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.

How do you control the locations? There is only one A/C system and it is either on or off. I guess he could install variable remote controlled registers as well. Or He could install a second A/C unit, which might be cheaper...

In hydronic systems there are zone valves which control where the heat is going. You could have a heat zone for each room in the house, which would require a thermostat in each room..

Most systems here are broken down into two or three zones..

I think forced air systems, which is what Mr. Masraum is asking about can be zoned in a similar fashion.. solenoid controlled dampers would need to be installed etc..

Retrofitting something like this may not be cost effective...

If your system can't accomodate two stats, go ahead and add the second one and run the wire to the unit. Pull the old stat wires off and attach the new wires. Nest xinter, swap the wires again and make the downstairs stat functional. 5 minutes, twice a year and your problem is solved. You still may need to do a little air balancing. with the registers.

JR

I would love to go with a more modern system, but I'm not going to put that much money into this house. This house has also got Aluminum wiring, blech!

It'd be really easy to put a second thermostat in even if it's the only functional thermostat in the house.

Thanks guys.

I was going to recommend adding a second HVAC thermostat upstairs too, but you said you only wanted to control the fan motor, not AC or heating, so instead I suggested adding a separate fan-controlled thermostat circuit upstairs.

I don't think adding a second thermostat will automatically switch the fan ON without also delivering cool or heated air.

Confirm this action using your primary HVAC thermostat.

Sherwood

Have you installed the kits in each outlet? Except for the risk of an electrical fire, there isn't anything wrong with Aluminium wiring...:eek:

Aluminum to individual outlets? That is where it gets the bad reputation. Aluminum is common in high-current circuits like A/C, etc, and shoul be problem free if installed properly. (conductive grease)

Depends on the age of the aluminum wire. The early wire was prone to cracking over time wherever it go 'nicked' during stripping. The later stuff was fine.

If you have had no issues so far then it likely won't ever be a problem. But all joints/connections should be coated in No-Al-Ox.

The entire house, every outlet, switch, box, everything is Al from 1967.

Because we bought the house with an FHA loan, we had to have the proper co/alr outlets put in everyplace including for GFCIs and switches. I've still had a few a couple of "sparky" incidents in the last 14 years. I've gone back through the switches and outlets (originally done by an electrician) and added noalox to all of the connections and tightened them up, and yes, I've had wire break near the end where it gets bent and unbent to wrap around connections etc....

I know that properly done Al should be fine, but in the future, I'll stick with copper since it's more robust and you don't have to worry as much that it's been "properly" done.

Ask this old house did a feature on retrofitting forced-air HVAC with zones. Clever system. Found the site by searching the this old house website. Check here:

http://www.homecomfortzones.com/mytemp.htm

the Tstat should have a switch just for the fan, then when the temp gets too hot, the compressor will kick in, this will run the fan 24/7 though

i dont think you need diodes. the Tstat is just a switch, unless you have a digital one. even then, as long as the 24vac is from the same source it sould not matter. plus, the controls are AC not DC.

the foam is pricey. i paid around $600 to insulate a 400 sq foot room. but if you are going to keep the house, it may be worth it.

Al was only allowed for a couple of years in the 1960's for house wiring. NEC doesn't allow it now.

Right, I'd like to not have the fan running 24/7. Right now that's what I do, or I turn it on when I leave for the day and then turn it off in the evenings. It'd be nice to have something that could automatically do the trick. Moving the thermo upstairs, I think would be an improvement, but even then, I couldn't set the fan to come on based on the thermostat, it'd be the AC.

I was thinking diodes would be necessary if I had 2 sets of switches/thermostats in case one was on and then the other came on, but maybe not.

Sounds like the foam would be pricey to do our roof. I'd love to vacuum all of the old crap out of the attic and put some new insulation in. Hell, I'd think that even some foam panels between the rafters would have to help.

I guess we just got lucky! :rolleyes: ;)

Another suggestion. Install a whole house fan or even a ceiling fan with it's own thermostat/timer. Energy usage might also be less than the HVAC fan.

Sherwood

I have had the same problem, solution was to put in a Honeywell Perfect Climate system. It allows two thermostats and averages the temperature, plus it offers a bunch of other options such as running the fan 20 minutes out of every hour, all the set back stuff, humidity and ventilator controls etc.

I think they still sell them at:
http://customer.honeywell.com/techlit/PDF/50-0000s/50-8151.pdf

Easy wiring....a unit goes next to your furnace, it uses the existing 4 low voltage wires to run the head unit, I think it is just two wires to the remote temperature sensor.

It has a neat outdoor temperature feature that varies the humidity based on outside air temperature so you don't get frost on the windows, plus it monitors the run time on the fan and reminds you to replace the filter when needed.

Good system...I have two...

Dennis