Furnace Amp Draw vs Blower Consumption

[skunkiechris]

How much power does a typical furnace draw beyond what the blower requires? I'd assume not much...

Here's the situation: As many know I recently had dual 80k XV95's and 3t XL16i HP's installed in my house. (I know, I still have to post pics. This weekend!) On the first floor, I replaced an 80% furnace and an XR12 AC (that is being re purposed.) After receiving the first new electric bill covering the first floor only, it's approx 60% higher than the same period last year. I checked weather data, and we had 156 cooling degree days both years. There have been no significant changes to my consumption patterns other than this equipment change. (Actually, I used to have a server that lived on 24/7 that is no longer in service...and I was out of town for 7 days this year...so if anything usage should have decreased!)

I've been attempting to narrow down where the power drain is coming from by checking electricity usage of everything I can think of, using either a Kill-A-Watt type meter for plug devices, or a clamp meter for hard wired. The XL16i came in pretty close to its performance data, around 6-7 amps on stage 1, 10-11 amps on stage 2.

The only surprise I had was the furnace, which appears to be drawing around 5-6 amps itself. I know there's a power factor which I don't know, but that seems awfully high to me. There is a CleanEffects in there, and it's difficult for me to find exact power draw for those, but it seems to be pretty low.

So back to my question: Should I assume that number is almost entirely blower consumption, which would imply very high static pressure (based on the blower performance data) or some other anomaly, or is it normal for the furnace to use a few amps in addition to the blower?

This is not a DIY. Believe me, for what I spent I have ZERO intention on messing with anything in the system lol. I just don't want to point fingers at the HVAC without some evidence that there is an issue there (even if doesn't explain the whole increase.) The contractor also redesigned/replaced the first floor duct system.

Thanks!

[beenthere]

The furnace transformer uses very little elctric.
Around .3 amp.
So your blower is whats using most of that amp draw.
Undersized ducts can cause that with VS blowers.

[a0128958]

Here's an example of current draw for a 1/2 hp variable-speed fan motor as a function of static pressure, where air flow is constant at 1000 CFM:

IWC Amps
0.12 0.32
0.18 0.36
0.20 0.39
0.27 0.48
0.46 0.82
0.55 0.93
0.74 1.13

At a constant air flow, current draw varies as a function of SP.

Best regards,

Bill

[beenthere]

Ok.

So whats the amp draw at 1.1" blower set to 1000 CFM, but only able to deliver 900 CFM.

[skunkiechris]

The furnace transformer uses very little elctric.
Around .3 amp.
So your blower is whats using most of that amp draw.
Undersized ducts can cause that with VS blowers.

Nuts. That's what I was assuming, but grasping on a little straw of hope that I was wrong. The performance data tops out at .9 for static pressure, with 3 ton unit set at 400 CFM/TON, drawing 420w. Taking a wild guess at a power factor, seems like mine might be pulling ~500

I'm thinking the return is where my issue lies. All of the returns (7) feed into two fairly large trunks (don't have measurements, at work,) but those feed into an 8x24 drop to the furnace. The elbow at the furnace does have turning vanes (at least I have one thing going for me, lol.) The supplies are 9 6" runs off of two 8x16" trunks.

I guess it's a Monday call to the installer.

[beenthere]

9, 6" supplies for 1250 CFM is also under sized.
That would average out to about 139CFM per supply. And 6" is too small for that CFM. That would come out to a FR of .17"(not to be confused with static pressure)

Your installer should take individual static readings of both the supply and return.

[skunkiechris]

9, 6" supplies for 1250 CFM is also under sized.
That would average out to about 139CFM per supply. And 6" is too small for that CFM. That would come out to a FR of .17"(not to be confused with static pressure)

Your installer should take individual static readings of both the supply and return.

Thanks beenthere. I actually questioned the size of the supply lines before they were installed and was assured they knew what they were doing, and they were the correct size to ensure good air mixing in the rooms. I looked at the guy skeptically, but figured "Eh, what do I know, really" and obviously conceded...

Wouldn't it be 1200CFM if it's 400CFM/TON and a 3ton unit? I'm just curious as to whether that was a math error or if there is another factor in there I'm not aware of!

I have no concrete reason yet to think they won't be responsive to the issue, but the fact that it happened at all leaves me very uneasy....

[a0128958]

So whats the amp draw at 1.1" blower set to 1000 CFM, but only able to deliver 900 CFM.

Don't know. For the example above, I believe the fan 'curve' is constant only through 0.50". I thought the example would still provide good illustration, without actually including a few footnotes, to keep it simple.

Best regards,

Bill

[beenthere]

Don't know. For the example above, I believe the fan 'curve' is constant only through 0.50". I thought the example would still provide good illustration, without actually including a few footnotes, to keep it simple.

Best regards,

Bill

The example is ok. If the blower is delivering set CFM.
But misleading for VS blowers that are working against static beyond what they can deliver set CFM

[a0128958]

9, 6" supplies for 1250 CFM is also under sized. That would average out to about 139CFM per supply. And 6" is too small for that CFM.

But wouldn't 6" be fine if it's round galvanized pipe versus flex? I.e., 175 cfm for 6" pipe (at 900 fpm)?

Best regards,

Bill

[beenthere]

Wouldn't it be 1200CFM if it's 400CFM/TON and a 3ton unit? I'm just curious as to whether that was a math error or if there is another factor in there I'm not aware of!

LOL.. Don't ask me why I used 1250. I think I was thinking of 350 per ton, and just added the 50 to 1200.

But that only drops it to an average of 133 per supply. Which is still about .145" FR.

