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Thread: New HVAC system observations

  1. #1
    DIY Senior Member Runs with bison's Avatar
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    Aug 2009

    Post New HVAC system observations

    Background: I had my HVAC unit replaced this past December to take advantage of the tax credit, utility credits, and because I knew the AC was on its way out and the furnace was showing signs of needing some minor maintenance. I've been in this house for 3 years, but it was built in 1994. In 1995 they apparently replaced the outside AC compressor/condenser, I discovered this checking serial numbers--my guess is that the old one underperformed, so they replaced and maxed out at 4 tons with the ductwork/air handler. Judging by their summer electrical bills they were running the hell out of it (about twice as much as us in comparable weather.) My primary goal was improved comfort, with a close second being some efficiency improvement.

    Sizing: I sized the replacement empirically...no Manual J calcs since I had data in hand. I determined max furnace duty based on daily gas use (minus WH) vs. average daily temps. Made a nice linear plot with an exceptionally good fit over an entire season. A 50,000 Btu/hr furnace would handle coldest anticipated temps easily. AC was done based on the original unit being 4 ton...and having trouble keeping up on back-to-back 100+ F days when we first moved in and before I did some house-tightening and insulating. Even after that and some service work it was hard pressed on the hottest days. Didn't want to risk going down to 3 or 3.5 ton with the 4 ton having trouble in the past...although I suspect I really only need 3 tons (next smaller air handler/blower size.)

    I got three sets of quotes for 95% eff. two stage furnaces with ECM blowers and single or two stage AC compressors. I eventually went with a Ruud system using the dual stage AC--$1,000 rebate on it made it almost the same price as the single stage. The Ruud installer did a better job of figuring out the new venting runs for the condensing furnace than the other guys did, plus he was considerably cheaper and he's done good work for me keeping the old unit on life support inexpensively.

    The real size of the furnace was set by the AC air handler requirements. (Has to do with our home's configuration and the climate--results would be reversed in a lower profile home or farther north.) This became apparent with all three quotes. They weren't fudging, it was the pairing of the air handler/blower/furnace heat exchanger with the 4 ton AC coil that determined minimum furnace size. So I ended up with a two stage 90k Btu/hr input furnace (vs. 110 k input single stage originally in the house.) It always runs in its low fire mode--other than recovery when it has been off or temp set down for an extended period.

    When they tried to run my AC old unit to pump out the refrigerant (on an unseasonably warm early December day) it didn't want to run and I could hear the contacts clicking. They got it to run briefly, then it spit out some orange glop and siezed...apparently I got every bit of life that was left in it.

    So far I'm generally happy with the result of the new system, but I've noticed some real problems with the industry.

