HVAC Heat pump guys I have a question

[Blown71X]

Ok for my second post tonite (i`m having a bad day bare with me)
First a little back ground:
My shop is 1920 SqFt with 12 foot ceiling R19 walls and R30 Ceiling with 15/32 BC plywood on the walls and 1/2 OSB on the ceiling, it`s fairly air tight with the exception of the freeking outlets (another post) and the overhead doors which i`m working on at the moment.
Furnace is a 4 ton 12 seer air to air orfice type coil with 20kw of electric backup that I installed, mounted at ceiling hight on a platform with only about 5 feet of plenum on the outlet with a spread diffuser pointing at the center of the building.
For the most part it has done well, but last winter it about killed me cost wise during real cold months and I have been watching it and it seems to be running alot so far this year, I checked the charge a couple of months ago with hot gas method and it was close to spec (ambiant plus 110° on the liquid line at the service valve) but i`m only getting about a 13-14.5° rise at the air handler and I seem to remember originally it was in the 23-25° range which I was happy with, All the HVAC guys I know around here are about worthless when it comes to heat pumps and i`m at a loss to figure out what has changed.
Any idea or suggestions on what to check, I know my HVAC background is automotive but this isn`t my first home heat pump install and I have done alot of research due to the lack of knowledge in my area about these and this is the only one that is giving me fits, For some reason in the back of my head I keep thinking that slowing the airflow down a tad across the coil might help but that is the automotive part of my brain kicking in and may not apply.

Thanks in advance
My hair may be gray but I don`t want to pull it all out...HELP
Rick

 

[hobie1dog]

I know this is a late reply but if you have lost 10 degrees or so of drop across the coil, you could have a refrigerant leak. If it is a r-22 unit, then an electronic 120volt leak detector should be used on it...like a GE H-10B or equivalent H-10B aftermarket/Japanese version...the battery models aren't worth getting out of the box. Any leak is a 24/7 non-stop leak(except for the rare high side leak that only leaks when the unit is running).,,,,so that would be the first thing to check for. I would not alter the fan speed.

Using the 110 degree method of checking the charge is usually a pretty vague way of determining the charge.


If you do find a leak and repair it, then you MUST use a Micron gauge to measure how deep of a vacuum you end up with...this is the only way to gaurantee that you have the leak fixed and that you have boiled away any moisture in the system...moisture is only boiled away below 2k microns. I always get a system down to 200 microns before I re-charge a unit, or I keep on evacuating and purging the system with R-22 between vacuums to get the moisture out. I just had my Thermal Engineering Vac-Check Micron rebuilt at the factory and was talking to the head tech on Thursday. I told him I always get to 200 microns or so and he said," you are one in a thousand who take the time to do that". I always took the attitude that when I take the valuable time out of my life to fix something, I want it to be the last time I have to work on it.

Hope this helps

 

[theblur98ss]

Make sure both coils are clean as well. 13-14 is a bit low while 23-25 may be a bit high also. Is the insulated line warm or hot coming into the unit? Is it defrosting? Any ice building up?

 

[Maulerman]

Is the outdoor ambient the same now as when it was 23-25? If its colder you won't get the same delta T. The probable reason for the high cost is that your resistance heat is running a lot. Air source heat pumps aren't too effective below freezing.