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How small is too small to fire an Indirect?

This is my setup:

Lots of pipes into my chimney. 3 individual gas-fired water heaters, 2 gas-fired WM CGA-25 boilers and 1 Laars NP-85 oil-fired boiler. The Laars will most likely be replaced with a WM CGA-25 or equivalent when the time comes.
When the water heaters go, my preference would be to replace them with modestly-sized indirect heaters. Probably something like 30-gallon units. With priority control, would a small boiler like the CGA-25 (38MBH Net) be able to handle an indirect? Each water heater serves one ~1000 Sq ft apartment, 2-bed, 1-bath, washing machine, no dishwasher.
You can see the last vestige of the original one-pipe steam system: The trusty WM SGO-4 that was converted to hot water when the heating was split up for separate metering. It has since sprung a leak, and was replaced with a correctly-sized CGA-25. The Superior gas boiler (far right) was replaced with a CGA-25 as well. It was a really horrible boiler, standing pilot, noisy, and had a no-heat issue almost every season. AND it was 3x too big.

Lots of pipes into my chimney. 3 individual gas-fired water heaters, 2 gas-fired WM CGA-25 boilers and 1 Laars NP-85 oil-fired boiler. The Laars will most likely be replaced with a WM CGA-25 or equivalent when the time comes.
When the water heaters go, my preference would be to replace them with modestly-sized indirect heaters. Probably something like 30-gallon units. With priority control, would a small boiler like the CGA-25 (38MBH Net) be able to handle an indirect? Each water heater serves one ~1000 Sq ft apartment, 2-bed, 1-bath, washing machine, no dishwasher.
You can see the last vestige of the original one-pipe steam system: The trusty WM SGO-4 that was converted to hot water when the heating was split up for separate metering. It has since sprung a leak, and was replaced with a correctly-sized CGA-25. The Superior gas boiler (far right) was replaced with a CGA-25 as well. It was a really horrible boiler, standing pilot, noisy, and had a no-heat issue almost every season. AND it was 3x too big.

Nick, Cranston, RI
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Comments
IDK about your locality, but around here, you can't vent different fuel fired appliances into the same flue.
You can choose to do what you want, but you cannot choose the consequences.
I don't like the baro's on the end of a tee, not code for oil. Also, the pancake reducers add alot of Total Equivalent Pipe Length to the fixtures.
I wasn't sure if it was a true 1:1 relation between the gas-fired heater BTU rating and a boiler/indirect rating. That's VERY good news.
As far as height, it's a 3 story house. Haven't actually measured it, but I'd guess about 30'? It's an exterior chimney so I'm sure that's not helping things.
I'm pretty sure code here allows for oil and gas in the same flue. I see it a lot here in RI.
I did buy Dan's book Pumping Away, and I plan on relocating the circulators. I calculated the head loss of the 3rd floor loop with all the 3/4" PEX, copper and fittings and the supplied 007 wasn't enough. I ended up installing a 00R 3-speed and reversed the supply and return to better balance the heating (Hottest water to the end-of-loop living room with cathedral ceilings, and coolest water to bedroom nearest to the riser from the basement, which is also over-radiated. Worked perfectly!
Oil & gas in the same flue are permitted by NFPA 211, with some notable rules and regs. Just a snippet:
9.8 Interconnection
9.8.1 Connectors serving appliances operating under natural draft shall not be connected into any portion of a mechanical draft system operating under positive pressure.
9.8.3 Gas appliances and appliances burning liquid fuel shall be permitted to be connected to one chimney flue through separate openings or to be connected through a single opening, provided they are joined by a suitable fitting located as close as practicle to the chimney and provided both of the following apply:
(1) Sufficient draft is available for the safe combustion of each appliance and for the removal of all products of combustion.
(2) The appliances so connected are equipped with primary safety controls and all appliances are located in the same room.
9.8.4 If two or more openings are provided into one chimney flue, the following stipulations shall apply:
(1) They shall be at different levels.
(2) The smaller connector shall enter at the highest level consistent with available head room or clearance to combustible material.
I've heard a rule of thumb about oil being below gas when they enter the flue. While that's often correct (oil appliances are typically larger than the gas appliance in this situation, right?), it may not always be.
With low mass like fin tube, best to use outdoor rest and run it at 140-160F most of the time except the coldest weather depending on radiation vs. heat loss. Rather than cycling it on and off and having the room temp swing up and down. With cast iron radiator you can cycle them because they stay hot for 15-20 minutes after shutdown. Fin tube will cool off a lot faster.
Outdoor chimney is definitely an issue. Maybe this is a good application for a combi? Switch to PVC vents. Otherwise, power vent boilers and use stainless steel?
Could go to a single boiler and just use 3 secondary pumps with 2 zones on each, 1 for DHW. Then bill the unit based on a hour meter on each pump. Not as simple as utility bill I guess but reduces maintenance costs and venting issue goes away.
But for some, a bill from the landlord could cause some whining and phone calls. Having been down that road I would keep the 3 units.
Plus if one unit down the building might not freeze up with 2 others working. Common walls, floors, ceilings etc.