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Old 01-20-12, 05:40 PM   #1
abogart
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Default Ideas for zoning a forced-air system

I recently had an idea to add zoning capability to a central warm-air/AC system. Of course this is all hypothetical at this point... unless I win the lottery, or land a high-paying job some time in the near future.

My thinking is that hydronic systems have zone valves with individual thermostats to control separate parts of the building, even individual rooms. Electric baseboard or radiant heating can use individual heaters with independent thermostats to manage heating of different parts of the house.

The problem with forced-air is that airflow must remain relatively constant through the system in order for everything to work properly and efficiently. Motorized dampers could be used to manage airflow to individual rooms. But this changes the heat rise of the whole system. If only one room needs heat, the furnace or AC unit must run at full rated capacity, but only enough airflow for one room is passing over the heat exchanger or evaporator coil. This would obviously not work with a traditional gas or electric furnace, or a single-stage AC or heat pump unit.

The idea is to retrofit the existing forced-air system without remodeling the entire house (running water lines, etc.). I have come up with two options so far.

First, one could install electric heating coils into each register of the house, each having its own thermostat. The central fan runs full-time, but only the rooms that call for heat get hot air via individual room heating coils. This would require some wiring and possibly modification of the registers/ducts in the rooms. The other downside being that gas heat is much cheaper than electric heat in this area. Also, this setup doesn't allow for zoning of the cooling system.

The more viable solution (although more expensive) would be to install a gas hydronic boiler in the basement next to the furnace. Instead of running water lines to each individual room, water-air heat exchangers are installed in the basement in each duct that runs to an individual room. Again, the fan runs continuously. Zoning valves controlled by room thermostats heat individual rooms via the ductwork. The boiler runs as necessary to keep the water at the set temp. Zoned cooling might be a bit trickier. If there was a way to cool the circulating water via a refrigerant/water HX (desupercooler?) Then a separate aquastat on the boiler could be used to maintain a low temperature in the circulating water and the room zone valves could operate normally, cooling individual rooms as required. Unfortunately, I would need at least 11 HX's for each room or set of rooms in my house. Although it wouldn't be much different than purchasing 11 convectors if installing a full hydronic heating system, albeit slightly more expensive.

I'm sure there are some better (cheaper) alternatives out there, just haven't thought of any yet. Just another one of the many ideas that pop into my head from day to day.

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Old 01-21-12, 02:05 AM   #2
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Interesting concept!
The only drawback that I can think of is that the one room that is demanding a temp change will be losing it's temp to other rooms, unless the door is closed, of course.
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Old 01-21-12, 03:36 AM   #3
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Interesting concept indeed! They have solutions for this already though. They make modulating furnaces, where both the blower and the gas valve can run though a range of speeds. Zoning works very well with these because if only one room calls for heat, that damper opens, and the furnace runs on a low fire and blower speed.

The Carrier Infity system is one that comes to mind as being very popular.

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Old 01-21-12, 08:30 AM   #4
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Quote:
Originally Posted by launboy View Post
Interesting concept indeed! They have solutions for this already though. They make modulating furnaces, where both the blower and the gas valve can run though a range of speeds. Zoning works very well with these because if only one room calls for heat, that damper opens, and the furnace runs on a low fire and blower speed.

The Carrier Infity system is one that comes to mind as being very popular.

Adam
That would make a forced air furnace solution feasible. The trick would be making the furnace modulate heat and blower settings based on how many thermostats are calling for heat, not just the temperature differential. Then motorized dampers would work. But from what I understand, these furnaces are designed to operate with only one thermostat, I don't think they support zoning very well. A separate (expensive) controller might be required.

Ideally, I would want to go with a geothermal heat pump system. I still don't think it would be very easy to modulate central heating/cooling with automatic dampers, though. In this case, a desuperheater/desupercooler with a hydronic system and zone valves would work quite well. The heat pump would run as required to maintain water temp in the tank. This system could also be used for DHW heating as well.
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Old 01-21-12, 09:34 AM   #5
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The idea got me thinking about these Booster gizmos again..
Flush Fit Register Booster Fan



But, what if you installed some duct fans in-line with the rooms
you wanted to control (adding more heating or cooling).?.


(Maybe with less blades, for easy flow-thru when off).

Connected those fans to a thermostat (or off-on switch) in each of those rooms..?.

Installing a few of these Booster fans would mean re-balancing the whole system, with the fans off..

But, this seems like a low cost way of gettin er done..
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Old 01-21-12, 09:55 AM   #6
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How would this improve upon just having electric heat or radiators in each room? If you put it in the register, you have to wire/plumb all the way to the room anyway. The only significant potential gain I'm seeing is if you can zone the cooling as well.

I'd also worry about packing enough heating for a whole room into a duct. If for whatever reason you don't get enough airflow through that duct, it could quickly become a fire hazard.
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Old 01-24-12, 04:33 PM   #7
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I have been doing some research on this subject. It seems that zoning forced air systems is entirely doable and actually not all that uncommon. I found an interesting design manual about it here...

Zoning System Design Manual

Apparently, proper airflow over the heat exchanger or evaporator coil is maintained during periods of low CFM demand by incorporating a static pressure bypass duct, which reroutes excess pressure in the supply plenum back to the return duct. There are automatically modulating bypass dampers that use a pressure sensor, but they are pricey (about $250 each). I would rather go old school and use a weighted-arm-type damper for about $85.

Honeywell SPRD12 static pressure regulating damper, counter balanced, weighted arm to control bypass

This setup would definitely work in my current system. The trick would be to redo the ducting so that all of the upstairs rooms are on one supply trunk, and the downstairs rooms are on the other. That way the house can be easily divided into two zones, upstairs (during the night) and downstairs (during the day). I would only need two rectangular zoning dampers and a static bypass damper to pull it off. Factor in another thermostat and the HVAC zone controller, and it looks like it could be done for around $500. Not bad!

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