Based on the local weather stations in the area, the HDD shown are between 4,446 - 4,849.

My question is in regards to the Base Temperature. Typically what is used for the base temperature, is it 65F?

Is that base temperature fixed or can one adjust that variable?

If you are inquiring for heatloss, use the temperature you normally keep the house at. Our code requires that the heating be able to maintain 22ºC (71.6ºF) in all living areas. So your might require a specific minimum as well.

Posted By FBBP on 13 Oct 2013 12:04 AM
If you are inquiring for heatloss, use the temperature you normally keep the house at. Our code requires that the heating be able to maintain 22ºC (71.6ºF) in all living areas. So your might require a specific minimum as well.

My family likes it warm, so 70F or 72F is where they comfort level is. I know some people pile on and wear 2 sweaters inside the home and keep in the lower 60's during winter but that doesn't work for the elderly, young kids or those who are sick.

The HDD showed a 65F base temperature so I will go back and compute to see what it will be at 70F or 72F.

On a similar note, for a Zone 4 (non marine) climate the 2012 IECC limits the SHGC of windows to <0.40

If you are covered by the IRC, the language is along the lines of "......68F, 3' from the floor, 2 feet from exterior walls in all habitable rooms....."

The default base temperature for heating degree days in the U.S. is 65F, but that does not imply an indoor temperature of 65F, but rather about 70F. The difference between these temperatures is due to the normal heat gain in a house from heat losses by appliances like refrigerators, hot water heaters, etc. and solar gain through windows. When the outdoor temperature is above 65F, you would not normally use the furnace to heat the house.

Use http://www.degreedays.net/ to determine the effects of different base temperatures (any temperature you desire to input) on the heating and cooling degree days in your area.

The ideal indoor temperature depends somewhat on how active a lifestyle that you lead. If you spend a fair amount of time outdoors hiking, skiing, etc., then you do not want to spend a lot of time adding layers or taking off layers every time you go outside or come inside of your house.

If you spend a fair amount of time outdoors

See, I had pretty much reconciled myself to the fact that life was all about putting on and taking off layers.

This is good site to get customized CDD/HDD data:

Weather Data Depot

You can select a specific base temp, location, and year which is very useful when doing an existing building energy usage analysis:

Local code will typically define the required design parameters, however, it usually is not very difficult to get a variance if you can successfully substantiate the reason (e.g., we often get a variance to use greater than 0.40 SHGC for our south wall fenestration passive solar designs).

Posted By sailawayrb on 13 Oct 2013 10:30 PM
This is good site to get customized CDD/HDD data:

Weather Data Depot

You can select a specific base temp, location, and year which is very useful when doing an existing building energy usage analysis:

Local code will typically define the required design parameters, however, it usually is not very difficult to get a variance if you can successfully substantiate the reason (e.g., we often get a variance to use greater than 0.40 SHGC for our south wall fenestration passive solar designs).

The default "Balance Point" temperature is 60F. Would it be out-of-line to set it at 70-72F if that is what the indoor temperature would be set at during winter?

Here are some of my Manual J specs:

Heating 99% Dry Bulb = 18
Cooling 1% Dry Bulb = 96
Interior Design Temperature: 72F (family likes it hot, medical reasons)
Delta T = 56

As far as a variance goes, I will see what the county says but my architect bumped the southern glazing exposure to around 8% of the total home square footage. In addition I will have thermal mass on the south side of the home via exposed concrete floors. With 30" overhang protection and thermal shades for summer use, I should be good to go.

As Lee stated above, the "Balance Point" is not the temperature inside your home, but the outside temperature at which point you don't need to run your heating system. Due to normal internal heat gains (people, appliances, lighting, solar gain, etc.), an outside temperature of ~65F will result in an inside temperature of ~70F or slightly higher without requiring supplemental heat addition.

Precisely, the CDD/HDD balance point is simply the outside temp datum from which the CDD/HDD data is based/calculated. In addition to the indoor temp cooling/heating system set point, the building assembly R-values and the amount and location (i.e., interior or exterior of the insulation) of the building assembly mass also need to be considered before selecting the proper CDD/HDD balance point for a given building.

 

The ability to obtain a variance often depends on the local government development/planning department and on the credentials of the party making the request.  We have been able to get approval to use thick single pane glass by showing a net positive heat gain during the heating months (using automated insulated drapes at night) and low heat gain during the cooling months (using a properly designed passive solar roof overhang).

Posted By arkie6 on 14 Oct 2013 06:57 AM
As Lee stated above, the "Balance Point" is not the temperature inside your home, but the outside temperature at which point you don't need to run your heating system. Due to normal internal heat gains (people, appliances, lighting, solar gain, etc.), an outside temperature of ~65F will result in an inside temperature of ~70F or slightly higher without requiring supplemental heat addition.

Arkie - that is why if Bear likes/needs to keep his house at 72º, he will have more heat degree days to deal with then someone who keeps it at 70º. He should probably try to find the hdd under 69º if his was a stick built home to get a truer value of heating costs. But as Sailer points out, R value and mass also play. So if Bear is doing an ICF home, I doubt any of the traditional hdd days will give him an accurate return.

Wind, leakage and solar gain also add inaccuracy. And the occupant's tolerance for variations in temperature (which strongly effects how useful interior mass is).

