Would you modulate?

[frugal_guy]

I've seen some posts praising the comfort of modulating gas furnaces over two stage furnaces.

On the other hand, few major brands (Rheem/Ruud) or minor brands (Coleman/York) sell them.

Considering reliability, efficiency, and comfort, would you go with modulating over two stage for your home if you were an average homeowner?

I live near Portland, Oregon and have comparably priced bids for:

Ruud Achiever 90 Plus with Ruud (White-Rogers?) thermostat
Coleman Echelon 9.C with Honeywell 8000 thermostat
Trane XV90 with Honeywell 8000 thermostat
Bryant Evolution 90i with Honeywell 8000 thermostat

In addition I have another higher bid with the Carrier Infinity 96 with the Infinity Controller that includes the installation of a new basement return duct, replacement basement ceiling registers, and moving the thermostat. The other three contractors did not mention the need for these changes.

All are sized at 100K BTUs. (Two contractors did a Manual J)

All four contractors appear to be reputable and competent. (clean records with the state contractors boards and under the same management for at least 15 years)

Which system would you put in your home?

[jacob perkins]

if you are frugal, then you will let the temperature
"modulate", like 65-72-65-72-65-72....


just make sure 100btu is what you need

[frugal_guy]

if you are frugal, then you will let the temperature
"modulate", like 65-72-65-72-65-72....
Well, for many years, we've set back the temp to 65 at night, up to 70 from 6-7:30 AM, back to 67 during the day, up to 70 when my son gets home from school, etc. This is considering my wife and I are both retired and home most days. We're more likely to just get a sweater than turn up the furnace if we feel chilly during the day. Is that frugal enough? 8^)


just make sure 100btu is what you need 100K BTUs sounds about right to me. The current 16 year old 80% furnace is 154K BTUs (input) and cycles a lot. The house is a ranch with a daylight basement, totaling 4200 square feet, built in 1964. I'm currently in the process of bumping the ceiling insulation from R19 to R49. I'm confident that two of the contractors bidding on the furnace used Manual J.

[wyounger]

Did the contractors who did the Manual J know about the insulation going in?

154k input * 80% = 123k out (and this is too big, you say)


Potential choice:
100k input * 94% = 94k out.
This is 25% less output than your current system- might be a suitable correction for the current oversizing- but you're also reducing your heat loss by adding insulation. Just how much difference that will make can be easily calculated as a revision to your Manual J (assuming that the contractors saved their work).

Because of the difference in efficiency, it sounds like you're dropping a lot more capacity than you are. It's the output that matters, and high efficiency furnaces put out almost everything they take in. You may find once the insulation is done that a 100k high efficiency furnace is still more than you really need. In some lines, the next step down will be 80k; depending on the updated Manual J that might be too much of a cut. The Rheem mod goes in 15,000 BTU size steps, though, so you can get a 75k, a 90k, or a 105k.

Smaller is better for fuel efficiency and comfort, especially for the "almost always home" crowd. You can do reasonably well with some oversizing if you set back the thermostat a lot because you're gone a lot. The unit doesn't have lots of time to cycle on and off because it does a big part of its work in a couple big goes each day (before you get up and before you come home). But when you're always home, though, and keep to a more consistent temperature, you're better off with a unit with minimal excess capacity. It won't recover from setbacks very quickly, but it will keep temperatures more even and steady by running long, slow, and steady to maintain temperature.

Furnace efficiency is like your car's fuel economy rating; you only get the rated efficiency if you go on a road trip and drive a steady speed on the highway for a long time. If the furnace is rated for good efficiency but it's too big for the job, it's more like driving in a convoy by alternating between going 120 and stopping on the shoulder for your buddies to catch up. You get to the same place, but the trip isn't as smooth and it takes a lot more gas.

As for modulating, I'll point out a distinction that is often forgotten between modulating and two stage furnaces. The two stagers can run at 100% (high) and 65-70% (low). The modulating units are of course almost stepless in comparison (they actually have 13 stages). What's forgotten is that their lowest stage is only 40% of full capacity. So even a modulating unit will have to cycle on and off some in very mild weather, but those cycles will be slower and lower than what a two stage can muster. Then when your need for heat is 40-70% of full blast, they will be running steadily when a two-stage unit would still be cycling. Once you're needing 70+% of your design load, there's not as much difference; the mod keeps modulating and the two stage starts switching between high and low (which is at least done without stopping first). The other thing is that the Rheem/Ruud modulating furnaces control the heating airflow rate differently than everyone else. They use a sensor and continuously adjust the airflow rate so that the heat is as slow and hot as allowable (ie the minimum airflow required to get the heat out without overheating the furnace). Everyone else just has a factory fixed flow rate setting for each stage (with variable speed blowers) or the installer selects a blower speed at the time of install (with regular blowers). In most cases the modulating furnace will do the same job with less airflow and have hotter output.

Even with the ability to modulate down to 40%, though, it's still best to keep to the smallest possible furnace size. You want the to maximize the time the furnace can spend operating in that 40-100% range, where it can run nonstop; the rest of the time it cycles on and off about like everything else.

