Ideas for zoning a forced-air system

abogart · 01-20-12, 04:40 PM

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.

01-20-12, 04:40 PM	#1
abogart Efficiency Afficionado Join Date: Jan 2012 Location: Michigan, US Posts: 68 Thanks: 21 Thanked 6 Times in 4 Posts	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.