Understanding temperature profile

Just a quick question about this temperature profile graph:

The downward slope seems to be saying that the furnace can heat the house faster when it’s colder outside? Seems strange.

I am wondering if the fact that I have a second NG heating plant in the garage might be contaminating the data. The sensor is configured as “information only” in the ecobee, it is not used as an input to the control loop. In fact only two or three sensors shown are used for control purposes. The garage is the orange sinusoidal trace.

I replaced an 80% efficient 80 kBTU unit with a 97% 60 kBTU unit, last fall. I did a heat loss calculation that said a 60 kBTU unit should do the job, and it’s only running 7 hours per day on the coldest day so far, so I think it’s alright. I just want to get a more quantitative evaluation.

Hey!

The garage sensor doesn’t directly affect the profile; they are generated from whatever average value your ecobee uses. Only sensors in the active comfort profile get used.

Usually inverted profiles like this caused by external heat sources (fireplace, space heater) that are used more in cold weather. My initial guess was the garage heat source, but that doesn’t seem to be affecting the other temperatures in the house. Do you have any other sources of heat?

If you PM me your serial number I can take a closer look at your data to see what’s up.

Is it a two stage unit with the thermostat controlling the staging?

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The furnace is a Goodman modulating type, has about 5 stages as far as I know. The thermostat only uses one heat demand signal, but the ecobee may have been incorrectly configured for 2 stages when it was installed in October 2020.

I looked at your data and it’s really all over the place. I don’t know exactly what logic the furnace uses, but I did some reading on modulating valves and your heat profile matches the way they operate.

So to answer your original question:

Yes, that’s absolutely correct. Technically the furnace can heat the house by around 1°C / hour no matter what, but it tends to work at about half that rate when it’s warmer out.

If it’s modulating and you’re using the Ecobee to control it you kinda defeat the nice modulating features. Most units that I’ve seen that are modulating have a communicating stat that can be replaced with a conventional stat at the expense of losing the nice modulating features that you paid for.

I can’t say how your unit works exactly but it could be as simple as when a call for heat is made it starts on low fire then progresses up to maximum output the longer the heat call is on. This would mean your furnace would only make it to maximum output on a long call for heat like a recovery from a setback. Since you only have heat calls long enough to make it to high fire when it’s cold outside you get faster heating at a lower outside temp. Some are a little smarter and have more complex logic than that but they definitely perform better if you can tell them ramp up and down.

There should be a guide for setting up this sort of thing. Lots of installers just go the tried and true simple as possible method where you get weird results and don’t really get to use the features to their full potential. Ideally with a modulating setup it should run almost continuously and be adding heat energy at the same rate heat escapes. This isn’t always possible with an Ecobee but sometimes you can get close enough to the same behavior as a communicating stat.

Yes, the ecobee predated the furnace and I know I should buy a Goodman communicating stat. I love my ecobee so I’ve been putting it off.

I might get a Nest, Goodman collaborated with Google to make a new model but there are no details yet about whether it can control my furnace and take advantage of all the features.

Your goodman might have an option to use all stages and ramp between them using a two stage stat. You really need to find the manual and see what your options are. Most installers just slap it together the second you ask for something they don’t normally do which kinda sounds like what your setup is. Wiring it as a single stage is about as poor as it gets and there are likely better options they didn’t feel like implementing because it would have required reading the manual or pulling more conductors between the stat and furnace.

Modulating units usually have conventional staged controls too which would allow it to run the higher output stages when W2 is energized instead of just waiting for W1 to run for some arbitrary amount of time. Post your furnace model number and I’ll take a look.

Goodman collaborated with Google on a new Nest model but I can’t find any information on whether it performs all the “communicating” functions. If it does, I’ll regrettably be switching from ecobee to Nest.

The system is performing very well right now though, tracks the setpoint really closely.

Before dropping a couple hundred dollars on new parts you might want to check out the installation manual. On page 29-30 it goes over the different configuration options for thermostats.

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.alpinehomeair.com/related/Goodman%20GMVM97%20GCVM97%20Installation%20Instructions.pdf&ved=2ahUKEwiroNCA3PvuAhWsFVkFHa-7Bd8QFjAAegQIAhAC&usg=AOvVaw3E5vGPX6hY7eaSgoyadLEU

You’d get most of the Modulating features if you switched it to a two stage unit so it goes from an on/off switch to having a few different steps to ramp up and down to. The only thing I see that the communicating stat does different is it uses comfort profiles and can adjust the cfm to reduce noise and improve comfort. The cfm can also be adjusted on the board so you could just find which setting you like and leave it there instead.

On page 39 figure 45 you see your current heating profile. It starts at 80% ramps down and then progressively ramps up to 100% after about 20 mins. The crumby thing with this is that you’re always going to end heat calls at 100% and then have it shutdown. If you look at figure 46 you’ll see that if it is setup as a dual stage and only stage 1 is called it will modulate up to 60% and stay there so you get longer cycle times. If it’s really cold out and stage one can’t keep up it will ramp up to the second stage and run at 100% after a given amount of time and then ramp down to stage 1 if you use reverse staging.

IMHO I’d say save your money as the main feature you’d gain is the comfort settings and those are just cfm settings you can change on the control board if you find it is too noisy. On page 40 it goes into how the communicating stats behave and it doesn’t sound like they are all that different from a conventional setup. If you had a confortnet compatible AC you might get a little more functionality but seeing how you’re not using it right now I doubt it matters.

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Sorry if it wasn’t clear, I just want to know if the furnace might be slightly undersized because I did the Manual J calculation myself. The ecobee is saying the furnace only ran 7 hours on the coldest day (-20 C) but I think what we’ve learned is that the ecobee does not know when the furnace is running.

