This has discussion has a lot of great information. I'm not sure if everyone's questions have been answered but we have a very in depth article on our site regarding the difference between the Delta 17 and 17T series trim kits. It gets pretty heavy into the details. Others above have given a pretty good explanations in this discussion but I think some aspects may still be a little unclear. Here's a summary of the differences detailed in our article. 1) Water pressure: 17 Series cartridges operate using pressure balancing techniques as opposed to actually sensing temperature. This means that water pressure from a 17 Series trim will be less than a 17T Series. 2) Temperature limit stop: If you live in an area where seasonal temperature differences are greater, you may have to adjust the temperature limit stop in your 17 series shower periodically. You will never need to do this with a 17T series. 3) Accuracy: 17T keeps the temperature in a tighter range. If you adjust the temperature handle more often the 17T will give you more predictable results with less fiddling around required to get exactly the temperature you want. 4) Shower systems with multiple spray outlets: In the case of a shower system you will require more water pressure and a 17T will be better. You will also get better pressure from a standard showerhead with a 17T. These are some of the reasons the price is higher. For more info check out our article: https://faucetlist.com/blogs/buying-guides/delta-thermostatic-shower-faucet-trim-kit-cartridges-monitor-17-series-vs-17t-series-tempassure read the guide

Just some thoughts on improving your existing setup (it's probably got 20+ years' service life left): 1. Don't use a heating boiler for domestic hot water, especially in the summer. 2. Convert to gas if you're currently burning oil (and be sure to carefully clean heat transfer surfaces). 3. Provide as many zones as you can afford, schedule the temperatures with programmable thermostats; use one circulator with multiple zone valves instead of multiple circulators. 4. Arrange for fire on demand (if you have a cast iron boiler - don't do this with a steel boiler). 5. Provide a reset control to limit boiler temperature when it fires (I always liked Honeywell T475A); install the water sensor in the pipe, don't just strap it to the pipe surface. (This is another one not to do if you have a steel boiler). 6. Put in a stack damper (make sure it works properly). Had a good time with this stuff in my first home, a 1930's semidetached; started with a new Weil-McLain oil fired and went from there, over a 24 year period.

Okay, So to bring a conclusion to this thread: An application issue for gas tankless heaters is if the summertime water inlet temps are likely to get into the 80's. What can happen, particularly in situations where there is a low-flow shower, is that the minimum btu input of the heater can't be sustained and the heater shuts off. In other words, there isn't enough water flow and temperature rise to allow the burner to run at its minimum btu rating. If you are in the southern US or otherwise have high summertime cold water input temps, a gas tankless unit may not be appropriate. Pay attention to the minimum BTU inputs. As Dadoes has pointed out, that may not be a problem with electric tankless units. However, electric tankless whole-house units provide no real energy savings to speak of and require large electrical service. Except for point of use applications or where continuous hot water is needed, consumers wanting an electric water heater would probably be better served with an electric tank unit.

I despise random malfunctions which can't be duplicated while troubleshooting. I suspect this has become more of a who done it for you rather than an inconvenionce but I understand how that can happen. How much do you feel the recirculation helps? Have you tried installing a ball valve so as to (A) see how much difference it makes in hot suppy delay and (B) if problem is eliminated ( which might be difficult since problem sometimes come's and go's for months)? Do you ever wake in middle of night thinking about it? LOL