    What have I learned so far?
    1. New AC compressors have heaters that should be shut off in winter unless it is a heat pump--by flipping the breaker on the AC. The case heaters are there to prevent liquid accumulation in the compressor that is hard on it during start up-- this is really intended for heat pumps...not for AC-only applications like mine. I measured the draw as 40W per compressor continuous. In my dual stage AC this was 2x40=80W 24/7 in winter--nearly 60 KW/hr per month (about $8/month at current rates here.) In summer this isn't much of an issue as the heater uses a timer and only runs when the shutdown exceeds 2 hours. "Smarter" control boards are needed for these...as they defeat the energy savings of higher SEER AC units. Various folks have told me that the heaters have limited lifetimes anyway...so shutting them off all winter should double or triple their effective lives.
    2. ECM blowers in existing (as in undersized) ductwork probably won't gain any efficiency. ECM blowers will save money/electricity when the blower is "loafing" at low load. Old lower SEER systems demanded less blower air. I've taken run data at the meter on the original and new blower, I'm not saving electricity there because both are/were working hard. If one has properly sized ductwork, the ECM blower should reduce electrical load. Undersized main trunks = noisier operation than properly sized trunks.
    3. Standard air handler filter ports/boots are vastly undersized for the flows needed They seem to be designed for dual inlet (two filters)...but this is rarely possible in existing homes. As it is with single inlet they produce far more DP than they should, and therefore far higher load on the ECM blower than should be required. I know because I built a manometer to measure DP with various filters, and read the electrical load for each case. I've honed in on a specific filter that works best with my system. Oversize the hell out of the filter box if possible. My old system used a 20x25" filter in the air handler opening for a blower that seemed to require about 1600 cfm for 4 tons AC. The new one is only 16X25" with 2000 cfm demand for 4 tons AC. 20x25 is no longer a standard boot dimension, so I'm measuring a serious DP hit (and it makes the unit louder than it would otherwise be.)
    4. My old 81% nominal furnace was doing better than rated. It was operating in a partial condensing mode because the flue ran through a long central, vertical return chase. I was seeing some evidence of this in the flue pipe corrosion. So the overall measured gas reduction vs. temperature for the new unit is about half of what was predicted for 95%.
    5. Low stage heat is nice in winter. In my case it is only about 33% lower than the output of the original, but that means it runs 50% longer each cycle...noticeably smoothing the temp profile of the house (yes, I've measured it.) I'm not sure that it even kicked into 2nd stage when we hit -14 F this winter, the coldest we've had in recent years.
    6. I'm liking the dual stage AC so far. Since my climate is humid during the summer, but with usually moderate outdoor temps, I've reduced the AC blower setpoints (board jumper switches) by 10% to improve moisture removal...which is what I really need rather than absolute temp reduction. This will cause some efficiency hit at higher loads, but the old unit was running sub-10 SEER and this one is rated 16.5 to 18.0 SEER nominal, so I doubt I'll notice the slight loss.
    7. The low stage cooling is very quiet outside. My neighboor said he's glad the old noisy unit is gone--went on to describe the sort of noises it made.
    8. Installer did a good job reconfiguring humidifier properly (builder's local contractor's install was a bastardized contraption that didn't work well and entertained my bidders.) It can actually control humidity levels in deepest winter now--I had to turn it down as it was performing too well relative to my old settings. I've argued here before about how humidifier configuration will make a difference. This experience leaves no doubt for me. Original install had humidistat in wrong location, and the box inlet was making use of only about half of the humidifier screen.
    9. Having the blinking CFM approximation visible on the board for the blower call is handy for figuring out what the system is doing. It isn't the actual flow, but is a measurement of the blower speed call. With a dozen dip switches on the control board that impact this in conflicting ways, and various thermostat options that can also change it, this makes it far easier to configure the system properly and verify you got it right. (The manual has a number of errors in it referring to the wrong tables, etc....took some experimenting to figure out what it meant vs. what it said.)
    10. A heat pump with gas back up might have worked for winter. However, it seemed a poor fit in this home for several reasons: a. Lower delta T meant high blower requirements and problems at the corners of the home--comfort would likely have been poorer as other heat pump owners warned me. b. Probably wouldn't be the most efficient during mid-winter high demand when reduced COP would make gas backup competitive. c. Our local electric rates are on a steady upward arc (30% more than when we moved here), while nat. gas remains cheap (20% less than when we moved.) d. Since it would run at least twice as many hours each year, it probably would last only half to 2/3 as long.

    Now to see how it does if we get some 105 degree days this summer.

  2. #2
    DIY Senior Member Runs with bison's Avatar
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    Aug 2009


    Well, I'm still collecting data, but we've had our hot weather already. Unfortunately, I wasn't home when the local temp recording station hit 108.5 F (multi-decade record) to do a full evaluation. No...I was camping in 112 degree heat...not heat index either. But the wife was home and the AC kept up in stage 2. From what I've seen 4 tons is still overkill for this home, 3 tons might have done the trick although it would be close at 108.5. (3.5 tons would still have been the same frame size as 4 ton so nothing gained there.) When I was home it seemed to keep up in peak temps well over 100 F running in Stage 2 off and on for a few hours. Stage 2 would start to kick in late afternoon when the mercury crossed about 95 F. Balance point in summer is about 65-68 F...so 3 tons might just be adequate at 108.5 to hold my 79 F set point.

    Before the hottest part of summer hit I cut back the blower to 400 cfm/ton from the 500 cfm/ton factory shipped default in 2nd stage. This improved dehumidification by a few percent, and it reduced the blower load (my vents are marginal at best at the higher flow.) Overall, it seems to run more energy efficiently and with greater comfort at this lower blower setting so I'm sticking with it.

    The unit uses about 36% less electricity than the original when corrected for temp delta...even though the EER rating should be reduced somewhat in our several degree hotter weather compared to previous years. Might be better than that now that I've got it fine tuned. My back of the envelope SEER rating is around 16. It's been a pretty noticeable energy reduction even compared to last July which was hotter than normal but still over 4 F cooler than this July.
    Last edited by Runs with bison; 08-17-2011 at 11:24 PM.

  3. #3
    DIY Junior Member tempritespokane's Avatar
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    May 2011


    Thanks for the observations very insightful.

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