What I do know is that the setpoint is being tracked very closely, including on the coldest day, so I’m not too worried.

It doesn’t run if there’s no heat call, you just have an oversized furnace. Here’s the graph of how it ramps heat output when used as with single stage controls from the manual:

modulating graph

  1. Starts off at 80% output to warm up ductwork
  2. Ramps down to ~30% for 2 mins
  3. Increases to 57% at 1% per second for 8 mins
  4. Increases to 78% at 1% per second then waits 10 mins
  5. Ramps up to 100% only if a heat call lasts for more than 20 mins
    As you can see it doesn’t go to full 100% heating output until it has been running for more than 20 mins. The reason why you have faster heating at lower temps is because most heat calls when it is warm out are <20 min and never use the full capacity of the unit.

This is an example of a better sized unit that ramps up and down it ran for about 11hr then runs intermittently for a total runtime of ~18hrs over the course of a day at -25C. It still has lots of capacity as most of the runtime is on stage 1 which is about 60% of the capacity of stage 2.

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Your idea that anybody is going to install a furnace less than 60 kBTU in a 2150 sq ft house in Canada is amusing! It’s the smallest one Goodman makes! The manual J calculation came out to 46 765 BTU at a setpoint of 19.5 degrees. I claim it is not massively oversized and it replaced a 64 kBTU (80% of 80k) Bryant. All the contractors wanted to install another 80k unit because they just match the plate on what they see in the basement.

The heat flux is a function of the temperature differential between the inside and outside. The slope can’t be downward. It could mean something is burning that the ecobee is not aware of, or there’s a bug.

I’m sorry I can’t really make this any simpler. There is no bug or other heat source causing it to have a negative slope. That’s how your furnace functions and if you looked at the picture from the manual you’d see that when controlled by a single stage stat it tries to use some of the modulating features like ramping up output the longer a heat call is made.

Another fun way to think of this is when it’s a nice 0C outside and your stat makes a heat call the furnace will click on and starts running. It is warm up so it clicks off after two minutes. In that two minutes it ran at 80% for a few seconds then went to 37% output as per the pretty picture. So for two mins it was mostly producing 20K btu.

Now when you have a really cold day and the furnace clicks on and runs for two hours you get very different behavior. First it clicks on goes to 80%, then 37%, then 58%, then 78% and finally 100% after twenty minutes. It then runs for the remaining hour and fourty minutes at a whopping 100% so let’s say it averaged 50K BTU over the whole cycle.

Now if you look closely you’ll notice 50K is a lot bigger than 20K. This means that when it is having short cycles and satisfying a heat call in less time it is actually heating the house at a slower rate because it never reaches the twenty minute mark and really opens up.

That’s why you get a negative slope. You are heating faster when it is cold outside like graph is showing. You’re free to disagree with me but that’s what is happening

Long cycles = Reaching 100% output and staying there
Short cycles = Heat call is satisfied before reaching 100% output
Cold days = longer cycles
Warmer days = shorter cycles

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That’s not the correct, or at least not the current, manual for my furnace. I think the correct doc is below based on what the terminal block looks like. There is only one heat demand input, and the furnace adapts the gas flow according to an algorithm - no fixed schedule. (page 40) There is a warmup period, but I’d guess many furnaces do that.

Beestat should probably report the highest rise rate it sees on the graph, or the average of the five highest. At least, if the intention of the graph is to show “rise rate versus ambient.” Maybe beestat already does that and the graph will become accurate over time, because it’s not correct now.

I claim my furnace is not oversized because the lowest ambient in my area in the recent past is around -35 C. You have to size the furnace for the lowest expected temperature. Also, the setpoint we use is 19.5 C but a potential new owner would probably set it for 21 - I did not want to have problems if we sell the house during the expected 10 year lifetime of our furnace. It was not based on a guess, it was based on a detailed Manual J calculation taking into account the triple pane windows, R60 insulation in the attic, etc.

It looks like the 2019 installation manual did away with a lot of the thermostat info. Have you checked to see if you have a W2 terminal in there somewhere? I am a little surprised that it is in the same family and has different inputs but manufacturers do stuff like that all the time.

But to get this straight you have a 2019 manual where they decided to get rid of all the staging logic info because it was a waste of paper but you’re convinced that it doesn’t behave the same way as a goodman unit from 2016. On top of that you have a data collecting system that is graphing how it is behaving and you’re convinced that it is dead wrong even though it correlates with the graph from the 2016 manual.

I’m in Prince Albert Sask in a house with single pane windows, R14 walls, R20 ceiling in a 1900sqft trilevel and a 60K is running 18hr/day on the coldest day and it is still running on the 36K first stage most of that time. I don’t know how much on/off time manual J is expecting (or if it changes depending on control type) but it is a fair bit bigger than the recommendations for local guidelines here. Most guys are running 60K stuff for the bigger air handlers so they can keep up with bigger AC units. New bigger builds here with full basements and ICF foundations and spray foam walls are way more efficient and they still are using 60Ks for them even if they are only running 6hr a day when it’s -40.

Weird I haven’t seen a modulating setup without a W2 before. That’s a new one for me. I guess they’re really aiming for those rebate programs and making them pretty bare bones but still modulating.

I’d say it’s probably using the same logic as the single stage control from the graph. It might be a little different curve but you could check it for yourself.

It has bluetooth diagnostics you can watch the static pressure or blower motor rpm go up the longer it stays on to see how it increases output as it runs longer. I think that’s a pretty clear way to see how it is working. The quick and dirty formula for BTU output is:

BTU = CFM x ∆T (deg F) x 1.08

If it provides you with cfm and temp rise you can calculate the heat output if you’re really determined.