As for ductwork changes, are there any comfort-related issues that go along with what was proposed? Also, newer furnaces typically move more air than old ones (all else being equal), so ductwork improvements are often needed to prevent noise and restriction issues. Infinity systems measure ductwork restriction and will complain on the display about airflow problems that are often otherwise invisible to the end user (it's just hard on their new equipment). It could well be that what he's proposed is the soundest because the Infinity systems will snitch to the homeowner if the airflow isn't totally up to spec.

[frugal_guy]

Thanks for the knowledgeable and informative reply!

All of the contractors were aware of the additional insulation.

The current furnace cycles even when it's coming up from 65 to 70 first thing in the morning. It will run for a minute or two, stop, then start right back up again.

The guy who bid on the Ruud system mentioned that he did the calculations with the software but didn't like the result because he thought it was more oriented to new construction, so he redid the calculations manually. I don't know which Manual J software he was using. If I remember correctly, he said that the heating load was 65K BTUs and that he normally added 30% to ensure adequate capacity in unusually cold weather. This would mean I need about 84.5. I suspect that since he had bid 100K BTUs, he had forgotten that Ruud has, as you mentioned, 90 and 105 systems. (He clearly had years of experience and was very knowledgeable, but had only been with this company for a short time. His primary option was for a Coleman modulating system, but I thought the Ruud was a better choice). I will ask him today whether the Ruud 90K system would be sufficient for our needs.

I appreciated your description of how the Ruud selects fan speeds. I had been wondering about that.

The house is a ranch with a daylight basement. About 500 square feet of the basement is an unheated store room. (the actual heated space for the house is 3700 square feet) Though there are 3 return registers upstairs, there are no returns downstairs. The Carrier contractor thought we needed to add a downstairs return to improve comfort. He didn't mention whether it was required to resolve an airflow restriction.

I had noticed the added diagnostics available with the Infinity system and certainly consider that added value. The Ruud diagnostics seem to be limited to an LED display on the controller board. Do you know if it would notice a flow restriction and report it on the LED?

Thanks!

[beenthere]

The Coleman can be set for a temp rise of either 55, or 65°.

It comes in both VS, and multi speed blower versions.

The control board has a memory for the last 5 errors, for dianostic aid.

Which is better, the one that is installed by a contractor that has more then your wallet on his mind.

[wyounger]

Following the design specs to the letter means selecting the next-size-up furnace; in other words no more excess capacity than what comes from one step to the next, with no fudge factor. So with a 65k heat loss, technically you should select the 75k size (72k output) in the modulating furnace. An 80k furnace in the other lines would be good too. I don't think 90k would be criminal either. Sticking with the letter of the sizing guidelines leaves you with such little reserve that you can't really recover from a setback in the worst weather of the year; if you're attached to the setback for comfort, you might actually be happier with a slightly bigger unit.

I'd try to avoid avoid the 100k units if you can get a little closer to your actual demand. For the brands that go 60-80-100-120, the 80s would be better in my book. Rheem/Ruud's 75k is cutting it a little close, but their size steps are smaller than usual, and the 90k is still a reasonable match.

If you really want to cut it close (like the 75k), and you are willing to be patient with slow recovery from setbacks or are willing to use a smaller setback when you know it's going to be super-cold, you'll do fine, and get the best operation in more typical weather. But you'll have a hard time selling a contractor on the idea. If you think about their incentives, it makes sense. If they sell you a unit that's a little oversized, it's not quite as comfortable (most people don't know the difference) and not quite as efficient (but it's still always better than their old clunker was, so they don't know what they missed). But if they go the slightest bit too small, they have irate customers calling about being cold when an Arctic blast blows through.

Adding a return in the basement will help keep the basement warmer in the winter. It is also a must if you have a door separating the basement from the other floors- the air you send down there needs a way to return to the furnace, either through a duct or through a free opening. Otherwise you end up sucking air through dirty places like walls and floors, which picks up fiberglass and dust and other such yuck.

I have an Infinity furnace and control myself. They have good self-diagnostics, somewhat more than others. A big part of the difference is the diagnostics are accessible from the "thermostat" part, rather than just from the furnace circuit board. They are the only ones that can also communicate the same kind of data with outdoor units (AC, heat pumps) too- but only if you buy the matching high-end piece for outside. You can use mid-range outdoor units with an Infinity setup, too, but then you have to fall back to the old-style connection between inside and out (no diagnostics data). The other thing is that they have better cooling season dehumidification control than you can get with other equipment. If that's important to you, Infinity/Evolution equipment is great. If you're more concerned about the best-possible comfort in heating season, the modulating furnace is still the best (but only if you use the special modulating thermostat).

[docholiday]

Originally posted by frugal_guy
I had noticed the added diagnostics available with the Infinity system and certainly consider that added value. The Ruud diagnostics seem to be limited to an LED display on the controller board. Do you know if it would notice a flow restriction and report it on the LED?

Thanks!

Yes, it will give you a fault code 66. Troubleshooting a furnace should be done at the furnace, not the thermostat. I mean it's good in theory, but the real deal is God has blessed a good tech with 5 senses and he should be using 4 of them (we try to leave taste out) and you cant use those senses in another part of the home.