I have a Customer that has a very open floor plan.  2 story opening in the middle of the house. 

When you walk up the stairs you feel a dramatic temperature change when you reach a certain point up the stairs. 

What causes this and how can it be corrected ? 

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Stagnation / inversion layer - common with radiant heated buildings where the heat stays low & it is cooler the higher you go - awesome in commercial & other areas as you aren't heating everything. The reverse can also be in effect also. I talked about it some in http://thehtrc.com/2012/common-sense-radiant-heat-faq

The cause - lack of airflow, bedroom or other doors closed up top which might be conditioned. How do you fix? Use a ceiling fan to stir the air, open doors, windows, etc...

Are these winter conditions, is it colder or warmer upstairs, is it forced air or radiant heat?

It could be a variety of things.

This scenario occurs in the Winter Months.  It is warmer upstairs.  This is a forced Air System. 

Thank you,  Kevin 

I also should have asked where the furnace was located,basement,1st or 2nd floor. In any case the simplest solution is to close some of the supplies upstairs and make sure they are fully open in the lower levels. The previous homeowner may have adjusted the distribution of conditioned air so it would be cool upstairs in the summer,

We see this in the mid-Atlantic with ceiling feeds on a hot air system, and usually with a taller-than-average ceiling height.  The duct design is poor and the warm winter air is not traveling fast enough to reach the floor to mix properly, and tends to accumulate at the top of the space.  Having a return up high forces hotter air into the return side so supply air gets hotter and hotter until temperature at thermostat level is satisfied.

Solution - If it is a 2 story house with a system for each floor, be sure the upstairs system is set to the same or lower thermostat setting as the first.  With just one system, first try speeding up the winter time fan speed which is usually set at a medium speed. If that does not work, make sure that both the air handler and the duct system are the right size to achieve a high enough velocity.   You may be able to get that velocity with a change of register grilles.  Also, make sure you don't ever install a duct system in an attic.

If it is radiant heat - I can't help - we don't use them much here

It is interesting that people respond to questions like this with answers that make some significant and often incorrect assumptions.

The OP did not state what type of heating system there is or whether the temperature rises or falls when ascending the stairs. It would also be helpful to know how cold it is outside when this is experienced.

Thank you Brad... Like many forums on the Internet the question does not contain sufficient information for an answer. 

So, what changes were made or what happened next?

Hot air goes up and cool air goes down.   I put in returns high and low.  In summer use the high return and winter use the low return.   Leave fan(S) on 24/7 when windows are closed,  Higher or lower temps are worse spread in temp - I have seen clouds form in homes.    I have seen 110' temp differential  in top to basement with a high presser steam system.    Just mix up the air to keep the with in 3' temperature change is what I plan for.   Look at chimney  effect.  

With enough details, I can now provide a practical response...

This is what HVAC guys call "stratification". Warm air is less dense than cold air, it is pushed up to the highest level in the home to which it can easily flow. The only solution is air mixing sufficient to distribute the warm air evenly throughout the house. 

I designed and built a home exactly like this; with an open-air loft looking out into the 25-foot high cathedral space above the great room. That loft was five degrees warmer than the first floor when it was freezing outside; 10 degrees warmer when it was below zero. Fortunately, we anticipated this. A couple of ceiling fans operating at low speed all winter, pulling air up from the first floor, were sufficient to keep the entire house at the same temp. (In the winter, we would turn off the fans in the evening -- the first floor might get a little cooler, but that loft was turned into a toasty reading room.)

No ceiling fans? The furnace air handler, operating 24/7, will provide the same mixing. But the electric load can be substantial -- 100 kWh per month or more. (Isn't it amazing how many design flaws can be fixed by throwing energy at them??)  

Fortunately, stratification can be defeated (usually) with shorter cycle times. An Aircycler control will allow programming to run the air handler for shorter periods. Fine tune to operate the air handler JUST enough to make people happy -- that will minimize the addition to the electric load. You will probably find that running the FA furnace air handler 10 minutes per hour or so will reduce the effect to the level that most people won't notice it. If that's not sufficient, try 5-10 minutes out of every 30.

Cheapest way is 1/50 HP ceiling fans put on low this will give up to 10,000 CFM movement.
If the HVAC system has a ECM motor and on low low about 200 FPM will cost just $200 yr in power with high end UV light and electronic air cleaner.

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