I have a US Powers 4 ton gsdx system that we installed when I built my house in "93. It has been amazing over the years(see customer reviews). But it is 15 years old and Little things are failing. I replaced the compressor 2 years ago so that part of the sys is in good shape. My straight to the water heater desuper heater was turned off last year because of a failed tempering valve. The circulating pump is leaking water as well now. So something needs to be done with that. Either full disconnect or upgrade. I now have a small (they added 1lb out of 22 total of r22) leak in the A coil in my single speed Trane air handler. That A coil needs to be replaced. I'm talking with a couple of folks about options of repair or replace. I DO NOT WANT A TOTAL SYS OR NEW FIELD AT THIS TIME. So I'm seeking opinions on benifits of:
 Replace A coil in my existing air handler or  replace the unit with a new variable speed unit.
 Repair the water leaking componets in my desuper heater or replace with what.
     New style desuper heater.
     New on demand system adapted to my compressor unit that uses heat pump for hot water year round.
     Replace with a solar hot water system.
     I have a reasonable new 50 gal "smart" conventional electric hot water heater.
Our water usage is low and we are on a well. Would one of the above options also be able to partially heat a possible future hot tub addition?
I'm looking for a GENERAL cost versus adavntage opinion over the long haul without getting into alot of heat value math. Pros and cons of the above. My system has been very efficient over the years and obviously I'm not going to see the dramatic operational cost savings of someone converting over from conventional hvac to geo. I'm sold 100% on DX so that is settled. Just like the idea of slowly fixing what is broken with current technology which has changed since my sys was new. However money is tight right now as I have a son in out of state college. So what makes more sense slowly upgrading into a new sys as major part of the sys fails or just fixing what I have until replacenment become inevitable. I'm also checking with my tax guy to get his take on what qualifies under the new laws. I would like to hear from knowlegable folks whose opinion is not too tainted by the money they'll make on the work. So far I'm working with good folks but we all need to make a living and they sell this stuff. Thanks in advance.

I'd just have the A coil replaced.

Next, I'd put in a new desuperheater.

As far as installation vs. maintenance costs, it seems to me that the solar hot water would not be of benefit to you largely because you have a small hot water usage.

I would replace the A coil only if the fan motor has been replace already , if it hasnt at that age it might not be to far off . At which point you have pretty much a new unit with out the upgrade .

The only plus of solar over your current set up is the tax credit , which from my readings repairs/ partial system upgrades likely would not quilify  .

Circulator / and tempering valves are pretty simple . But Im going to defer on that one upgrade desuperheater
Can't comment on the hot tub with out more info ( size / hot water production)

Could addition of a buffer tank allow the present desuper to be restarted without a tempering valve?

Geo fan, The fan has not been replaced and there is some surface rust in the air handler. This is a humid area and this thing gets cold. Often if the sys is turned on(ac) during humid weather after being off a couple of days the condensate tray cannot handle the volume of condensate generated till the house levels out. So I get some water in the handler and floor. Over the years it has left it's mark. That's why I thought about the upgrade. That fixes the fan and A coil and should save a little elec at the same time. Depending on replacement cost and tax deduction as an energy efficient replacement/upgrade of course. The other thought is that if I end up with Major failure in a couple of years I already have an  up to date air handler  that won't need to be replaced. So that will reduce the price at that time. Sort of upgrade a little at a time until sys is new again. Keep the replacements/upgrade a piece at a time and pay for out of pocket instead of one big bill. I guess the big question is would I likely see much benefit from a variable speed on the monthly bill over what I have if repaired?
 
Engineer, The desuper heater I have is pretty simple. As I understand it that tempering valve is my temperatue control. It has an adjustable dial with temps in the same range as the water heater. Set the temp close to WH and it keeps the tank warm. Or in the case of when it started acting up it was heating the WH to extremly high(unsafe) temps. I have checked and that exact valve is no longer made but a similar replacement can probably be found. The circ pump shouldn't be a big deal. All I did to shut off desuperheater was to turn the valve all the way down so it won't pass water. sensers don't see cold water so they don't circulate refridgerant or cycle the pump in the DSH. But with some water drips from both parts I need to do something before water leaks escalate or elec gets wet. At this point I have replaced the electrical contacter on the compressor recently, the compresser and dryer at the same time 2 years ago,  and a whole lot of air filters, and redid the condensate lines to drain. This along with regular cleaning and inspection. That's it since new.

The company that installed it is checking out for parts to repair; desuper and A coil along with pricing a new air handler and options for replacing DSH. I have a couple of other folks doing the same.  It seems the new dx systems are pushing hp on demand hot water over desuper heater. How does this figure price wise as an add on vs a new DSH? Can someone here recommend a DSH that could used to replace mine. Elec rates are jumping fast here as we are getting creamed by the end result of energy deregulation in this state and the price caps were lifted last year effectively doubling out bills. So I would like to get this thing back to tip top shape and take advantage of all it has to offer for saving money every month. Even if that requires a little up front cash being laid out. At this point I have not seen any prices yet as I just had these folks out last week. I'm a technican in another field and a dyi'er but in HVAC I guess I fall into the sorta knowlegable layman catagory. I let other people work on this stuff normally. I haven't done too much homework on this stuff in years as I've had no need. There are a lot of new ways to do stuff that I'm just now getting exposed to. Thanks again for the schooling.

The big benefit to a vari speed fan in a new airhandler would be more even distribution of air through your home. Since I haven't sensed a complaint in that dept. it's a shame to spend the extra money.
Tax benefits only make sense if you are already buying something. I wouldn't stretch the budget over it right now.
Fix the DSH if you want, or do as little as possible with an eye toward replacing everything but field before 2016. Your field is currently compatible with other dx equipment.
Good Luck,
J

Thanks for the feedback folks!