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selective room heating? Answered

I just turned on the heat before going into my room to go to bed. My roommate and I are in about one fourth of the whole house (and will be for about six hours), yet every room is being heated. This seems like a huge efficiency problem. Are there commercial systems out there that can heat selective rooms? If not, are there ways of making such a system? Can I just tape cardboard over the vents?


Baseboard electric or baseboard radiator heating systems are normally wired separately for each room, so one can heat certain "zones" being occupied, rather then the whole house.

Alas, I don't have this kind of system so I can't apply this (admittedly very simple) solution.

Electrical floorboard heaters aren't hard to install really.

With radiators you could add bypass pipes and electronically actuated valves...

Think it would be worth trying? I don't personally have a radiator around here to mess with.

Oh I was guessing you had radiators and central heating built in if your whole house is already heated... Kelsey's Ideas would definitely be more simple to introduce to a duct system than installing radiators...

Yeah, there's a typo in the topic heading; I'm actually in a student apartment so I can't make permanent/overt changes. I'm leaning towards the branch-shutoff and fan speed-adjustment idea.

That does seem like an easy option, unless you can find an easy way to heat the single rooms without burning too much electricity...

. Most vents have an adjustment - cut back on the unused rooms. If you reduce the flow through the heat box too much, it can cause problems, but shouldn't be a big deal. . To find out how the pros do it, search for "zone heating".

I'd completely forgotten about the built in vents...hm, most of mine are stuck. Yeah, we've been discussing the basics of zone heating to some extent.

Yes, there are certainly commercial systems. They are called "zoned HVAC". They work by having a thermostat for each zone connected to a butterfly valve in the duct leading to that zone.

It is inefficient to retrofit an existing forced-air furnace/cooling system for zones. The problem is that the fan is usually optimized for a particular volume flow; by valving off significant portions of the ductwork, you can introduce an impedance mismatch and backpressure, unless the system is able to adjust the fan speed.

It shouldn't be too hard to adjust fan speed, based on the number of vents open at a given time. If you closed the ductwork cleanly at the base of the branch, wouldn't it eliminate any backpressure problems?

Ok, I'm not sure what you mean by impedance mismatching and backflow. To think aloud: my HVAC system expects to push air through a certain distance of ductwork. By considering this length and frictional losses, the fan is calibrated to spin such that it does not over- or under-power the system. I guess I need to read an HVAC guide..

Yes! Impedance is fancy-talk :-) for "friction" or "resistance to flow." The HVAC system expects to push air through your ductwork at a certain rate (volume per unit time, like cubic feet per minute). If you consider the cross-sectional area of your ducting, then you can figure out the velocity the air has to have to meet that volume flow rate. When you have ducting with branches (or zones), the the cross-section of interest is the total of all the "main" branches. If you block some of those off, then the same volume of air is going to have to be pushed through fewer branches, meaning higher velocity.

Oh, it clicked! If I was to physically block a vent, air would still travel down that branch and then, potentially, turn around and interfere with the main branch = backflow = inefficient.

I'm actually taking a course in fluid mechanics now so this is fairly familiar territory. Final's coming up on Saturday, as a matter of fact. Maybe I could do an HVAC project instead..

Bingo! If you're taking a fluid mechanics course, you should already be more familiar with this than I am (my undergrad was...ack...24 years ago).

You could tape cardboard over the vents, but it would probably be better to block the air closer to the source.

As KJ said, you could be selective rather easily with a radiator-type system. My house has radiators along the floor, through which heated water is pumped from the furnace. Electromechanical valves could selectively turn on individual rooms.

You could rig it up with a switchboard, and keep it completely manually operated, or attach it to a small computer/microcontroller, and set everything up on schedules. If you wanted to get really into it, you could carry or implant an RFID chip, and have your presence in a room override the schedule. Apparently Bill Gates' house has a similar system to adjust temperature, music, and lighting.

I was definitely considering using a PIC or something. Hah, RFID would be fantastic. A group at my school was working on a "wisdom door;" it analyzed body characteristics when you passed through the doorframe and appropriately tweaked the space's temperature/music/light/whatever. It's a neat idea.