[JRMichler]

When we left for two days, I shut off the HRV. Unfortunately, my fat finger also shut off the boiler. The result was a golden opportunity to measure the thermal mass of the house.

House temperature when we left: 70 deg F.
House temperature when we got back: 50 deg F.
Average outside temperature: 16 deg F.
Calculated heat loss at -25 deg F: 13,000 BTUH.
Estimated average heat loss at average 60 deg inside, 16 deg F outside = (60 - 16) / (70 - -25) X 13,000 = 6,020 BTUH.
Total time without heat: 54.5 hours.
Total heat loss = 6020 X 54.5 = 328,000 BTU.
Thermal mass = 328,000 / 20 deg F = 16,400 BTU per deg F.

The house is 1294 square feet over a crawlspace. Construction is conventional stick built with normal drywall on the inside. Walls insulated R32, ceiling is R96, windows are Andersen 400 series, blower door tested at 0.85 ACH50.

Since the heating system was sized for the house, it can only raise the inside temperature at a rate of about one degree F per hour. Fortunately, we also have a wood stove.