Zone heating with a single furnace

[mheslep]

Given: a two-story residence currently with separate basement and attic gas forced air furnaces-chillers. The basement furnace is a recently updated high efficiency (96%) hybrid heat pump/aux gas heat at around 80,000 btuh (out); the attic unit is much less efficient and maybe 60-70,000 btuh (out), gas only. The home insulation and construction is oldish - walls, windows, etc. With the house as is, a Model J analysis tells me the current total house heat loss is ~110,000-120,000 btuh. In looking at a serious improvement in the insulation - walls, new windows, R-30 attic - it appears as if I could easily bring down the loss to ~60,000 btuh. The heat gain numbers present a similar case in that the entire house, well insulated, could be supplied by just the basement chiller equipment.

In that case, I'm wondering if I could find away to stop using the older, more expensive attic furnace and chiller, along with its duct system that runs in the outside ventillated ~ambient attic space. I'd especially like to know that answer before replacement time of the old attic unit anyway at end of life in 3-5 years. Currently the basement unit duct system supplies just the basement and the first floor. I understand that to maintain comfort in all rooms of a such a multi-story house, it will need at least a two zone control system. How might that be done with one heat source? I'm aware of some controlled damper solutions, but I also read, vaguely, about over heating or freezing problems caused by damper on/off instead of furnace-chiller on/off. Is there no answer but continuing to run the attic unit, and having to replace it in a few years?

 

[russ_watters]

If you have a good location where you can cross-connect the two systems, you'll have a vastly larger ductwork system than a single unit was designed-for and as a result, when feeding air to either level, you'll have plenty of airflow to avoid the problems typically seen with zoning. You'll create a new problem, though, which you may or may not care about: your cooling supply air temperature will be too warm to adequately dehumidify the space and your house will be more humid than is typically designed-for.

Try browsing smarthome.com for some ideas on solutions. Basically, though, you get a controller with two thermostat inputs and one HVAC unit output plus multiple damper outputs (you can actually slave all of the dampers for each zone to a single output). The controller decides whether/when to heat/cool and sets the dampers appropriately to send air where it needs to go.

 

[mheslep]

If you have a good location where you can cross-connect the two systems, you'll have a vastly larger ductwork system than a single unit was designed-for and as a result, when feeding air to either level, you'll have plenty of airflow to avoid the problems typically seen with zoning. You'll create a new problem, though, which you may or may not care about: your cooling supply air temperature will be too warm to adequately dehumidify the space and your house will be more humid than is typically designed-for.

Try browsing smarthome.com for some ideas on solutions. Basically, though, you get a controller with two thermostat inputs and one HVAC unit output plus multiple damper outputs (you can actually slave all of the dampers for each zone to a single output). The controller decides whether/when to heat/cool and sets the dampers appropriately to send air where it needs to go.

Thanks russ_watters. Regarding the humidity problem, the issue there, if I understand correctly, is that the humidity tied to vent outlet air temperature, and that by using a larger duct system on a single chiller the outlet air temperature must inevitably increase, and any increase will raise the humidity over what it would have been previously with a smaller duct system?

 

[russ_watters]

Correct. A psychrometric chart will tell you the resulting relative humidity from a certain dew point. http://docs.engineeringtoolbox.com/documents/816/psychrometric_chart_29inHg.pdf

Room (dry bulb) temperature is at the bottom. Dew point temperature is horizontally across the chart. When you cool air, it becomes saturated, so the supply air temperature is roughly equal to the dew point (plus a degree or two due to heat gain in the duct and inefficiency of the coil). Where the dew point line and the dry bulb temperature line cross, you can read the RH. For example, if the room temperature is 75F and the dew point (supply air temperature) is 55F, the resulting RH is 51%.