I am (still) trying to decide btwn a 4 boiler or 1 boiler solution to my apartment renovation project.  My heat loss is 15k and 25k. 

The options are:

1. 4 boilers/indirects.  The lochinvar wbn050 is the smallest I've found (modulates to 10k); i'd get a 40gal indirect with these.  My concern here is that any boiler will be oversized for this application and I will be partially defeating the purpose of investing in high efficiency boilers. 
   
   
2. 1 boiler with btu metering.  I'm looking at the Veissman Vitodens 200 and triangle tube 120 gal indirect.   In this case I'd pay for the hot water but not the heat.  

All equipment will be in the basement.  

I will be using lower temp (150 design day) rads for this job.


Any feedback appreciated!

I suspect it'll be more efficient and less money up-front, less maintenance down the line to go with the 1-boiler metered-heat approach. Unless the apartments are occupied only by 16 year old cleanliness obsessives who take 3 hour showers it'll be hard to rationalize the up front & maintenance expense on the landlord-savings of letting tenants pay for the hot water.

If taking that approach, a 4 x 48" or longer drainwater heat exchanger as pre-heat to the HW heater will probably pay for itself in two years, since if it's serving 4 apartments you'll likely be seeing something on the order of an hour/day or more of shower use.

Design-day temps of 150F means you'll be running ~85% efficiency for those few cold hours, but well under 120F most of the time, for good condensing savings and efficiencies in the low-mid 90s. If it's just 4 zones (and not micro-zoned within the apartments) it won't short-cycle much, especially if you're using radiators with some volume to them rather than minimal-mass convectors. If it short-cycles during the shoulder seasons, 40-50 gallons of buffer tank plumbed in series with the output should take care of it. (A good system designer could figure out ahead of time if this would be necessary based on the radiation volume & heat load.)

I think the at-shower/sink version might be my best option for wastewater heat exchangers. My wastewater will probably end up in a 4" pvc with everything in it... I don't have the money to separate out drain lines. Or I probably won't have a plumber whose willing to do that...

But that means I'd have to buy four... Hmm. Maybe I could find a way to separate toilet from sink and shower: to run the 4 baths to one exchanger; plumbing the sink and dishwater drain to all hit the exchanger is going to get expensive...

At an installed price of about a grand each it's likely to be cost effective to do some drain separation to use one heat exchanger, if it's required by code.

And if it's just one indirect, there's no logical way to plumb 4 heat exchangers in series with the cold feed to the HW tank unless you put them all in series, which adds to head loss, and the active HX would be giving up some of it's heat to the other HXs along the path.

Most drainwater heat exchangers are double-walled, sufficient to allow both blackwater & greywater in most jurisdictions.