EcoRenovator - View Single Post - Heating Degree Days NOT useful for sizing

stevehull · 03-06-16, 06:40 AM

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

03-06-16, 06:40 AM	#2
stevehull Steve Hull Join Date: Dec 2012 Location: hilly, tree covered Arcadia, OK USA Posts: 826 Thanks: 241 Thanked 165 Times in 123 Posts	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 __________________ consulting on geothermal heating/cooling & rational energy use since 1990