I'm doing an audit on a client next week and I thought I'd ask this question to help me prep for it.  The customer is calling me in because one bedroom on his second floor is to hot in the winter while another one 20 feet away is too cold.  He wants to know:

1) what he can do to make the 2nd floor they're on have a consistent temp.

2) how he should set the upstairs thermostat vs. the downstairs thermostat.


It's a 4,100 s.f. house built in Austin, TX around 1991.  It has two HVAC systems; one upstairs and one downstairs.  40% of the year is spent heating; 60% of the year, cooling.


To answer the first question, I'm going to investigate:

- attic insulation, ventilation, air leaks, thermal bypasses, etc.

- wall insulation in the cold room.

- where the return is located in relation to the two bedrooms.  

- whether or not the doors in the two bedrooms are undercut enough to allow air to circulate.

- the balance of the dampers in the supply vents of the two rooms.


Are there other things I should be thinking about other than air circulation, insulation, and ventilation?


As for the second question; I'm assuming setting both the HVAC systems at the same temp during the day should prevent them from fighting each other (as both floors should be the same temp), while setting the upstairs system higher than the downstairs system should work in the winter to save electricity while keeping the occupants warm while they sleep.  (You don't need to heat the downstairs in the middle of the night.  Because heat rises, any excess heat added to the downstairs will float to the top.)

During the summer, it may make sense to have both systems at the same temp at night instead of having the upstairs colder than the downstairs.  Doing so prevents the upstairs from continuously running as it cools the upstairs and whatever hotter air would have floated in from the downstairs.


Any other suggestions on debugging strategy?


Thanks for your thoughts.



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I Like reading these questions. I think it is just the flow to that room that needs to be adjusted. I live in a home similar to this. In the summer both my HVAC units are set at 79. I have my thermostat  programed for our usage thruout the day. We work out of our home.  My upstairs unit goes on more often, due to it is warmer up there, but we are never up there during the day. Downstairs rarely goes on at all. I have a delemia there. I thought of turning the thermostat higher upstairs. In the winter, the thermostat is on 69 downstairs goes on and our upsatirs is set at 65 and while we sleep we perfer it cold and than at 4am the heat kicks in, so when we wake up it is comfortable. Please tell us what you found as the problem so we can learn from this. Thanks for sharing.

Judi Lyall


In most cases, issues like this require a real good HVAC guy that knows his Manual J, S, T & D. Are the ducts properly sized for that area, sealed up or disconnected (check CFM of air flow coming through vents for a quick check) You might also check out a zoned ducted system where each room has a thermostat in it - works great with multi speed units


Quick gut check says, IR pictures might show one wall with no insulation or not enough if next to an attic, or if it is over garage no insulation between garage and room


I've decided to recommend an HVAC guy that's NCI certified to do air balancing. 


I've also recommended they install jumper ducts in the bedrooms, but only if they need it after air balancing.  Two clues: a) one of the bedrooms was more comfortable when they left the door open over night, and b) the master bedroom door would close all the way if you closed it 80%.


As for insulation, they have it in the walls and the ceiling, but thermal imaging shows the typical thermal bridging across the top plate.  Insulation teams don't like so much to scooch close to the eves, so they don't end up covering the top plates of the walls.  See below.


Before blower door:

After blower door:


I think air balancing will solve most of their problem.  Once the HVAC system and duct work is tuned up and running like a top, they can decide on other measures, but they may not need to.  Air balancing may solve the problem enough.


The wisps you see in this image indicate air leakage causing room to be cold.  The tthermal bridging would not flare out like this.

If the envelope is tight it may be an equipment oversize issue.  I always ask if equipment ever runs continuously.


Like many building science truth's that are intuitively backwards, undersizing equipment is better than oversizing.   Oversized equipment short cycles, satisfying downstairs but delivering warm air upstairs because the duct doesn't have a chance to cool. A good way to tell that equipment is oversized it check humidity levels.  If RH is higher than outdoors, you are cooling too quickly.  


If the envelope is messed up, this really can't be properly fixed until envelope is fixed.  In winter heated air goes up on it's way out, somewhat correcting load to distribution imbalance's.  In summer cold air dropping sucks superheated attic air into upstairs, and poor insulation allows it to radiate, exacerbating imbalance.  




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