accurate charging in heat mode
in heating season what is the best method of adjusting and/or charging system. I do not get the subcooling readings in heat mode that unit is requiring but pressures are accurate. learning everyday but i guess i make things too complicated
Pull the charge and weigh it back in per the manufactures specs.
A good HVAC tech knows how, an educated HVAC tech knows why!
magac, I'm sure you'll get a lot of different opinions on this topic. By far the best way to be sure you have the correct charge is to weigh the charge in. If you can't weigh the charge in you must use the manufactures recommended method for charging. Often the directions for charging are posted somewhere on the outdoor unit. The problem with not weighing the charge in is that there are many variables that affect the pressures and temperatures. The airflow, pressures and temperatures are very dynamic and can result in false readings resulting in inaccurate charges.
If you have to charge a system that usually means that it was never charged correctly to begin with or you have a leak. Find the leak and fix it then weigh the charge in. If there is no leak you can try to adjust the charge using the manufactures method.There can also be other problems with a system that can appear to be a low or overcharged system. If you find your self adding refrigerant and the pressures are not responding the way you would expect, STOP! Verify airflow and if airflows are good you might be over-charged or have a metering problem. Pull the charge and weigh it in and eliminate the charge. If pressures are still not right then you have other problems. Metering device could be having problems or mis-matched equipment just to name a few possible problems.
Good luck and remember to always read the instructions.
"The bitterness of poor quality remains long after the sweetness of low price is forgotten". --Benjamin Franklin
"Don't argue with an idiot, they will drag you down to their level and beat you with experience". --Mark Twain
If this post is related to the other ("outdoor coil frosting") post, and if "pressures are accurate" is based on service literature included with the unit, that's about the best you can do without weighing in the charge.
Originally Posted by magac
But, "pressure vs temp" service literature data is based on a "frostless" coil. Same as with the cool cycle and frosted indoor coil, you have to defrost the coil before you can evaluate system operation.
I don't know of any mfg's that provide "target" subcooling values for the heat cycle, since it can vary significantly with outdoor temps. But extremely high or low subcooling values can be helpful in evaluating over/undercharge or liquid restrictions.
If the outdoor coil is TXV fed and you can measure superheat, that will also tell you something, since the superheat should be within a "typical" range of normal superheat.
Unless the manufacturer has a pressure chart for heating you've pretty much got to wing it. The way I do it is to ballpark what the equivalent outdoor temps would be between cooling & heating & use the cooling head pressure for reference. I figure 95 in cooling is approx. 55 in heating for head pressures. So if my system would run 240 head at 95* OA in cooling I would look for 240 head at 55* OA in heating & interpolate from there. Suction usually runs about 10 psi lower in heating too. I know its we're supposed to use SC & SH but until manufacturers publish some kind of data for it in heating I just use pressures & ballpark it.
I've been doing it for a lot of years & I rarely have to come back to adjust the charge in the summer. I know manufacturers say weigh it in but that's a little impractical every time you need to adjust the charge in heating.
An engineer designs what he would never work on.
A technician works on what he would never design.
I totally agree with that. I mentioned subcooling in terms of a diagnostic "indicator". If I decide the suction pressure is "low", add a pound or two and it's still low, then measure 30˚ subcooling, I start looking for a liquid restriction.
Originally Posted by garyed
I think most give you a chart, graph or something. Still wouldn't hurt to return and recheck in the summer. Today's units tend to be rather touchy with huge outdoor coils.
I watch my delta T on my indoor air and when it reaches it's max somewhere around 25F I call it good. I than go back in the summer and recheck it with SC & SH values. This is not the scientific method but it works.
As mentioned above, weighing in the charge is by far the best method.
But this is what I do on mild days: Here in GA, we have relatively mild winters so when it is around 45/55 degree's ambient - I operate the system in cool and I then wrap the condenser coil with a trash bag/cardboard box. This elevates my head pressure and allows a better ballpark charge. I wouldn't suggest doing this in weather below 45 degrees due to the fact that your head pressure may never get high enough.
And I then always offer a "free" refrigerant check in spring.
Take a picture on your phone of a r22 and a 410 heating chart and use it to get you close. They say they are for piston only but it is still relatively close on a txv and will get you by. If its warm enough throw it in ac mode and use SH/SC.
Originally Posted by magac