We use a Ruud hot water heater currently in our process to develop sparge water. it is not an in-line unit and does run out with extended use.
We are planning to change this over to an in-line unit in the future, but are doing the research now.
It really depends on the size of your mash tun, batch size, etc., that sort of thing. In our 7 Bbl brewhouse we probably sparge over about 124 gallons (4 Bbls) in 45 minutes. This equates to around a 2.75 - 3 gal/min demand for 170 F water.
Figure that your ground water is 55 F in the winter, and 60 F in the summer (for buried water lines entering into the building), you'll need an in-line unit that produces a 110 F - 115 F temp rise @ 3 gpm.
That's the type of data you'll need to select an in-line unit.
So.......it's kind of a serious of assumptions and calculations:
1.) Select or know the size of your brew system. this is where you get your mash tun size.
2.) Make some assumptions about how much strike water you will use for your mash.
3.) Make some assumptions on how much of the strike water will remain in the spent grains.
4.) Make some assumptions or know how much you collect to your brew kettle at the start of boil.
5.) From 2.), 3.), and 4.), you will know how much water you need for sparge over a minimum 45 minute time. By dividing the gallonage by the amount of sparge time you will have your gallon/min (or metric equivalent if using that system).
"......supplimenting an undersize boiler"............do you mean a steam boiler, or a brew kettle, or a hot water heater?
If you mean you have an undersize hot water heater, then there are a few threads on this site that answer questions regarding alternate methods for storing sparge water heated in the brew kettle.