DIYbanter - View Single Post - gas furnace register temperature question

Speedy Jim

wrote:
Lately I had been noticing that the temperature of the "heated" air
coming out of my gas furnace was barely above room temperature, 75
degrees by my infrared thermometer. Filter is nearly new, flame is a
perfect blue.

Turns out that the problem was I'd opened up the registers to a couple
of rooms, and the "redumbdant" register in the living room. After I
closed the registers again, the temperature of the heated air shot up
to 110+ degrees at the furnace.

Now, my gut is telling me that this furnace is undersized for the house
if I can't have airflow to more than one register in the living room,
one in the breezeway and one in the main hallway without the register
temperatures dropping to barely-above-ambient. Any expert opinions on
this? House is 1300sq ft, furnace is 64,000 BTU, location is Rochester,
NY. Former owners were snowbirds and the furnace never had to do more
than keep the house at 50 over the winter.

64K BTU may not be unreasonable for only 1300 sqft.
I'm in Cleveland w/similar weather and 69K (95%) on a
somewhat larger house.

Instead of the IR probe, take a common outdoor thermometer
or similar and place it right in the air flow to check temps.

If this furnace is a high efficiency condensing model,
they are designed to have quite low plenum temps.
The air from the registers may indeed "feel" cool
if you are used to withering blasts of 160F air from
an old furnace.

If you want to find out what size furnace is needed without
all the complicated measurements (and assumptions),
record what percentage of an hour the thing burns when the
outside temp is (X) deg F. Do this after the house has
stabilized at some comfort level. From here you can
extrapolate the BTU loss for the house per degree (you will need
to know the furnace efficiency, or at least assume some number).
From there it's easy to get the BTU required for the coldest
day expected.

I've used this scheme and you can get numbers (BTU loss) which
are very repeatable and represent actual losses within a few
percentage points.

Jim