Tankless Water Heater

[Cedric Van Duyn]

I have a 2 bedroom, 1 bathroom unit with a short water heater under the kitchen counter. The tenant is saying that the water heater is making some water noises. I previously discovered some condensation related moisture under the counter and thus considering switching it out for a tankless water heater. It would need to be able to fit in the same space as the current short heater. Is there anyone with experience with these tankless water heaters that can tell me if they are able to handle the light demand of a 1 bed, 1 bath + kitchen? Any recommendations? (The tenants are male, female and baby who occasionally have a guest staying the night). Thank you for any input.

[Malcolm Boston]

I have experience with gas and electric versions of these. After much deliberation I elected to revert to a conventional tank unit for a property that is intended to be a rental.

First the technical information:

Electric Tankless

With limited exceptions and circumstances; you will not be able to do a drop-in installation of an electric tankless without upgrading your electric service to your home. The absolute smallest unit on the market at the time (two years ago) required 50 amps at 240v. As a result, I needed #6awg wire. I got lucky because the electric service to the home had recently been upgraded to 200 amps and a load calculation revealed enough capacity to allow for the electric draw. That being said, it was a small house and was rated for 1 or 2 simultaneous fixtures but it was located in Kentucky where the water temperature input was much lower. This led to a water heater capable of supporting a single person but not more than that. You MIGHT find enough amps if you have gas service to your house and converted everything else to gas (e.g. Heat, cooking, clothes dryer).

Gas Tankless

You will be stymied by the minimum size of the gas line and/or the required vent material. If you can still find a Tankless unit that allows a 1/2" gas line it will likely be older and require special stainless steel vent pipe which costs more than the heater. On top of that the heat exchanger will likely not be as efficient so you won't get enough gas no enough heating to allow for more than 1 or 2 maybe three fixtures depending on the water entry temperature. The newer units all require a 3/4" gas line from the meter which you probably won't have because no typical residential gas appliance requires that much btu/hour. You will need a concentric vent which is cheaper than stainless but more expensive than B vent. If you didn't have gas service at all you would need to have it installed ($$$$$$$) plus venting ($$$$$). You could go very expensive and get a condensing unit which requires only cheap PVC pipe for venting (very efficient heating) but you still need a gas line.

Why I went back to a tank unit before I rented out my former residence:

My experience with Tankless units was with both retrofit and new build applications. In the case of the retrofits, I self installed the units into the existing non-home-run plumbing. This was technology that is now old relative to the state of the art stuff you can get at the big box stores today. The units activate based on water flow rate (basically how many and how far you have the hot water fixtures open). The issue I had was the 50 year old mixing valves required the shower handle to be placed in the maximum water temperature position to get the HwH to activate. Same for the sinks. This would probably have been less of a problem if the plumbing were updated to a home run manifold system but I had not done that work yet. There were some other issues that were not a big deal to me (but I'm an Engineer). I decided this was too much to require of a tenant so I ultimately R&Red this unit with a high efficiency high recovery tank unit (Rheem XR90). So I get a traditional operation method for the tenant with a slightly less efficient but highly reliable tank unit that has an energy factor that qualifies me for a big tax credit. In addition it is compatible with the existing 1/2" gas line and b vent. I also get a significant credit from the gas company for replacing a gas appliance with a gas appliance. By self installing the unit I basically got it for free.

Modern Gas Tankless with a home run manifold plumbing system

This unit requires a 3/4" gas line from the meter and concentric vent pipe. I self installed this item in a rental home that I took down to the studs. I had the opportunity to install up sized gas pipe plus a pex home run manifold water pipes. I also installed the vent pipe through the roof. This unit works very well with no issues at all. It is sensitive to very small flow rates so there is no difference in the method for getting hot water at the fixture.

Bottom line:

I recommend you replace your existing water heater with something similar or identical in size and power draw. It sounds like you are constrained by the electric power supply at the water heater and the dimensions of the cabinet it fits in. Your earliest opportunity to make a bigger upgrade is likely the next to the studs remodel. Even then, I would suggest the most efficient, highest warranty (typically best insulation) tank unit that is compatible with the existing power/fuel supply. If space allows, I would relocate the water heater from under the cabinet because that greatly limits your replacement options.

Good luck