What temp. at the warm air plenum?

Discussion in 'Maintenance and Troubleshooting' started by donnie p, Dec 28, 2013.

  1. donnie p

    donnie p New Member

    My warm air temperature measured at the plenum is about 79 degrees. The cold air return plenum is at 66.5. I keep the house temperature at 69. The system is having a hard time keeping up even in milder near freezing weather. I am thinking that the warm air temperature is too low. If that is the case what could be the cause(s). Any comments will be greatly appreciated and I can provide more information on the unit if needed. Thank you.
     
  2. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    We need water temp in and out to guess at a cause.
     
  3. donnie p

    donnie p New Member

    Hello Mark...thanks for responding. A reading I took earlier today was 39.5 in and 33.5 out. Warm air plenum temp. was 79.9.
    The unit is in a basement that is chilly...about 50 degrees. After my first post I taped some cabinet seams and it seemed to help a bit.
     
  4. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Make sure your return air temp is measured downstream of any possible return air side leaks from that cold basement. System ought to be able to manage 25*F or higher airside temperature rise.

    Find the "problem checklist" here or at Green Building Talk and provide us as much of the info asked for therein. As Mark noted, water temps are important, though yours seem fine.
     
  5. donnie p

    donnie p New Member

    Thanks Kurt. I measured the return air temp at the cold air plenum before the filter. I guess the 25* ballpark number is what I was wondering...not sure the unit is working to the full potential... but here is the pertinent info from the checklist.
    Unit is located in Western NY...14427. Unfortunately I am not and will not be there again for several months. I will try to have someone at the house relay any other info any of you need. The following is from memory as best I can.
    Heating about 3500 sf in a 1870 farm house. New windows, insulated floors above the basement... fairly well insulated otherwise. Can't recall the heat loss/gain numbers. It was calculated that the 4 ton Mcquay VFW 1048 unit installed would cover 80-85% of the heating load. This is the 3rd year running. When it gets into the low teens and below we augment the geo with gas space heaters and sometimes a wood insert in a central fireplace.
    The loop field is horizontal slinky loops 6-7' deep. Hit rock gravel at this level. Appox. 760' per loop I believe. 4 loops.
    Average of 5.5cents per KWH. Cheap electric is good. During our coldest month the geo has cost about $110. to run. Maybe $40. for gas. We now use very little wood...the wife likes a fire once in a while.
    The last readings I took after sealing the cabinet...38.5 ewt, 32.5 lwt....the warm air plenum was at 81. Did not recheck the cold air....probably still around 66. I might add that the entire duct system was built from scratch as there were previously radiators and a gas boiler. $600+ a month to run that 25 years ago when I shut it down.
    This unit has really been a Godsend for us. Previously we were burning 15 cord of wood and heating a couple rooms. Gas space heaters were also used at about $200. on a cold month. Still, much of the space was barely heated. Now the entire house is comfortably warm for a fraction of the cost and removing the danger and mess of the wood fire. Could not be happier unless there would be a way to increase the efficiency of the geo we have now. Hope I have covered everything. Thank you one and all.
     
    Last edited: Dec 31, 2013
  6. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Sounds like you are in fairly decent shape, given your cost data. Duct leaks / losses, perhaps too high CFM / ton might figure into the low leaving air temp.
     
  7. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    "the warm air plenum was at 81. Did not recheck the cold air....probably still around 66."

    One without the other has little meaning.
    Make sure you are checking both temps as close to unit as possible.
     
  8. donnie p

    donnie p New Member

    Thanks for the responses... I took the warm air plenum reading just above the unit and the cold air reading just before the filter that is installed in the side of the cabinet. 79/66.5. Sealed the cabinet up with metal tape and got the warm air to 81. The unit has an ECM blower if that would have any bearing.
    I am not at home now but my wife will keep tabs on the numbers. If something changes drastically I will post. Thanks for the safety net.
     
  9. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Airside delta-T should be higher. Could be excessive airflow, charge, or refrigerant metering issue. Probably need a competent tech to have a look.
     
  10. donnie p

    donnie p New Member

    Thanks, Curt. I will look into getting an evaluation onsite. Anyone know a competent tech that would service the 14427 zip?
     

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