A good set of applicable previous replies from others, here’s more… As noted, it would be most helpful to you and to us if you would include the model number and BTU of your heater and add it to the signature too. Another simple test for a DIY-er that would quantify any potential problem is to get the deltaT – water temp at inlet and outlet of heater. If you don’t have sensors on the pipes nearby, just use a digital meat thermometer (or better yet a multimeter probe) on the skimmer, shut off main drain input for the test (so you’re not mixing in that colder water), and compare that temp to the outlet temp at one of the jets. The last time I checked that on my 115K BTU heat pump I was getting a deltaT of about 9 degreesF with pump on low (40GPM) and about 4 degreesF with pump on high (70GPM). Heating impact is actually similar regardless of flow, although technically the heater is most efficient at the higher flow rates.
As others noted, ambient temp and RH affects efficiency. Ironically heat pumps work better at warmer temps, hence you can see COP ratings (Coeffecient of Performance) far higher (more BTU per watt) at higher ambient temps as pictured below. For those deltaT tests I noted above, ambient air was between 75-80 degreesF. You’re in NY, if the ambient temp is below 50 or even 40, the heat pump gets very inefficient, not sure it really works at all as you get to 40F and below. Again see pictured chart for BTU output at 50F, as it approaches only half of its BTU output at those lower temps. I also notice it’s better if the heater is in the sun – not shaded!
Yes it would be useful to use an amprobe (clamp-on inductive ammeter) to understand current draw from compressor and fan. The value won’t be definitive but something in the range of 20-30 amps won’t be unusual for combined compressor and fan on a 115K BTU heat pump. Nice try with the capacitors but in my experience those usually influence the ability to start up more than having impact of efficiency while running. It depend on the design. Also, many $20 multimeters allow you to check capacitor microfarad values. Watch youtube videos for safe capacitor testing because they hold a charge that can harm you.
If you truly find a very low deltaT – water not heating much – other causes can be clogged evap coils as others noted, low speed or nearly stalled fan motor, finally a low refrigerant charge will do it for sure. On many heat pumps, if the refrigerant charge is low enough the unit will totally shut down with an error code like (eg LO or LowP or LowPres). It takes knowledge of HVAC/R and proper gauges (and EPA 608 certification) to test for proper charge, to assess values of things like “Superheat” and/or “Subcooling”. If you do have a tech discover a low charge, it's always wise to figure out why - ie first find the leak, or else the trouble will recur. Could be as simple as a schrader valve, a schrader valves cap, a nosy neighbor letting charge out of your heater, or more serious causes. On the last 3 heater I assessed with low charges, one had a leaky schrader valve, one had missing schrader valve caps and leaky valve, and one had a loose sealing hat on the expansion valve. Easy fixes but you gotta pay the tech, the cost of refrigerant, and ideally they run a nitrogen pressure test. All pricey due to the time it takes. If it's an older unit that uses R-22 or something like that, the refrigerant cost is so high you'll want a new heater
Summarizing some statistics from my Builtright 5BR115 which uses R410A, running at ambient 75F and pump on high for 70GPM, deltaT of 4degreesF achieved, Superheat of 15F, total current 23 amps. I found a local tech who told me that 15-17 is the right target superheat value, but it’s not on the nameplate. I would have rather expected a “subcooling” target value based on the design that uses a refrigerant expansion valve. A service manual would have been nice too, LOL.