As far as good air mix. They didn't mislead you, its probably blowing out fast at most if not all registers. Which does give a good air mix.
Just it can be loud, and use more electric then need be.

WAG on my part. Your supply is exceeding .7" static.

[beenthere]

But wouldn't 6" be fine if it's round galvanized pipe versus flex? I.e., 175 cfm for 6" pipe (at 900 fpm)?

Best regards,

Bill

I've been basing the FR on metal pipe. If he has flex its even higher yet.

175 CFM in 6" metal, would be .22" FR. way too high, and too loud for resi.

[a0128958]

I've been basing the FR on metal pipe. If he has flex its even higher yet.

175 CFM in 6" metal, would be .22" FR. way too high, and too loud for resi.

Thank you. Looks like max CFM for 6" metal should be about 118, assuming a desired fpm no higher than 600. This would be equivalent to a FR of about 0.12, I believe.

Best regards,

Bill

[skunkiechris]

I've been basing the FR on metal pipe. If he has flex its even higher yet.

No flex, all metal! Pretty straightforward, one long main trunk with the furnace in the center, 4 runs on one side, 5 on the other, all running off the trunk sides straight out and up to the floor vents.

175 CFM in 6" metal, would be .22" FR. way too high, and too loud for resi.

What is FR? (Just curious.) As for how loud it is, well, there are two vents that are almost immediately above the furnace -- < 10 feet. They do make some noise. The remainder of them are basically silent -- so I naturally assumed the two loud ones are just a result of being so close to the furnace.

As far as good air mix. They didn't mislead you, its probably blowing out fast at most if not all registers. Which does give a good air mix.
Just it can be loud, and use more electric then need be.

Well, sort of. The 6" runs lead to floor vents that are very large -- smaller ones are 8x10, larger are 12x14. Immediately after the install, I had terrible stratification, somewhere around 5 - 8 degree difference between floor level and the 6 foot mark. (All ceilings are 9 feet.) Feet were freezing, head was warm lol. This is a 100 year old house, and the floor vents are basically built in and "framed" by the hardwood floor, so reducing the size is not an option. We blocked off roughly half the surface area of them from the underside, and it was a huge improvement, there is now about a 2 degree difference. (Is that reasonable?) They are the decorative type old Victorian grilles, so they really do nothing for the throw either. Oh -- and all returns are also at floor level, in the baseboards.

If they were to increase the size of some of the 6" runs, thereby reducing velocity, would I likely be getting back into stratification problems? Or should they be able to overcome that? I can live with the sound, if I'm going to be more comfortable. It's still WAY quieter than my old unit!!

Thanks again for all the help!!!

[beenthere]

Although larger supply runs will lower velocity. It will allow more volume(CFM) to the registers. And with more volume t the registers, the throw will increase.
You should also have balancing dampers in the supplies

6" metal pipe is good for around 100CFM. (varies alittle by length, ells, etc)

Good chance that many of those supplies should have been 7", and possibly 8" on some of them.

[skunkiechris]

A
You should also have balancing dampers in the supplies

6" metal pipe is good for around 100CFM. (varies alittle by length, ells, etc)

Good chance that many of those supplies should have been 7", and possibly 8" on some of them.

They did install dampers on every run as it leaves the trunk, so there's at least one point. :-)

The registers all serve almost the exact same amount of space. There is a central wall that runs down the length of the house and divides each floor (~1800sf) in half. Each side is divided into four 12x15' "areas," with a register in each of those areas. (The living room comprises 3 of these "areas," etc.) The ninth run is the front entry hall. Every non-bathroom/kitchen has a return in it as well.

In my initial skepticism of using the 6"ers, I did purchase HVAC-Calc and painstakingly enter every room, window, and wall in my house. When I used the "Size Ducts" option, and left speed of air at "900 Recommended" it gave a mix of 5" and 6" sizes. Does that make sense? Do you consider 900 FPM too fast/noisy?

Most of my pre-install conversations with my installer revolved around equipment -- they were the only ones in the area who didn't seem intent of stopping me from getting heat pumps. They flat out told me when they didn't know the answer to some of my questions, but said they'd talk to their Trane distributor and get back to me -- and always did. They successfully hooked up the XLi's and IAQ's correctly on their first attempt, which was one of my worries as they had never installed those combos before. And the whole thing is very neat, ducts are meticulously sealed, etc, overall a great job (in my opinion.) It seems as though perhaps planning the duct work was not their strongest suit, although they did specifically mention "balancing airflow and ensuring proper static pressure," even if they didn't do it!

Too bad you weren't designing my ducts. If you lived just a bit further North...and West...

[beenthere]

It the time hasn't run out on it.
Use 600FPM, and see what size it says.

Even though those rooms are teh same size. The calc should have had different CFMs for most of the rooms due to orientation, number of outside walls, etc.

[skunkiechris]

Yep, changing to 600 fpm gives me a mix of 8", 7" and 6" actually. Is 600 fpm what you would normally design to?

The room do have varying heat loads showing, but they're not too far off from one another. All have similar exterior wall space, and all four sides of the house on the 1st floor are completely shaded (porches & other houses.)

I'm home now, and of course now that you've brought it up, the noise is bothering me lol.

[beenthere]

I use 500 and 600 mostly. Depending on the type of install. What I need the air to do after it leaves the register/grille.

I very seldom use 8". Once in awhile though.

2-6", or a 6", and 7" in most cases, instead of an 8". Again, varies with the room.

[skunkiechris]

Beenthere -- your advice is much appreciated. I'll update this thread next week when I've had the installer back to take a look!