Understanding HVAC and heating/cooling zones
I posted a question a few weeks ago about radiant floor combined with forced air:
Taking the advice on that thread I’m moving away from the radiant floor concept and going either forced air or a mini-split setup.
One comment from Dana on that thread requires more clarification for me and I’m hoping the readers can help me understand. Dana said:
Designing the mechanical systems and deciding the zoning BEFORE the Manual-J is done is a serious waste of effort!
I’ve now got my Manual J back and I don’t understand why planning a zone based on intended use is a waste of effort. When I wrote that thread I had planned zoning based on the following use:
Zone 1 – South side of the house is guest bedroom and bathroom and laundry area. This will typically be used less than the main part of the house and so I anticipate keeping the thermostat in that zone around 60F in heating season and 80 in cooling season. When guest are staying they can choose a temperature that suits them.
Zone 2 – Great room, kitchen. This is a large volume and typically we would want it a little warmer than our master bedroom in both cooling and heating season. We typically set our living area thermostat to around 65F in heating season and I’ve never had AC before but I’d imagine about 75F in cooling season.
Zone 3 – Master bedroom, bath, closet and office. Typically we’d have our master much cooler than the rest of the house. We’d normally do a setback (in our old leaky house) to 55F but I anticipate having our thermostat set to about 60F during heating season and 70 during cooling season.
When looking at my manual J that I’ve just received and totaling up the heating/cooling loads for each of the zones I was planning I get the following:
Zone 1 – Heating – 11,600 Btuh , Cooling – 6,200 Btuh
Zone 2 – Heating – 32,300 Btuh, Cooling – 22,300 Btuh
Zone 3 – Heating – 25,300 Btuh , Cooling – 11,100 Btuh
Can you help me understand why designing zones based on usage is not a good idea and how to design the zones based on the manual J?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part