiabatic cooling. When outside temperature is too high air for condenser is evaporatively cooled. A bonus is that the air is filtered so that condenser doesn't foul. I know that we aren't supposed to discuss $$$$ but now that labor is so expensive why don't customers pay for better equipment?

jumper · July 11

Design temperature can get tricky. In good old days residential heating was sized to maintain 70° inside when it was 0° outside. So most of the time heating system is over sized. Then to save energy old homes were tightened up and insulated. So now heating system is grossly over sized. So when boiler is replaced it is down sized. Radiators are now over sized. Is that good in theory? How about in real life?

pecmsg · July 11

https://forum.heatinghelp.com/discussion/comment/1806061#Comment_1806061

It allows lower supply water temperatures.

DCContrarian · July 11

https://forum.heatinghelp.com/discussion/comment/1806021#Comment_1806021

Don't get me started on "thermal mass."

DCContrarian · July 12

This is a screen shot of a sensor in a room in my house on a day when the outside temperature got up to 6F above my design temperature. Thermostat was set for 77F.

It's not a perfect line because it actually got down around 73F overnight. But ideally the line would look like a speed bump, flat while the cooling can keep up and then bumping up when it can't.

Jamie Hall · July 12

https://forum.heatinghelp.com/discussion/comment/1806065#Comment_1806065

Why not? Adds to the fun. One can either make this whole business of designing heating or cooling equipment remarkably complicated — or just give up, calculate the still air load (Manual J or similar) for a given design temperature (look up the nearest city — maybe add a fudge factor for odd local environmental conditions), check the maximum and minimum heat sink/heat source conditions if you are planning on a heat pump or similar cycle equipment, find a catalogue, select a unit which fits, and that's it. Design the installation by the book, and follow the design, and you're done. We all do it. Most of the time it works just fine, and if it doesn't we can say — truthfully — that we followed the proper procedures.

Something to be said for that! It would be a rare installation which required a more complete analysis.

If it cheers you up any, highway engineers do the same thing for bridges. Most of them stay up… architects and structural engineers for whole buildings. And so on.

What does Design Temperature mean in practice?

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