Saturday, October 20, 2012

Steel Ducts Heat Capacity, Forced Hot Air Heat

I am enjoying steadier, quieter heat, where I replaced the leaky old steel warm air ducts in my crawl space.

I don't want to remember found conditions in my crawl space, where an impassable 14" diameter trunk fed variously-wyed 6" ducts at mid height. Here is a less-traumatic view, ducts down this past Summer:

Here is the new trunk 10" by 20" by 10 ft, lined.

Here are views, with flex ducts attached.

The crawl space will be conditioned and warmer. I won't need precarious duct wrap, except at the floor boots. There is ruggedness, simplicity, and very assured sealing of ducts. All good, but I wonder, have I done even more good by taking away waste of heat in warming the old ducts in each furnace heat cycle?

Surely the internet will tell me. Google-search ""heat capacity of steel" steel heat ducts", and get;

  1. PDF] 

    Fourier-based Calculations of Temperatures of Structural Steel
    File Format: PDF/Adobe Acrobat - Quick View
    Structural steel, when exposed to a uniform heat source at temperature T, will, in the absence of .... specific heat capacity of steel ..... AC and heating ducts, 4.

Modern Hydronic Heating: For Residential and Light Commercial ... - Page 127 - Google Books Result
John Siegenthaler - 2011 - Technology & Engineering
by 1°F. The heat capacity of a material can be found by multiplying the specific ... theheat capacity of steel, which reflects both its specific heat and density, ... water can convey the same amount of heat as a 14- by 8-inch duct conveying air, ...

No, it is not a well-expressed topic. Nothing more relevant further down, within my limits of patience. The second hit is interesting in questioning acceptance of forced air heat and consequent badly-placed ducts, but not on-topic.

Here are my assumptions doing this afresh, by myself:
  • Crawl space at 200-day heating season average of 50°F.
  • Average air temperature within ducts in a cycle is 90°F.
  • Wrapped ducts heat inertia is such that temperatures are followed 50%.
  • The furnace cycles every half hour, 48 times per day.
  • The total mass of ducts is 120 pounds.
The temperature change of the total mass of heat ducts is 10°F in each cycle.
The heat consumed in each furnace cycle is:
m * Cp * delta T
where m = 120 pounds mass
Specific Heat Capacity, Cp = 0.12 BTU/pound/°F
delta T = 10°F

BTU wasted in each cycle: 120 * 0.12 * 10 = 144 BTU
Times 48 cycles
Times 200 days
Result: 1,380,000 BTU per year

Times 0.00001 to convert to therms, is 13.8 therms.

Times $2 per therm is $28 per year.

I think this is low by a factor of two or more. The average cycle is perhaps by more than 10°F. I would believe 30°F is common for a well-ventilated crawl space in moderately cold climates.

I think the heat capacity issue outweighs savings in stopped leakage. Heated air doesn't cost much, with its tiny heat capacity.

Surely there are savings to justify the new ducts, beyond making them airtight and assuredly wrapped. I like the near-instant response, warm air out of ducts.

Google now finds this post, top of list. Bing is out to lunch.

At 10/1/2013, insert a test phrase, to see when it becomes detectable:

"heat up cool down HVAC steel duct mass energy used " I am at the top of search results of course. Little else that follows, is on topic. NREL, US DOE mentions the concern , but does not offer the math.

Please find more posts on this subject: 

At 8/8/2017, add some content about coating of duct interior, with flexible grout, as means of reducing dynamic thermal mass. My flexible grout is half ceramic microballoons, as aggregate content. Tiny, hollow glass beads, must reduce surface thermal conductivity, and retard cyclical heat transfer.

This 3 1/4x14x48" steel duct and two register-mating elbows, weigh eleven pounds. I should of course do anything to avoid waste of heat in swung metal temperature.

Two successive coats of flexible grout brushed on a wet surface by palm, go on quickly. I give up a total two hours of progress in the coating, mostly in wait of drying.

I can't claim how much energy this saves, but it must be worthwhile. Measured numbers would be achieved in a sponsor-worthy experiment at some University.


heatingontario said...

A heat pump works best in moderate temperature climates.

Furnace Toronto

Mignon Her said...

That's good to know. It's a great move that you chose to replace the old ducts with a high quality one. At least you wouldn't have to worry about anything in the attic this time, especially that you never see this part of the house unless you go up there and check it.

Mignon Her