gas heating

john_183

gas heating in 2 story home ;is it possible if you shut of some of the heating ducts completly;
could this cause the furnace to go on and off more often; becuase some areas are not heated ; the warm air goes to these areas; reducing the temp at the thermostat; is this possible.is this a good idea to shut. thank you for any help ;;;

Brad White

A few variables to consider...

If your furnace is a conventional on-off type (no staging or modulation), this is roughly what happens:

You start with a fixed temperature rise, say 50 degrees at full airflow. Call it 70 in and 120 out, just for discussion. If you reduce that airflow appreciably, say by 30 percent, your temperature rise now goes to roughly 71.5 degrees (in at 70 out at 141.5 more or less). This may be within the range of your furnace as most have a variable temperature rise within their CFM range. 40 to 70 degree rises are not uncommon.

If you continued to drop your airflow to say, 50 percent, your initial 50 degree rise goes to 100 degrees. This may be above your safety high limit and your furnace will trip out, requiring a manual reset.

If you are away and this happens you risk a freeze. If you are away and this does not happen, you will be looking for a new house. Or in Canada, as you say, "hoose". Pick any colour.

From a practical and control standpoint, you have to consider which space has the thermostat and what your adjustments are doing to that. Cycling is a function of the thermostat being satisfied, typically.

Key is, how and when does it become satisfied compared to the rest of the house?

If you do reduce airflow (less air but hotter), the heat delivered is about the same to each room on a per cfm basis.

But if you shut off more remote rooms (your mother-in-law's bedroom, will she never leave?), the rooms closer to the furnace will get more -and hotter- air hence more heat. This may shut down your furnace sooner if one of those rooms happens to have the thermostat or is close to that. This may cause cycling and that is an efficiency loss. Long and steady operation is one key to efficiency. Starting and stopping? Not so much...

In short, isolating too many rooms as a way to save money is not an effective strategy in my opinion. It usually forces your furnace to cycle more. Rather than shut off rooms entirely, I suggest dampering them down partially so that they are cooler than other rooms. Bedrooms are good candidates for this treatment as are basement rec rooms and your indoor curling rink.

My $0.02 (with currency parity no less!)

Brad

Empire_2

Great point's Brad

Additionally, Dont forget that when shutting down heat registers, the least amount of insulation is between interior ceilings and wall's. Some homes do not even have the insulation inside the inner walls Now you have a really cold room directly adjacent to a heated space and that heat will try to go to the cooler room's. Your furnace still has to get rid of so much air and by choking it down, you will definitely change the way it runs.

Mike T.

Mike T., Swampeast MO

Short Answer

Rare for me...

Shut down heat supply to a room and close it off with a shut door (especially with forced air with no return in the room) and you reduce the load on the furnace and it will cycle more frequently because it is now more oversized (especially in moderate weather).

Unknown

John, the best thing to do is balance the

registers to your comfort and desired cycle rate of the system. If one area is getting warmer than the other then close that one slightly leaving the cold area fully open. It takes a little time but with patience and willingness to make adjustments over a period of time it will all work out.

I did my house 25 years ago and have never touched the registers since. I also run my blower on my furnace on Constant Air Circulation with a very low blower speed, this is very economical and again with some work it can balance out.

Many warm air systems also have inadequate return air. The ratio should be 1 1/2 times more return air than supply. A real easy way for a home owner to test this is to go to the farthest register in the house with the fan running and put you hand down near the register. Now go down to the furnace and remove the blower door, jump out the interlock switch and get the blower running, now go back to that farthest register if the flow velocity increased then you probably need more returns in the house. Ideal would be one in every room.