Suggestions for Lowering Fan Speed
Just wondering if anyone has any suggestions on how to lower my furnace fan speed so it'll deliver approximately 525 - 550 CFM in cooling mode. I'd like to maximize the humidity/latent heat removal of my A/C system becuase here in Winnipeg, Manitoba, Canada the outdoor temps don't get that high but we have high humidity.
A/C - Ruud UAND018JAZ - 13 Seer - TXV - 18,000 BTU (requires 525 to 600 CFM)
Furnace - Ruud Super Quiet 80 UGPN07EAMER - current low speed approximately 700 CFM @ ESP .8 (derived from Magnehelic readings before blower and before coil and manufacturer’s blower data sheet)
Static Pressures - Duct System = .295 Filter = .225 Wet Coil = .280
I'm not concerned with lowering the ESP because I don't think the system is loud, in heating mode (medium high) it's within the temp rise specs and lowering the ESP would increase the CFM.
I would prefer to lower the cfm to 525 without using a rheostat (which appears to be common industry practice in my neck of the woods) and unfortunately replacing the blower with one for a UGPN05EAUER won't work because the CFM would be too low at 485 on low and too high at 770 CFM on medium low @ .8 ESP.
I would assume you have a PCS blower motor in your furnace at the moment ? If that is the case I don't believe you will be able to get your blower any lower than what it is already set to deliver for the pre-set speed settings that the furnace was on when it was installed. The only thing you would be able to do is to have your service guy come and move the jumpers around to the lower fan speed settings to try and give you the optimal fan speed for your circumstance. Give the guys here what you have for a make and model numbers of your equipment and someone will chime in with a thought or 2. If your equipment is 10 yr's or older I would think about upgrading to something with a VS blower and a good T-Stat controller and properly sized equipment and you should be good to go.
Was that static taken at low speed.
Was it taken with a dry or wet coil.
DanW13 - Motor is 1/2 HP PSC (4 speed) currently set on low (lowest speed) so there is no lower speed available. The equipment makes and models are posted and both the A/C and furnace were new in Sept. 2007. Seems as though my problem is a manufacturer oversight. The furnace manufacturer did not provide an optional air flow/CFM solution for those of us that require larger furnaces with smaller air conditioners (more heat and less A/C). Thanks for your reply.
beenthere - Static was taken on low speed with a wet coil. I know the static is higher on the heating speed (medium high) but after experiencing a heating season it's not a concern. It's ironic that the filter (1" merv 6 - pleated) and coil alone eat up the nameplate static (.5). Sure hope the same guys don't design aircraft.
Since your equipment is that new maybe you should get some quotes on getting the motor replaced with a ECM motor and have a new circuit bd put in to accomodate the new ECM motor will give you VS, you also may want to put some dampers in your duct runs to all the rooms, and upgrade your filter with a 4" merv 10 filter, as far as the changing out of the motor I am not sure it is possible but being that the equipment is fairly new it is a strong possibility.
If you aren't cocerned about higher static,simply increase it to the point you get the cooling cfms you want.
A restriction of a piece of metal ,slide into the supply or return plenum ,usually does the trick,remove when the furnace is operating.
Or a more restrictive filter ,might do the job.
This assumes that you have read all the footnotes on the fan data ,and applied them correctly.For example,fan data usually inclides the factory filter,this must be accounted for when using a different filter.
Not interested in a merv 10 filter, don't mind dusting when required and the cost for the filter and mod has little benefit. My 19 year old system that was replaced in Sept. 2007 had the same filter - no problems - coil and fan were clean as a whistle at time of replacement and heat exchanger only had a little rusting. My duct runs already have dampers and I can't see how they would aid in bring my CFM into spec and still provide acceptable air flow to each room for both heating and cooling. The dampers are set for heating and they performed quite well with the old A/C system and furnace combination. I would be better off dumping air into the basement from the supply before the coil (by using damper before the coil) to reduce the CFM over the coil but this is very much a hack method and not for me. I'm a Chevy type of guy, doesn't have to be too fancy to please me but it better work correctly.
If there's no magic solution, as I had suspected prior to posting, my plan is to rheostat the motor on low speed and when/if it fails due to low voltage effect have it rewound so it will provide a more acceptable RPM/CFM. I think this is a more practical and cost effective approach than converting to ECM at this time.
However, it sure would be nice to know if there is a drop in replacement motor that would meet my needs once the motor fails (after rheostating it). Too bad Ruud does not provide owners of their equipment access to their company for issues such as this. They may learn something from a consumer's point of view that may benefit their business. You have wonder about companies that insolate them selves from the consumers of their products.
Thanks for your suggestions, they are appreciated.
Is it possible for you to get another blower (not the motor but the fan part) that will give you less CFM per rpm? You could then change the heating tap to high and all should be fine.
Just a thought.
There are 2 possible solutions.
1. Your contractor can install a damper, or slide plate to increase the static on the return when in cooling mode. That you can either remove or open for the heating season.
2. Your contractor can drop the blower HP. The next size down. On low speed it will move less air then the current, and on high will move about the same as the current on med high.
dash - although I'm not concerned with the present ESP, I'm certainly not interested in increasing it. The fan data was obtained by cross referencing the ESP to CFM on manufacturer's data sheet.
It's funny that you mentioned the manufacturer's foot notes because the following is an excerpt from my furnace manual:
"Size the ducts according to acceptable industry standards and methods. The total static pressure drop (including evaporator coil if used) of the entire system should not exceed 0.5" w.c. Be sure to have adequate space for unit filter. Note: Airflow external static pressure measurements do not include filter or coil."
So, if there's a Ruud dealer or factory rep that can interpret the intent/meaning of the above statement I certainly would like to know what it is - with or without filter and coil?
Thank you very much for your suggestion dash, I must confess that I had considered it.
It means that you have to measure the static between the filter and blower, and between the furnace and coil. And add them to gether to get TESP.
They consider both of them external devices. And have made no allowances for them.
Nope the OP says number 1 that won't work!
Originally Posted by beenthere
" My duct runs already have dampers and I can't see how they would aid in bring my CFM into spec and still provide acceptable air flow to each room for both heating and cooling. "
Don't ya just love it when they know the answer better then you.
One other thing ,a 4" round bypass duct can be installed between the supply and return,with a damper to be set by a Pro.This will increase dehumidification in cooling and be closed of in heating.
You stated your static in heating was higher then cooling and you were happy with the heating static.
Check the chart on low ,what static would be requied to get the cfms desired???Then decide.