low pressure lock out, changing filters prevents clogged coils

Discussion in 'Maintenance and Troubleshooting' started by zacmobile, Jul 26, 2010.

  1. zacmobile

    zacmobile Guest

    I'm working on a waterfurnace ATV057 with a low pressure lock out condition, I'm normally a hydronics guy so could use a little advice.

    outside temps are a little above average here in interior BC 35°C (92°F) unit was installed in 98' & runs for a good 7-8 hours before locking out. here are my readings while running in second stage, all seem to be within range to me:

    supply air: 53.5°F
    return air: 63°F
    source T in: 53°F
    source T out: 58°F
    source P in: 25 psi
    source P out 15 psi
    compressor amps: 13.8A
    unit voltage: 241VAC

    The air side delta T seems OK but I tried increasing the fan speed anyways which didn't improve the situation, could it just be a weak pressure switch?
    Last edited by a moderator: Jul 26, 2010
  2. Bergy

    Bergy Member Industry Professional Forum Leader

    Why is the return air temp so low?

  3. zacmobile

    zacmobile Guest

    low return temp

    when I took the readings it was early morning and the house was cool from the night, they had the unit switched off for a few days prior to my first visit. also, on closer inspection of my notes it should read 63°F.
    Last edited by a moderator: Jul 26, 2010
  4. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader


    Your readings don't point to this, but low pressure cooling lockout points to inadequate air.
  5. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    I've been replacing a rash of NAFTA era pressure switches.
    Try a new switch Teed in on the service valve. If it doesn't work you can still dig deeper.
    Good Luck,
  6. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Normally, hanging a set of gauges on a geo system is the last diagnostic step to take owing to loss of refrigerant via gauge hoses upsetting the charge, especially on a package system.

    That said, given the nature of the lockout, you might consider gauging just the low side - that hose fills just with gas, so loss is negligible, and it could support or rule out the pressure switch hypothesis advanced by Joe

    I share Bergy's curiosity about low return air temp, even if the house is cool from overnight. The airside split seems quite low as well

    In my experience a system inhaling return air in the low 60s would be on its way to an iced evaporator, and that will really depress low side pressure, among other problems.

    Compressor amps seem low for a nearly 5 ton system, but that may be explained by relatively low waterside temps (wish we had those in Florida...)

    Are the voltage and current readings US or Canadian units?
  7. WF_Inc

    WF_Inc Member


    You are welcome to contact our technical support staff at (800) 934-5667, as we would be more than happy to offer our assistance.
  8. zacmobile

    zacmobile Guest


    Thanks all! called WF this AM and got some good advice. they say air split seems too low as well, will put gauges on this afternoon, i'll let you know how it goes.
  9. zacmobile

    zacmobile Guest

    coil clog!

    the first time I was at the clients house the unit had been shut off for some time, this time they had been running it and when I pulled the filter out I noticed a cool vertical skating rink on their coil.:eek: I ran the unit in heating until the coil thawed out then pulled the return air plenum off and sure enough the coil is packed with lint & junk not visible from the filter opening. so, anyone got any tips for cleaning coils?
  10. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    My preference is foaming cleaners specifically for fan coils (condenser cleaners might be a tad caustic for indor use).
  11. geome

    geome Member Forum Leader

    I did some research recently on coil cleaners. The evaporator coil cleaners that I looked at seem to be more friendly to indoor environments than condenser coil cleaners. You would need to look at individual product information to make sure what you choose is safe. Some are rated for outdoor only.

    For any products that you are interested in, you can also check the MSDS sheet. I looked at the NFPA rating for Health, Flamability, and Reactivity. Here is some information on the ratings: NFPA 704 - Wikipedia, the free encyclopedia

    If you choose a concentrated cleaner, there may be directions for different concentrations to use depending on the amount of cleaning the coil needs. I like the idea of cleaners that won't harm metal.
  12. zacmobile

    zacmobile Guest

    clean coil

    I called around to about a dozen local wholesalers, hardware stores, auto mechanics & auto parts stores, none had any kind of coil cleaner. I couldn't wait to order any in so I ended up getting some non-acidic biodegradable de-greaser from an auto parts place. it seemed to work quite well, loosened up all the lint & grime then I gave it a few good rinses with a hose. I finally got the gauges on and called WF, seems like it is in fact low on charge. hopefully getting a refrigeration technician out there tomorrow.

    thanks for all the help!
  13. geome

    geome Member Forum Leader

    For future reference, you could try an HVAC supply house. Even the ones that say that they only sell to installers may still sell you coil cleaner over the counter. This is what one local company told me. An online company is Patriot Supply (in NY). No idea what their policy is on shipments to Canada. Good luck.
  14. Coil Cleaning

    I do commercial, but here is my two cents worth. 96% of all coil cleaners used in residential or commercial is purchased from local hvac houses (places that sell AC parts) Baker Distributing, CC Dickson, Johnstone Supply, etc.

    Canada has their equivalents. You could research online. For air to air condensor coils most techs prefer a "strong" alkalie or acid base FOAMING cleaner to start to loosen the impacted dirt, fruit flies, lawn grass, etc.. Has to be rinsed WELL, or the chemical can eat at the coil: not good. This is not hard to do outdoors. For those who use pressure washers, there is an art to it, or you will destroy to coil. Novices start FARTHER away. First, never over 1200 psi; the idea is high pressure, low water volumne. Some coils can be cleaned with good garden pressure from a garden hose, many cannot.

    Generally a milder foaming cleaner is preferred for evaporator coils, especially if that evaporator coil has expensive protective coatings to lengthen its life; the problem is that one, most evaporators / air handlers are installed in ackward places with little provision for access or drainage. In some cases you have to cut squares or 'doors' into the ductwork for access. So if you use a garden hose or 1200 psi pressure washer, where is all this dirty chemical 'wash' going?

    For that reason many techs use a milder spray on - walk away cleaner applied with a garden sprayer. You could use aerosols but they tend to cost $10.00 per can. The residue either goes out the drain, if you have a real drain, or leaves through a electric condensate pump if you use one.

    I personally use blue and white two layer commercial 'media' material on all the returns in my house. The media, which can be vacuumed or thrown away captures about 96% of all dust BEFORE it even hits my 2" pleated 18" x 18" filters. I replace the two layer media three times as often as I replace the pleated filters. That way only the smallest of particles finally hit the evaporator coil and it means that the coil can be cleaned or kept clean more easily.

    Last statement, a "packed up, neglected evaporator coil" can be the devil to clean up.
  15. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Ditto that.

    The key requirement for the homeowner / maintainer of the system is to arrange / maintain return air filtration so that the indoor coil does not accumulate dirt.

    Blue horsehair filters are right out. Pleated filters are much better, but if neglected will bow and allow dirty return air to bypass them. Pleated air filters that fit only loosely in their housing will also allow dirty return air to bypass into the evaporator - if I find the filter is excessively loose in its housing I add 2x1/8" foam tape to it to make it fit more tightly.

    I dislike cutting holes into return plenums so as to slide myself on my back into a position where I can clean the dirt and doghair off a clogged evaporator - much of the cleansed debris seems to fall on my face - so I tend to charge quite a bit for doing this...Maintaining good air filtration is pennies spent to avoid dollars spent.

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