The air conditioner started leaking water all over the place a few days ago, and not interested in being without AC in the 100 degree heat we’ve been experiencing, I promptly called in the pros to take a look at it. While it cost me $80 for some strategically placed tar tape, I did get to ask the tech a few questions. His immediate reaction when he saw all of the servos was “You should sell this to your neighbors, their house is burning up!” to which I replied “oh hell no, I lose sleep over just one of these things”. I asked him about how much pressure I should expect to measure in the main air plenum (didn’t know) and then got the sign off on putting a return vent in the basement. I was able to install that this evening and while it’s too early to call, I’ll be watching the graphs real closely tomorrow. Next step, controlling the thermostat!
6 Responses
Leave a Comment
June 26th, 2009 at 4:08 am
Always nice to get a little validation. Congrats! It looks like the basement trace started getting choppy this morning; any idea what’s causing that?
You might consider entering Digi’s design contest (digi.com/designcontest). If nothing else, the starter kits you’re required to use appear to be a *smoking* deal; if I’m looking at the right bits, for $150 you’re getting close to a grand worth of hardware. You’d be able to place sensors wirelessly, and perhaps set up a wireless thermostat controller. I think I’m going to enter, although my power monitoring project is far from original…
June 26th, 2009 at 5:33 am
Nice! ya know, I think I just might!
I have a theory on the saw toothing of the graph, but no hard data yet. It seems to be spot on with when the AC cycles. If you notice, those are spikes in temperature and not drops as I would expect. I believe that is because I have the sensors in the return vent, so when the blower motor goes to stage 2 and starts really moving air, it sucks in all of the warm air in the room first while the cooler air is still displacing it. I’m working on my amp clamp this weekend and integrating the pressure sensor into the graphs, so we’ll know for certain pretty soon.
June 26th, 2009 at 7:24 am
Not sure if this is applicable to your system, but I am also sensing zone activity on my furnace (it doesn’t appear in my graphs right now, because the furnace has been off for a couple of months). I used an MID400 AC optocoupler to sense when the thermostats are calling for heat. For me, that makes the connection between “what is happening” and “what is the system asking for”. In your case, you could see when the thermostat is energizing the compressor and fan (which are presumably separate functions in the thermostat). It’ll give you more information when it comes time to overriding the thermostat, too.
June 26th, 2009 at 7:26 am
I think I’m just going to start all of your replies with “That’s a good idea!” Right now I’m using the pressure sensor in the main plenum to tell me if we’re off, on stage 1 blower or on stage 2. I need to interface with the thermostat soon though, so might as well bite the bullet and start that direction.
June 29th, 2009 at 9:05 am
All of the pressures are documented on a plate on your HVAC unit. As well you want to watch for pressure drop between the plenum and return side.
Do yourself a favor and install a 4″ HVAC filter, if you have a 1″ filter it’s the suck. Especially if you bought a 3m sharply pleated filter. Only the rounded 3m filters have decent flow in the 1″ series.
If your going to add a return in your basement make sure you don’t create a negative pressure zone down there. Pretty easy to check if you seal up the basement (i.e. close windows) with the furnace fan on high see if the basement door sucks itself shut. i.e. close it very slowly and see if there’s any pull. Vacuum in the basement == bad. Suck radon in the house or backdraft appliances down there.
Most HVAC systems cheat by adding too much return in the basement and or have seriously leaky return vents since most HVAC systems are installed like crap and use the floor joists as returns.
June 29th, 2009 at 9:11 am
Ah, wise words from a man that I know know’s his stuff. Thanks David! It’s funny you mention that because I was just starting on the negative pressure test this morning and believe that I may have indeed caused a bit of a low zone in the basement. Not a huge one mind you, but the radon scare is what prompted me to start testing. It doesn’t look like it’s given me much benefit having the return in the basement anyhow, so I think I’m just going to close up those vents and focus on insulating the duct work. From the graphs, it looks like I’m losing a fair amount of cooling to the basement simply by having them exposed. We’ll see.
For those of you interested, David is a wizard at just about everything and has turbo charged his home HVAC (you should see it, it looks like it was built by Honda). One of those guys that when he offers advice, you take it.