You are probably past the useful life of a hot water heater. You don’t want to wait until they burst. The damage water can create, especially if you are not at home is ruinous. I would recommend replacing your heaters, soon.
I guess. The two water heater vents I have looked at appear to have been installed by barely competent primates, with a big gap between the top of the vent stack and the roof vent. I would think that any DIYer who feels confident enough to hook up a tankless heater could replace the roof vent and the pipe with the type you need for a gas fired tankless. (they sell these special roof vents and pipes with 2 lumens)
I’m not sure what you’re referring to when you say ‘2 lumens’. It appears this may not be the case anymore, but I thought all these units had to be vented out the side of the house. From what I’m reading, they can’t be vented into (most) brick chimney’s for the same reason. That being that they’re so efficient (turn a high enough percentage of the gas into heat and move it to the water) that the exhaust isn’t hot enough to rise all the way up to the roof line. It then cools and starts falling back down, bringing with it CO and water that enter the house through other appliances (ie furnace).
They may, at the very least, required a double walled (with insulation between them) dedicated vent to the roof line, but I really don’t know.
No, the efficient ones are called condensing. And those are $1000 instead of $500 on Amazon. I don’t recommend, tankless gas fired heaters are a fairly complex piece of machinery and they might need to be replaced before they save you $500 in fuel. (the efficiency is around 80% instead of 96% for the non-condensing ones…vs maybe 32% for a tank heater, assuming 40% of the generated heat leaks out before you use it).
The double lumen vents are for a different reason. There is a very high air consumption rate for a tankless heater, so they are required to suck their air from the outside. (not sure, this may be a code requirement for all combustion heaters now actually) So you can set it up either with two vents, one intake and one exhaust, but the easier way is you replace the vent pipe with a double lumen one, and you replace the roof vent with a double vent one. The roof vent now has a pipe that lets out the exhaust and a separate intake and some trick in the design keeps most of the exhaust from getting sucked back into the intake.
I’ve never heard it called a double lumen vent, but if you’re talking about what I think you’re talking about, the ‘trick’ is that it’s a pipe within a pipe. Exhaust is sent out the center pipe and fresh air is brought in though the space between the two pipes.