extra thermostat, electric dampers and 1 furnace

Question

I have a problem similar to others that have posted: balancing heating/cooling throughout the house. I have already played with adjusting the vent's air flow as well as the manual dampers in the basement runs.... nothing seems to work sufficiently.

So I am trying to figure out how to add both an extra thermostat for the bedrooms and electric dampers to control the air flow to just the bed rooms when needed. My thought is to install electric dampers on the main level HVAC branches off the main run.

The trick is I need something to help with the very simple logic:

• When both thermostats on are, all vents are open.
• When The Main is on but bedroom off, all vents remain open.
• But when just the bedroom thermostat calls for heat and the main doesn't, the dampers close to redirect the heat to those rooms only.

I have 7 vents in the main house and 5 in the bedrooms. I figure I can install a mechanical damper to control overpressurization if that becomes a problem. The real trick is something to control the logic. The electric dampers will run me around $400 for the 7 that need controlling so even another few hundred would be worth it for comfortable rooms. Any ideas?

EDIT: I have been thinking about this a lot the last few days and have bounced it off some especially handy family members. I think I may have some solutions...

I was thinking about it backwards. I would install dampers that are usually closed vs open. When power is applied, they would open. So when the main house calls for heat, the dampers open and heat goes all over the house (although mainly on the first floor). When the second thermostat in the bedroom calls for heat and the main doesn't the dampers would be closed and heat would pump towards the bedrooms.

It turns out wiring two thermostats in parallel isn't such a big deal as long as they are both set to heat or cool and if I install a backflow damper, I can ensure I don't do damage to the hvac system.

So I just need to wire the main thermostat to not only the furnace but also to a power panel that opens up the dampers

No computer controller needed, right? What am I missing?

Answer 1

If the main thermostat doesn't live in one of the bedrooms (probably doesn't), you'll lose the feedback loop that makes the thermostat work. In other words, the thermostat will stay cold, and keep calling for heat, while the bedrooms are broiling.

If the problem is that the bedrooms are always cold (assuming winter), and you want them warmer, another shot at balancing the system is probably in order. Get aggressive (I have one vent in my family room where I blocked the entire thing with a sheet of cardboard with one 2" hole). Try blocking 50% of the area of the register with cardboard behind the grate. Do this on the vents in the warm part of the house, and make sure the bedroom vents are open all the way. This is a weekend project. You'll put some baffles in, wait an hour or so, make some adjustments, wait, rinse, repeat. Buy a couple cheap thermometers and leave them in different parts of the house so you can check on them.

Just be careful not to block too much airflow. You should take the temperature of the air coming out of the vent closest to the furnace before you start. If it rises notably after you're done, you might want to undo some.

Also, if there's not a decent gap under the bedroom doors for airflow, leave the doors open, trim them, or install a through-door vent. If you sleep with the door closed and there's no way for the air to circulate out of the room, it won't get enough airflow, and the room will get cold.

Answer 2

Why re-invent the wheel? There are already parts to make this happen.

The way a multi-zone forced-air HVAC system usually works is the duct systems for each zone are separated early and only one damper is used per zone. Obviously if your duct system isn't designed with that in mind, you'll have to use more dampers.

Also, a zone controller is used to control the dampers. Some can handle multiple dampers on a single zone, otherwise you need a damper relay.

See Do thermostat-controlled, auto-adjust HVAC vents exist?

Answer 3

Try this site for the dampers: http://www.famcomfg.com/hvac-damper.html

They make a 24V version but I would power them from a separate transformer than the furnace power.

You can get them in either normally open or normally closed.

You said:

The trick is I need something to help with the very simple logic:

• When both thermostats on are, all vents are open.
• When The Main is on but bedroom off, all vents remain open.
• But when just the bedroom thermostat calls for heat and the main doesn't, the dampers close to redirect the heat to those rooms only.

The first two statements are the same so one is not needed. So if the main is on all functions as it used to. The last statement you made will control the damper to stop heat to the main room. So, you just need one damper that is normally open but will be closed when the bedroom-only thermostat calls for heat (or cool). Then the relay that controls the damper to the main has to be interconnected so if main thermostat is on then that relay cannot be actuated.

Something like this:

(It would be very convenient if both thermostats had an extra set of unused contacts. Otherwise you will have to actuate a separate relay with each thermostat to close and open the contacts in my drawing.)

You could do this with very small relays as the damper coil shouldn't draw too much current.

Good Luck!