Solar gain on building envelope assemblies will significantly increase the cooling load, but will not have a significant effect on the heating load. Solar gain (and convective/radiant heat exchange with the outside environment) is addressed by using the appropriate ceiling or wall type when doing the cooling load analysis. Having the mass on the interior side of the insulation reduces the cooling load. Having the mass on the exterior side of the insulation increases the cooling load. Solar gain on building envelope assemblies and the mass location within the assembly does NOT significantly affect heating load unless you are using the mass to capture/store/release passive solar heat, in which case the mass should be on the interior side of the insulation and should also be directly exposed to the irradiance.

Leakage/wind will increase building air infiltration, which will increase the cooling/heating load. However, using the correct CDD/HDD balance point for the building is all about improving the accuracy of the building energy usage analysis forecast. The CDD/HDD balance point does NOT affect cooling/heat load analysis accuracy (and is not even used for a cooling/heat load analysis) unless you are using the CDD/HDD data to reverse engineer the cooling/heating load for an existing building using the existing cooling/heating system and actual fuel usage over some period of time.

The balance point of 65F was standardized to be within a reasonable range of the actual measured balances points of houses built in the 1950s, with a 70F interior temperature. At that time R-values & air tightness weren't as good as today and window losses were higher per square foot.

But at the same time you had considerably less exterior surface area per occupant and comparable interior heat sources, so the balance points haven't really moved that much. Even though there is higher-R, there is more square feet of exteiror area to lose heat from per BTU of interior heat source. Careful Manual-Js estimate roughly where the balance point is based on the data entered, but balance points of 60F or lower are usually only found in very tight houses built to better-than code R. Most homes today will still be within very few degrees of 65F for a balance point, but it's a distribution curve- and I'm not sure where the 1-sigma ticks are. Clearly in a PassiveHouse or Net Zero Energy home the balance point will likely fall below 60F, but probably not below 50F, and maybe not even below 55F.

The error induced by ICF mass-walls in using fuel use to measure heat load is at most the very low double-digits, even in the desert SW- wouldn't sweat that one too much. The solar gain in many houses is a much bigger factor than mere wall-mass effects, and when looking at homes specifically designed as passive solar houses it's all over the place, rendering fuel-use data nearly useless for making that determination.

You can get a rough idea of how accurate it might be here. It appears that the 60F base used was too low for this house and that a +/-15% variation is common, even when many of the variables are constant or averaged out.

DD have been of little use for some time. I my everyday Manual 'J' work, I find windows and ventilation the weighed factors in new construction.

I working on a small home in N. Wisconsin now that will not go below 70°F while occupied. Too tight, carpet over slab, R-50 ceiling, two people and same appliances as a 3 bedroom house. Ventilation over-sized but under-ducted. Boiler over-sized and not wired to ODR. The heat output of light fixtures unaccounted for.

There is more to it the DD...

That's right- all houses have thermal mass, solar gain, and internal heat sources, and some R-value. The true balance point has quite a distribution, but it takes pretty much a tent without power for that balance point to be 70F, but a fairly high R to be below 60F.

And even when the outdoor temp goes below the balance point there's a time lag due to the thermal mass of the house, and even solar gains will often be sufficient during the shoulder seasons to keep tight higher-R houses comfortable in the shoulder seasons even when the daily average temp is running below the balance points. Mid-winter fuel-use/HDD ratios tend to be far more consistent than the shoulder seasons, and a better measure of the actual heat load. But the higher window fraction you have (any side of the house), with U0.40 or better windows, the more solar gain issues skew the result.

Precisely, CDD/HDD data and balance point are all about improving the accuracy of the building energy usage analysis forecast and little else. Yes, a well executed high R-value building will often have a balance point in the high/mid 50sF range. However, even if you have the precise balance point for your actual building, your actual CDD/HDD in future years will likely be different than the baseline CDD/HDD data that was used to create your forecast. Hence, it is unlikely that your forecast monthly/annual energy usage will match your actual energy usage in future years.

For a well executed passive solar building, you can’t treat the solar gain as just a secondary effect as is customarily done for a conventional building cooling/heating load analysis. You need to carefully determine the maximum BTU/hour solar heat gain during both the irradiance period and subsequent to the irradiance period (i.e., stored/released by the thermal mass)…and this maximum BTU/hour solar heat gain must always be less than the building heat loss to avoid overheating the building. Yes, you can use average monthly climatic solar heat gain data (i.e., expected solar heat gain after considering/accounting for local climatic conditions) in conjunction with CDD/HDD data to perform a supplemental heating building energy usage analysis forecast with all the aforementioned limitations. However, you do NOT want to use this average monthly climatic solar heat gain to size the solar fenestration area and to design the thermal mass unless you don’t care to maximize the passive solar heating performance and avoid overheating the building. This is a common mistake often made by novice designers who only use simple design rules of thumb like just sizing the fenestration area to be between 5 and 12% of the total building floor area and having 5.5 square feet of thermal mass for every square foot of fenestration area in excess of 7% without considering/addressing anything else (e.g., actual building heat loss, fenestration SHGC, fenestration orientation relative to true south, roof overhang design, local terrain obstacles, thermal mass properties, maximum clear sky solar heat gain based on latitude and local atmospheric conditions, etc). Just like for a proper building cooling/heating load analysis, you need to properly determine and carefully book keep all the passive solar heat gain BTUs.