Closing Vent Registers

[redpoint5]

I bought a 4 bedroom house 3 years ago and rented out the extra rooms. Now that I'm married, we only use 1 of the bedrooms. I have closed the heating vents to the 3 unused rooms and unused bathroom. In total, almost half of the vents in the house are closed. Would this cause excessive pressure on the ducting and place an unacceptably large strain on the blower motor?

In addition to closing the vents, I was thinking about taping over them so they don't even blow a small amount of warm air. Then I was thinking I would stuff a towel under the door gap to isolate the rooms from the rest of the house. Could this save any noticeable amount? The bedrooms are all on exterior walls.

Of course, I have a highly programmable thermostat set to suit our daily schedule, and we turn off the heat when we leave anyhow.

With an unusually low temp of 16 deg F, I've been thinking about this more recently. Hopefully the fuel in the truck doesn't gel, otherwise it will be a cold commute on the motorcycle.

[gone-ot]

You need to know how much air volume (CFM/min) the air handler (fan) is capable of at maximum. Then you need to determine the area of the vents that you have NOT closed off...if they are not BIG enough to handle the CFM, you run the risk of possibly "blowing-out" (a) duct connections to vents or (b) ducting itself (especially if old).

[euromodder]

A fan is a pump.
As long as they can transport some air, they'll be fine.

If there's less demand for air, it should slow down the fan.

[roosterk0031]

Some blower motors have multiple speeds, take a look at the motor, slowing it down will save a little power. If the furnace is new enough to have a circuit board it there might be a jumper there to change the speed.

[Frank Lee]

Originally Posted by euromodder

If there's less demand for air, it should slow down the fan.

It should speed up.

[user removed]

Don't think there would be any problem, just make sure it will not get cold enough for any pipes to freeze. You will get more air from the open vents and the system will increase the temperature more quickly and run less average time.
Just make sure the intake filter is clean. That can really cause problems.

regards
Mech

[Ryland]

If you want to know for sure, put an amp meter or a kill-a-watt meter on the fan and see how much more energy it uses, if any, when you close off the vents.
Guessing will get you no where.
A remote sensing thermometer might also be worth while, I have one that tells me humidity as well for both the indoor and out door sensors, knowing if a closed off room is damp or below freezing could save you a lot of money in repairs.

[ksa8907]

Careful not to let the rooms get stagnant. Blocking off the registers is good, but don't let the rooms get too cold or you may build condensation and spend way more than you would ever save.

[sheepdog 44]

I've taped over all the vents downstairs before. All the bedrooms are upstairs, so at night i was only heating half the house where people could appreciate the heat. It did create more pressure in the rooms where the vents where open, so i used a book to crack the door open to help the heat move through the room.

The only problem was the thermostat was downstairs in the cold. I set it to 50 degrees so the upstairs wasn't too hot.

[redpoint5]

The house was built in 1995, and I'm assuming the natural gas furnace and blower are original. As far as I know, the blower only has a single speed, and is electronically controlled.

I counted the open and closed registers today and found that 12 out of 18 are open, or 2/3. I'd guess this is enough to relieve the ducting pressure. I would also think the unused bedrooms would stay warm enough to stave off mildew problems just by conduction through the thin doors. My strategy is to block off many of the upstairs registers where the unused bedrooms are, and open all of the downstairs ones. Since heat rises, this usually does a good amount of warming the entire house. Besides, the wife likes to sleep at 60 degrees or less.

Today I went into the rooms and estimate the temperature was 60 when the rest of the house was 67 degrees. I blocked off the gap under the doors with towels and will monitor again tonight. It's supposed to be 13 degrees tomorrow night! That's low for the valley (~200ft elevation).

There was a thin layer of snow this morning that caused a lot of trouble for commuters. I was worried my Dodge/Cummins wouldn't start with the extremely weak batteries, so I plugged in the block heater for the first time. Any idea how many watts these usually consume? I don't know where my Kill-a-Watt is at the moment, so I can't measure it myself. How long should I plug in a block heater (I'm assuming that's what it is) before driving off?