how many tankless water heaters for quadplex?

[Andrej Walilko]

Hi all,

I'm looking to replace two currently functioning but well-aged tank style water heaters with longer-lasting and more efficient tankless heaters in my quadplex (3x1br+1x2br). This will also completely remove the use of the chimney, easing a future roof installation. Installation and connection of the system should not be a problem, but sizing the heater is turning out to be the difficult part.

Presently, there are two 40 gallon water heaters that take (or once took) 35k BTU each. They are split between the first and second floors of the quadplex. Each apartment has one full bath and one kitchen sink, no dishwashers or clothes washers provided (though at least one tenant has a portable clothes washer.)

My calculations with the 47 degree groundwater in Michigan lead me to believe that a single 199k btu system, the largest I can feasibly and affordably find, may not be enough to support 4 showers and 8 sinks, even with water savers applied. These tankless systems could be ganged, but this would require a gas meter upgrade, as the boiler is also a 199k btu unit, and the supply to the mech room is 1". The meter itself is only rated for 250 cuft/h. I could go without a line upgrade if only one tankless unit of this type was installed.

My question is this: should I size the system to support 100% usage on every hot water fixture in the building, knowing that it will probably never be used, or is 70-75% usage from a single heater going to be alright? I can't imagine a real life scenario where every unit's kitchen and lav sink is on hot, in addition to all showers being run, especially with three of the units being 1 bedroom.

[Mike Reynolds]

Originally posted by @Genny Li:

While on demand are super hot, they don't actually save enough over NG to justify the switch, and they have higher lifetime costs, so it comes out in the wash.  Electric on demand would actually cost more than NG tanks....  NG on demand requires more ventilation.  The new heat-pump electric tanks put electric almost on parity with NG, so I'm actually not inclined to do tankless at all anymore in any application.

And I like cool new things.  I have a PEX manifold in my house right now. lol.

I've never had an oil on-demand! I have an oil indirect now. lol.  Exotic. I thought I'd had practically every way to heat residential imaginable....

 I agree, the gas is better in the long haul. Plus they heat water better. Electric just is not feasible for most remodels anyway. Most old electric service is even 100 amp and many if you are lucky 150 amp. Back in the day we didnt have cell phones and PCs and every thing under the sun to plug in. Usually it was one plug per bedroom wall, no GFCI or arc fault. Dont even get me started on Zinsco breakers and fuses. 

I built a 19,000 foot party barn and the owner insisted he wanted electric on demand heaters. I cheated and ran a gas line to the unit also and capped it off. Come the following January he asked me how hard it would be to convert it to gas. I told him it would take a good day and a new heater. I never told him that I already had a gas line ran there. I knew he would want it sooner rather than later. He thinks I am the man and I have got so many recommendations from him I have lost count.  I just dont trust the electric ones at all but some people swear by them.