Regular Guest
Hello,
I am an AC contractor. I did a change out on an apartment complex of some air handlers (dx open case, twin squirl cage, ceiling mount, hide-a-way type.) The air handlers have electric heat strips with a r22 evap/coil w/ txv's. When winter began I got numerous complaints of no heat. The heat strips are cutting out on the high limit safety disk and the fuse-links. The manufacturer is not being very cooperative in rectifying the problem. The heating elements are the ones that look like long "U" tubes. They are over-heating from not getting much air across them coming out of the two 4" x 6" blower ports.
Has anyone else out there used these units and have had the same problem. I replaced one of the air handlers with another brand unit with the curly-cue type heating elements that are located right in the air streams of the two 4x6 blower ports and it runs just fine.
If anyone had or has this problem also, please let me know.
Professional Member*
Your talking about furdown units, if your have a high limit issue, I would think you probably have an air flow issue, either return or supply. You can check for return air flow issue by removing the access door, supply might be a little trickier, since most of those go into a ductboard chase hidden above the Sheetrock. Is the coils clean? I doubt it's anything to do with the equipment.
__________________________________________________ _______________________
“Experience is a hard teacher because she gives the test first, the lesson afterwards" ~ Vernon Law
"It's what you learn after you know it all that counts." ~ John Wooden
"When the teachers become unteachable we're all in trouble" ~ Mr. Bill
"Remember "Pro" is only a name, it's not always a mindset determined to do everything correctly" ~ Mr. Bill
Regular Guest
Thanks Mr. Bill for your reply. However, these are brand new units coils are clean. Air flow is very good Air flow was measured with a magnahelic actual clocked in at .2" wc which is the rated s.p. Motor rpm at or a little over. Amp draw at or a little under. CFM verified with capture hood. Heat fails on a table top test with no static pressure.
Professional Member*
are these 1st company units?
if so there are a special kit to use with heat pumps.
the new ones are as follows;
blower blows air thru the heater then the coil which causes high temps
Regular Guest
Originally Posted by gravity
Thanks Gravity for your response. These are not 1st Company manufactured units. Also, they are not for, or are they connected with any heat pumps. They are up to 8Kw heat strips, which for the few minutes they stay engaged the values of the strips actual read resistance coupled with the voltage yields the appropriate amp draw for the rated Kw's. Which also matches the data on the name plate.
Regular Guest
What is the actual CFM that you are delivering from the air handler based on a total sum of the supply air grilles?
Have you calculated your temperature rise based on return air and actual supply air temperature?
What is the high temperature limit set point? Are you exceeding this temperature limit due to low air flow?
Have you checked that you are running at the correct fan speed?
CFM= BTUs/hr (electric heat input) divided by 1.08 X Temperature Rise.
BTU/hr =CFM X 1.08 X Temperature rise.
Trouble shooting is not part of the repair…understand the symptoms and you will find a solution.
Professional Member
just throwing this out there, are they using stack sequencers? Are they wired correctly? 1st stage heat comes on and passes power to the fan, 1st on, last off?
Regular Guest
The actual CFM's captured is equal to the manufacturer's posted cfm output per the .2" wc. Heat strips also cut out while running the blowers in high speed and low speed on heat. Both heat strips only remain on for a few minutes, not long enough to perform a temp/rise test. We are certain about adequate air flow for two reasons: on a charge check sub-cooling and superheat measurements are right on the money, and; we removed this brand unit and replaced it with another brand (w/ curely-cue heat strips also reated a nominal air cfm's @ .2" wc) other brand runs perfectly in heat mode without cutting out everytime.
Regular Guest
Thanks for your comments, alcomech, This is what happens: T-stat request heat, Blower starts 1.5 amps for about one minute or so. Then amp draw clocks in 19 amps or so indicating first ht strip engaged. In about another minute or so amp draw is at about 36 amps second strip engaged. In a couple of minutes back down to about 18 amps, one strip and blower only. Unit blowing only luke warm air. Second strip never comes back on. Always find either high temp safety open or fuse-link failed. Also, have lost both strips at times on some units.
Professional Member*
For apartments are you sure the second set of strips are even needed? 5KW of heat is plenty for most apartments around here, not sure about where you live.
Regular Guest
J-Calc's indicate the need. Anger from the tenants verify the need. The 2 ton units have only 6kw's; also 2 strips, 3+3. Can't keep both strips running here either. Seems like if the other brands can run specified KW's so should these.
Professional Member
Sounds to me you've eliminated all but a design problem. Just what is the mfg offering for an explanation?
