Hi @Kassandra M
Yes, when I posted my comment, it shows for me but didn't increment the comment count. Something must be wrong... so here's a repeat:
Hi,
This heater was an undersized trd from the beginning and it's 17 years old. Unlike wine, it doesn't get better as it ages. It's probably limed up - but even cleaning it won't fix the fact that it's undersized.
So here's what's up: It has an input of 117,000 Btu/per hour and is about 82% effecient. That's enough to heat about 3 gallons of water per minute 55 degrees - so, from 50 degrees to 105 for a shower - which is what we design for. That's about enough hot water to take 1 shower (2.5 gpm). It's not enough for a standard tub filler (4 gpm). And I'm sure not enough for the Jacuzzi filler.
You didn't say what part of the country in which you live - it's very easy to get water that's colder than 50 degrees in the winter.
It's suppose to limit the water flow output in order to maintain the output temperature. What I suspect is that the hot water lines aren't insulated to the master bathroom - and so the water cools off too much also.
If you replace it with another tankless heater it needs to be one with 200,000 btu/hr input and 92+% efficient. This heater can deliver about 7.5 gallons per minute at 55 degree temp rise.
To do that means that you will have to make sure the gas supply is sufficient (to the house, and to the heater) and change the vent from the metal flue (Type B) to PVC. That can be expensive (or not) depending on what the setup is now.
The other thing you need to look at is if your hot water lines are insulated. Too often the heater is in the garage and the master bath is at the other end of the house AND the hot water line is uninsulated. I've even seen situations where the hot water line was uninsulated and in a concrete slab - which means that the water NEVER got hot because it gave up all of the heat to the concrete.
So, bottom line, This unit is undersized and 17 years old. It's an 82% non-condensing unit that wasn't a great unit to start with. You can't fix that unit.
To solve your problem will require a knowledgeable plumber who can tell you the cost / feasibility of installing a properly sized unit (200k btu/hr) paying special attention to the gas supply and replacing the vent.
The alternative is replacing the tankless unit with a tank heater.
Q
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Q