I just got a new 50 gallon water heater and my circulating pump would only keep the hot for 1 shower (progressively getting cooler until completely cold). When I turned off the circulating pump the hot water lasted long enough to take 4+ showers (obviously had to wait a few minutes to get the water hot). I was told by a plumber that circulating pump might be broken (15 years old with no problems), so I bought a new one and exact same issues have continued. Help!!
Rise boiler temperature to over 60°C, also ensure that the recirculation pump does not run too fast (or your hot water mains will act as crawlspace heaters). The other option could be that your new boiler is slower to recover: the heat-pump kind is more energy efficient but really slower to recover.
You could address it by setting higer temperature on the boiler (colder return water will mix with hotter water in the tank keeping the temperature correct)
Install an 'on call' recirculation: it starts when you switch on the light in the bathroom. Other option is with a flussostat: you open hot water for few seconds (then close it and wait a minute), pump kicks in bringing hot water to the taps, you open the faucet again and have hot water without having to keep the pump costantly running.
A simplification could be turning on the pump at specific times (in the morning and in the evening) so the tank during the day has only to heat itself and not the whole circuit.