Regular Guest
When I first complained to the manufacturer he said just like everyone else (in this forum above) must be air flow. After arguing the issue for a few months by phone and emails, reps from the manufacturer came out to several apartments and verified we were correct in that we had adequate air flow. The verbally told me just remove and replace the safeties with higher temperature values. I said that I didn't think it would be appropriate for me to do so. This is something the manufacturer should do In as much as they are responsible for the integrity of their ETL (UL) label. At this point they declined to replace the safeties. It doesn't appear they are willing to accept responsibility for the problem. I wrote into this forum to see if anyone has had a similar problem and what did they do about it.
Professional Member
Do you mind revealing the brand name/mfg?
Regular Guest
Static pressure does not indicate true air flow. If you do not have a flow hood than you should simply be able to calculate CFM by the Equal Friction method
Equal Friction Method:
Area (Sq Ft.) = CFM divided by velocity FPM
where as: area = 15,000 CFM divided by 1500 FPM = 10 Sq
10 Sq. Ft x 144 (Sq. Inches per Sq. Ft)/24" = 60"
Therefore, select 60"x24" supply duct
5400 CFM ÷ 1700 FPM velocity = 3.18 Sq Ft. Duct Area
Aspect Ratio = 48/12= 4
Aspect Ratio 4 = 144%
Round duct = 66%
Difference = 78%
Velocity FPM = airflow CFM/ area in Ft Squared
Velocity x Area = CFM
To Figure CFM by known Sq Ft area and Static Pressure.
To Measure Air Volume there are two easy methods:
1. Temperature Rise:
In the temperature Rise Method, the heating section is ran with the indor fan speed set to cooling speed.
The temperature of air entering and leaving the air handler or furnace is measured to find the temperature rise. A formula is computed and air volume is determined.
2. External Static Pressure Method:
This called total external static Pressure.
A differential manometer is used to measure the air pressure at the inlet and outlet of the blower assembly. This pressure is the same pressure a balloon has pushing against its internal walls. Air Blowers have the same type of pressure at the inlets and outlets. The measured air is compared to design charts provided by the equipment manufacture to determine how much air volume is flowing in cubic feet per minute. (CFM).
As the static pressure across the fan goes up, the ability to move air goes down. This static pressure at the inlet and outlet is called total external static pressure.
The higher the Static the less air will be able to move through the air handler. The less static pressure that is present, the greater the ability to move air through the air handler.
For every cubic foot of air, the air has weight.
Every pound of air carries so many BTU's of heat in it.
When a system is designed the amount of air is set and the heat in the air is measured. This is important as the system can only absorb as much heat as is available. There are two ways the amount of heat fluctuates, by the amount of pounds of air flowing and by the heat in the air. If the heat in the air the system capacity falls. If the amount of pounds of air falls, the system capacity falls.
Professional Member*
please tell us the brand you are having issues with so we can stay away from them.. thx
Professional Member
Airflow has been confirmed and re-confirmed...reply #'s 3 and 13.
Regular Guest
So what is the total actual CFM being delivered. What is the temperature Rise. What are the temps of the safeties. With out this info there is no way to judge whats really causing his problem..
Trouble shooting is not part of the repair...understand the symptoms and you will find a solution.
Regular Guest
There are 3 different tonnage sizes. Each size is delivering right at nominal cfm's per ton as determined by capture hood. Actual cfm output matches manufacturer's spec's at .2" wc. Remember I said above that heat strips are cutting out on safeties and fuse links. Both strips don't stay on long enough to take meaningful delta t measurements. As far as the cut out temperatures on the safeties and the fuse links they are what they are. I would never attempt to second guess or propose replacement of a higher temp after a units been tested and approved by ETL/UL. As stated in my original post my quest here is to find others who actually have installed like units with these unusual type heat strips as I have described above. I think I may have also stated above I removed and put in another brand unit with conventional heat strips and the units run every time perfectly.
Professional Member*
This is a very good question, forget about the airflow for a moment and check temp rise. This will at least tell you if there is enough air moving over the heat strips, I was told back in the early 70's by my instructor, before reinventing the wheel, check for the most basic things first. Please check this and get back.
__________________________________________________ _______________________
“Experience is a hard teacher because she gives the test first, the lesson afterwards" ~ Vernon Law
"It's what you learn after you know it all that counts." ~ John Wooden
"When the teachers become unteachable we're all in trouble" ~ Mr. Bill
"Remember "Pro" is only a name, it's not always a mindset determined to do everything correctly" ~ Mr. Bill
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