I was surprised at what some of the 4 gpm tankless
>> heaters are going for on ebay. I was also surprised by the 60A draw
>> for those units! Any experience or advice?
I have 5 THWH's in use now.
A few guidelines I developed for myself:
Tank-type: best for low install cost and frequent use/ big family.
drawback is wasted energy when "idling"; limit to total HW supplied with
high demand (e.g. everyone takes a shower at once).
not a great idea for infrequent (e.g. shop) use
Tankless (THWH): higher up front cost.
electric cheapest/ simplest. need large diameter wire (mine use 40A
sub-assy's; each gets 8 ga pair; one ground per device)...a cost factor if
long distance to breaker panel. Can draw a lot...120A for 4.5 GPM.
Interestingly, cost of energy / BTU heat delivered to water is same for LP
and electric where I live; LP availability a factor to consider.
need to calculate worst-case simultaneous water demand (e.g. shower(s),
sink, dishwasher, clothes washer).
examples off the top of my head:
shower with std flow limiter..1.5 or 1.6 gpm;
sink: 0.5 - 1.0 gpm
Washing Mach (clothes): 1.5 gpm
Electric specs/ flow capacity:
all are 240VAC
20A 2.4 kWatt 12ga wire 0.7 gpm
40A 4.8 kWatt 8 ga wire 1.5 gpm
80A 9.6 kWatt (2) 8ga wire 3.0 gpm
I made up a spreadsheet in Excell with details if anyone wants them.
compiled from avail on-line specs from several sources.
The electric units all have about 98% efficiency; some have thermostats;
quality of build varies.
electricity is $0.08/ kW-h in my area.
my dble head shower uses ~ 3 GPM
cost of heat is ~ $0.014 / minute.
I take long (10 - 15 min) showers; never run out of hot water; well worth
the 20 cents I spend for heat.
sink in outbuilding/ shop:
20A/ 2.4 kW; trivial cost to wash my hands in hot water or whatever.
unit cost $160 if memory serves.
contact me if you need the files