simple custom electronics HVAC interface ?
 

I've got an older (1967) 2 story house in Houston (HOT & HUMID in the summer).
The house is a tract house. Both floors are divided into 4 rooms with stairs in the middle. When you open the windows, especially upstairs, you may not get a fraction of the breeze that you feel outside in the house. It just wasn't designed to have air flow through the house.

I've opened the house when it was 68* outside and 72* inside and actually had the temp go UP inside the house because of lack of air flow.

That's the background.

I've got a regular gas furnace and A/C unit (not a heat pump, no dual level, nothing special) with a 7 day programmable thermostat that will auto switch heat <-> cool.

During the comfortable months, the temp downstairs can stay low enough that the A/C never kicks on, but the upstairs can end up 5-10 degrees warmer.

I'd like to set up a couple of temp sensors (one up and one downstairs) connected to a spare computer and setup a program that says "if the temp differential between the up and downstairs exceeds X, turn the fan on." Assuming that I can work out the temp sensors (easy) and the program (not so easy for me), how would I connect the two without frying anything.

It's my understanding that the green wire is the fan and I just have to join that to power(red wire) to kick the fan on. I assume that proper placement of a few diodes would also be necessary. I'd like to tackle this, and was wondering if anyone of you electronics gurus had any thoughts.

Thanks

I wouldn't use a spare PC, real waste of energy to run a PC 24/7 just to monitor a temperature sensor.

I can think of two approaches.

First one - buy a ready-made device. Google "differential thermostat" and you'll find products designed to close a relay if the difference in two sensed temperatures exceeds a user set level. These seem to be used in solar water heater systems.

Second one - build that device. Here is a DIY page
http://www.solaffect.com/

Thanks, I'll check that out. I'm actually going to be using the spare PC to act as the indoor unit/server for a home weather system. So the PC will be running and will already have the sensors logging to it. The only thing would be to do the programming which could probably be done with an alarm of some sort (if x>y+4 then alarm).

But, I'll also look into the thermostat that you mention. A reasonably priced commercial alternative would, I'm sure, be better than whatever I'd manage.

Look up Lion controls. They have a host of Temp transmitter/differential. They will also have your T/C's. They have numerous outputs from 4-20 to relay. You would only configure where to send the output from the transmitter. Make sure that you keep the run of TC cable as short as possible.

I know this isn't what you are asking for but this way could work as well.
If your furnace has a direct drive motor why not wire the motor so you have low speed always circulating air throughout the house? High speed would kick in when the a/c comes on. You would need a relay to so 2 speeds of the motor aren't powered at once.

Lots of interesting ideas guys. Thanks.

What about merging this extra bit with the existing bit without burning up the HVAC stuff or the house, diodes, etc...?

Look in the Grainger catalog. They might have a direct fan control relay and thermostat setup.

Sherwood

You might also just close the registers in the downstairs area so that all of the cool air is directed upstairs. The downstairs half should remain cool enough. Adjust the exisiting thermostat accordingly and you're done. I have a two story house in Tulsa with seperate systems for each floor and, while it doesn't get quite as hot here as it does in Houston, we never run both systems in the summer or winter. In the summer we run the upstairs unit and in winter we run the downstairs unit. Works fine.

You can also adjust the registers on both floors to direct whatever percentage of air you want to either floor, or to specific rooms. This assumes you have adjustable dampers in each register...a little fiddling and you should be able to get an even temperature distribution.

JR

The problem is that the AC doesn't run at all because the downstairs (where the thermostat is) doesn't get warm enough to trigger the thing. Otherwise, yes, during the winter I pump most of the warm air to the downstairs, and during the summer, I pump the cold air to the upstairs.

Add a second thermostat upstairs....

JR

Just move the Thermostat. Typically it is a 4 wire setup. Run a new cable through the ceiling. It is all low voltage wiring (24 VDC). You then adjust the registers downstairs so that you get enough cooling without freezing everyone.

Just note the color code of the wires, and the marked terminals. The terminals are marked the same on the thermostat and the HVAC control board in the furnace & A/C.

Post about your weather station when you can. That really interests me too. I'd like to get something like a Davis VantagePro someday, but if you blaze the trail I would consider a DIY setup too.

The right way to do it is zone dampers, and add a second thermostat with controls. Depends how accessible the ductwork is. But frankly, I would just move the thermostat, and balance the system with dampers.

I've seen houses where the thermostat was just in a terrible location. Sunlight, near doors, even on an outside wall.

this may be why most 2 stories have 2 units.
a better location for the T stat.

have you checked the insulation in the attic or added any? you may even look into the spray foam stuff, i have it in my music room. they can spray it on the rafters of the roof and it will drop the attic temp a lot in the summer. the heat from the attic will radiate into the house late into the evening.

i would think you could parallel 2 controls, one up, one down as long as the 24v power source is the same.

I think this is the best idea yet. Thermostats up and downstairs. I think this is doable, BUT I think it would require some diodes someplace, but I'm not sure where.

That's the question that I'd really like an answer for.

"How can I have 2 controllers without frying some part of the system?"

I've got 10" or 12" of loose fill insulation in the attic. I like the idea of the spray foam that sticks to the bottom of the roof. That should keep some heat out of the attic which should help.

This is the second best answer and might be the one that I go with.

I'd love to get more sophisticated, and if it was a 1 story, I probably would, but then a 1 story probably wouldn't have the same problem.

Ouch, yeah, at least mine is not that bad.

I've got 10 or 12" of loose fill insulation in the attic, but a foam that sticks to the bottom of the roof sounds good. That should stop some of the heat from getting into the attic space. I'll look into that.

Thanks for all of the suggestions guys. You've provided some better options than what I'd originally been thinking.

You need to go to you furnace control board and see if it allows a second thermostat. It may. Otherwise, I'd go with a "simple" solution.

Thanks, I'll check it out. Like I said, I may just end up moving the thermo upstairs. That seems like the simplest option.

You can get thermostats that can be networked..

http://www.tekmarcontrols.com

also you should zone the house, having the whole house controlled by one thermostat is not efficient.

With some of the network thermostats, each room controls its own climate, and you can access the system remotely... turn the a/c on in the den and kitchen 20 mins before you get home, etc.