Rite Temp GPMG8085C: Informal Review

Who would've thought that this blog would be one of the most often referred to sources of information about GPMG8085C... All right, if that's what y'all want...

Ever heard of "The Golden Triangle"? "Good, Cheap, Fast - Pick Two". So, this thermostat may just as well be the closest approximation. $20 cheaper than the next guy (Honeywell RTH7500, I'll come back to it another time), does the same job, if not better.

Comparable quality and length of DIY installation.

Installation complexity: no-brainer (YMMV). A few advices, though:

• Make sure you RTFM;
• Don't drop the wires into the wall;
• Programming is best done when the thermostat is laying on the desk before you, not when you're standing next to the wall with the thermostat on it;
• At least for me, the preinstalled battery still works after a few months of operation, may want to ignore that paranoid advice to remove it.
  

Significantly bigger and nicer backlit touchscreen than the competition has (to be completely fair, they do have a big touchscreen on VisionPro, but that one is almost twice as expensive, so no cigar), and it has a nice textured button that you can grope (English experts, give me a non-ambiguous word and have my thanks in return) in the dark without being afraid to push a wrong button and reset something.

You have to really hate your customers not to provide an option like that (example: RTH7500 needs you to press any button, and, invokes the action corresponding to that button, even in the dark. So you have to be really careful where you tread, and the fact that the screen is backlit is useless - you can't comfortably use it anyway).

There's a stylus and a holder, but feel free to ignore it - fingers and/or fingernails do the job just fine. No visible scratch marks after few months of use.

7-day programmability is certainly nice, but the schedule selection is really weird - you can't set the schedule boundaries at arbitrary time, but only at one of two settings (forgot what times exactly, will update later). Kind of inconvenient, and this is probably the most annoying feature, or, rather, lack thereof. Competing device shines here - you can have your heat and cold any time.

Side note: I'm wondering why nobody figured yet that anticipation is something you're quite willing to have for a period starting, say, at 6PM, when you return from work, but not really want for the period starting, say, at 9PM - when you are on a time of day electric plan, which makes energy after 9PM three times cheaper than before. Remember, you read it here first.

If you don't need 7-day programming, there are other, cheaper Rite Temp models that offer compatible feature set, but 5+1+1 or 5+2 day programming.

I'm quite willing to forgive lack of schedule configurability for including an extremely important feature - you can manually change the hysteresis. Setting it to the maximum value makes the temperature spread quite noticeable (up to 5°F), but makes the unit cycle about three times less often than it would otherwise do. Don't have equipment to measure the actual gain, but would certainly recommend this thermostat to people who are either energy conscious and don't mind little discomfort, and to people who have grossly oversized units - you guys may suffer from temperature swings even worse, but at least your electric bill will drop like a rock because the unit won't breathe like a dog outdoors in Arizona summer anymore.

Can't say anything about humidistat - not much use for it in the middle of Arizona summer.

Another useful feature - filter usage counter. Depending on the climate, your unit may cease operating for a couple of months in mid-season, so you can afford to buy a higher quality, more expensive filter without a fear that you're wasting money (just think of how much it'll cost you to clean the inner coil when the time comes, and reconsider buying that inexpensive $3 filter).

You definitely need to read the manual provided with it in order to learn to operate - but then again, this is true about any more or less complicated home appliance - it takes significantly higher IQ to figure out how to connect the 5.1 home theater and the rest of components together without electrocuting oneself, so no big deal here.

At least all the settings have icons assigned to them, and you can figure out what is what, unlike some other designs, where features are indicated with numbers (come on, it's not 1960 anymore...)

Bottomline

Well worth the money I paid for it.

Disclaimer

God, what have we come to... Anyway - use your common sense, don't do stupid things, and read this first.

The Dual Nightmare

There is an ongoing heated discussion among homeowners and HVAC professionals about what solution to the problem of properly balancing the house is better - zoning the house, or having multiple units installed. There are all kinds of arguments on both sides.

The proponents of zoning are usually saying that it would be silly to use one switch to turn on or off all the lights in the whole house, whereas the opponents are pointing out that it is silly to pump the amount of air intended for all ducts together through one.

The opponents of zoning are usually saying that it is more reliable to have two simple HVAC units than it is to have one complicated HVAC unit and one complicated zoning system, whereas the proponents are pointing out that maybe HVAC professionals may be simply out of their depth and are trying to cover that up by silly arguments.

The opponents of zoning are saying that having two units is an adequate solution to the balancing problem and point out to the fact that existing commercial zoning systems are rarely extending "affordable" options beyond four zones, to which I say they are completely nuts and give the room (read: zone) count for three quite average houses that I've lived in: roughly 11 zones in Iowa farmhouse (2 levels, 4 bedrooms, full basement), 14 zones in one house in Arizona (3 levels, 4 bedrooms, game room, library) and 13 zones in the house I live in now (2 levels, 3 bedrooms). Oh, I do include bathrooms, too.

But I digress.

Sometimes you just have two units - for example, when you simply buy the house with them installed - it would be silly to rip them out, wouldn't it? So one just has to deal with it...

Whereas having two units is certainly nice, it is by no means a guarantee of any improvement whatsoever. Like it was pointed out before, all your luck depends on having the job done right.

Sometimes your luck is improved by the fact that the house was designed with heat exchange physics in mind (little heat transfer between floors, narrow staircases).

