# estimating static pressure by furnace power draw

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• 06-05-2013, 11:24 AM
greenjp
I did not consider that, thanks for pointing it out. Here's the order of the test. The AC hadn't been running until I started the testing:
In one session I did:
Fan On @ 400 CFM/ton
80% of 400 CFM/ton
400 CFM/ton
This took ~20 minutes total. The time to note the data and swap the filters was just a couple minutes. Then I shut everything off, reset the switches, turned it back on, waited the 5 minute tstat delay, and got back to it.
80% of 350 CFM/ton
350 CFM/ton
15 minutes for this step.

I did the Fan On @ 350 CFM/ton at a later date.

So that effect perhaps resulted in greater static pressure as I went through the steps, but it does appear that the effects of CFM and filter type dominate. That effect would also support the ESP-mitigation steps I outlined.
• 06-09-2013, 10:30 PM
CraziFuzzy
What you are observing is effectively a 'system curve'. In any fan/pump system, there will be a fan curve, and a system curve, and where those two curves intersect will be the current operating point. The system curve is, in effect, a function of the flow resistance (pressure vs. flow). The fan curve is, essentially, the performance of the fan (also pressure vs. flow). The variable speed fan basically shrinks the fan curve along the flow axis, as speed decreases, causing the fixed system curve to intersect the variable fan curve at a different point.

Now, fan efficiency is a different thing altogether, there is a point on the fan curve, where it will be most efficient - and it is best to try to get the system curve to match as close to that point as possible - but in practice, the lower the overall resistance you can have, the better off you'll be, efficiency wise - meaning larger & shorter ducts, larger filters, larger coils, etc.
• 06-10-2013, 10:50 AM
motoguy128
I think something worth noting, is that your chart does a great job of showing what can be a possible impact of oversizing. If you have a 3 ton systme it might use 380Watts, while a 2 ton uses <1/2 the power at 160 Watts. So even if it runs 50% longer, it consumes dramatically less energy on the same ductwork. Meaning a properly sized system compared ot a oversized system, could operate at effectively 1-2 SEER higher without even factoring in longer run times and lower humidity levels allowing higher setpoints.
• 06-10-2013, 10:55 AM
motoguy128
Quote:

Originally Posted by CraziFuzzy
Now, fan efficiency is a different thing altogether, there is a point on the fan curve, where it will be most efficient - and it is best to try to get the system curve to match as close to that point as possible - but in practice, the lower the overall resistance you can have, the better off you'll be, efficiency wise - meaning larger & shorter ducts, larger filters, larger coils, etc.

Maybe the OP could add another axis on that chart for "watts/CFM". I think since it's a reverse inclined fan, it shold be pretty flat at lower RPM's and increase at hgiher RPM's sicne the return size begins to act as a restricter.
• 06-10-2013, 03:43 PM
greenjp
Thanks for the feedback guys. Crazifuzzy you are correct of course about this essentially being a study of my home's system curve and the air handler's fan curve. I'm more familiar with those for water pumps and ship's cooling systems but a lot of it is analogous.

motoguy I'll add that data or generate a new plot for watts vs. CFM. Good point that a smaller unit would require much less energy for the fan in addition to all the other benefits.

I do wish, based on what I've learned from the folks on this site, my observations over the last year with this new system (it was installed a year ago after the original system bit the dust), and this test data that I could send an email back in time to convince myself to insist, against 6 different contractors' recommendations (no load calcs, natch), to downsize to a 60k furnance and at least a 3 ton AC vs the 80k/3.5 straight replacement we went with.

I have some thoughts about adding return capacity (supply would be very difficult and expensive to address) for both reducing ESP as well as addressing some comfort issues that I'll make another thread about after I have a chance to take some photos and measurements.

jeff
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