Sometimes it is degraded by the fact that aesthetic considerations took priority over common sense - wide open floor plans with walls being mostly glass, rooms spanning several floors vertically, and different floor count in different parts of the house.

Which is exactly the situation I have to deal with right now. The north side of the house (1 floor, 2 rooms + 2 bathrooms) is served by a nice powerful variable speed high SEER Lennox split unit. The south side (2 floors, 4 rooms + bathroom + kitchen + laundry room) is served by a puny rooftop Trane and a set of drinking straws they called ducts.

So let me skip the rant about how unthoughtful that was, and get directly to describing various interactions between two units that may happen. Let's assume for simplicity that we're talking about cooling season behavior.

Unhappy Load Meter

If both units kick in at the same time, and you are on a load controlled electrical billing plan, you're not gonna be happy. Neither is your electrical company, but that's a completely different story for a different time.

Cold Spots

It may happen that both units serve the same room[s], directly or indirectly, and the room has enough or more than enough supply and return capacity, and neither of the thermostats is in it. If both units kick in at the same time, the room will get uncomfortably cold.

Hot Spots

It may happen that the cold air supplied by unit A drags down the temperature at the thermostat controlling unit B. In this case, it will not get unhappy and cause unit B to switch on until unit A is off. Which will cause the rooms normally served by unit B to become uncomfortably hot.

The Skew

To add insult to injury, it is quite possible that the layout described above also makes unit A serve, at least partially, the zone that is supposed to be served by unit B - and in this case, unit A will keep running and running, possibly not even catching up with demand - and it will not get any help from unit B because the effect described above robbed unit B of necessary control input and is preventing it from switching on. You end up with one unit trying to do the work of two, for which it is clearly undersized. To further aggravate the situation, it is possible that the overworked unit will go beyond its normal design range, and then you have a dead unit (ask me how I know).

For more than one floor, The Skew is rather a rule than an exception.

Ask yourself a question, don't you always have to set the thermostat downstairs a couple of degrees lower than upstairs, otherwise it feels hot and stuffy? The scenario above explains it vividly. All there is is a leak of cold air from upstairs through the staircase (or a tall room) that causes the thermostat downstairs to make the unit stay off.

Catch-22

To prevent The Skew (further defined as one of the units working significantly longer than the other), you need to carefully balance the thermostats in such a way that the race never starts. This will usually mean that you will have one of the thermostat set below the comfort level, and the other above. Also, this is going to cost you, because the difference between "comfortable" and "balanced" positions of thermostats is going to make a significant contribution to your electric bill. Also, it robs you of comfort. Also, it will make you fiddle with controls all the time, unless you have monitoring set up, in which case you'll just spend half of the time fiddling.

It would be interesting to know what side of argument described at the beginning of the article you are now...

Shut off the A/C or leave it running?

One of the most frequently asked questions is "Should I shut off my A/C when leaving the house, or leave it running?"

You see, it's not that simple...

This is one of those classical cans of worms that a lot of people (HVAC professionals in particular) have an opinion on, but just try to ask them to quantify the problem, and the result has all the chances to be hilarious.

Suppose, you go on vacation and shut off the A/C for a month. You've just saved a lot of money, except now you have to repair the cracked deck in your sixty thousand dollars Steinway grand piano you absolutely forgot about.

Suppose, you get out to walk the dog and shut off the A/C. I will call you paranoid and probably pound foolish, penny wise (all in all, having a dog is an expense you don't really have to incur if you're so concerned about your financial well being).

Suppose, you send the kids to school and go to work at 0800. Kids get back from school at 1400, you get back at 1800. This is where things get interesting.

Pretend the house is a pool. The HVAC is the hose. The walls are the drain. The drain is constantly open. The hose is open as it is needed to keep the pool filled up.

So when you shut off the hose, the pool starts losing water. The longer the hose is shut off, the more water you have to replenish (note that this is pretty much like cooling the house - yes, there will be a point beyond which all the water is gone, and from that moment on you're purely saving money). On the other hand, the longer the hose is shut off, the less you pay.

When you come back and open the hose, the water level goes up (and the house cools down). The bigger the hose is, the faster it will happen.

In other words, the bigger your HVAC is, the more sense it makes to shut it off or adjust the setpoint when leaving the house.

But wait, this is not the end of the story yet...

The drain is nonlinear.
The hose is nonlinear.

The hotter it is, the bigger is the drain, the sooner the water is gone.
The hotter it is, the more strain is put onto the hose (this is even worse for heat pumps in heating mode, because of the balance point).
It may lead to a situation when the inflow can't keep up with the outflow.


In other words, the smaller (or worse) is your HVAC, the less sense it makes to shut it off or adjust the setpoint when leaving the house. If this is your case, bite the bullet and pay the bill. Which reminds me, by the way, that poor people pay more.

But wait, this is not the end of the story either.

The highlighted cases above are for extremes. As the weather gets milder, efficiency considerations fade because the drain narrows (not as much heat is lost or gained). For this case, the longer you are away, the more sense it makes to shut off the HVAC or adjust the setpoint.

Now, let's remember the rule: whatever you cannot measure, you cannot improve. You may spend days and nights guessing, but you will never know whether you improved the situation or made it worse until and unless you have long term trends monitoring set up.

Which brings us all the way back, right here.

PS: Inquisitive readers will notice quite a few threads dangling here. Detailed explanations will go well beyond the point of loss of interest, but those who care to find out have probably figured out where to go to get the answers.