Hydro-air / Boiler BTU output

mikeInMA

I am having a tough time wrapping my head around this one and was hoping someone had a good answer. I have a customer with a 10,000 sq foot oceanfront home built about 12 years ago so 2x6 exterior walls but not spray foamed. This house has two 190,000 BTU output boilers to deliver water to 5 hydronic air handlers scattered around the house. One boiler has failed and they have been successfully heating the home with only one boiler. Figure each hydronic coil at this house to be 80,000 BTU so 400,000 BTU's connected load being heated with 190,000 BTU's of boiler.
I have another customer with a 3000 sq foot house built late 1800's but fully rehabbed about 16 years ago. This house has two hydronic air handlers and both are fairly close to the boiler. The combined connected load of both air handlers coils is about 150,000 BTU. What size boiler is needed at this house? Right now it is a Munchkin on it's last leg that I do not yet know the BTU output of but it is irrelevant for this discussion since I am trying to figure out what size boiler I should be using. I am guessing the boiler is their 129,000 BTU model but it may be one size down.
So the question is do you size a boiler heating coils different than a boiler heating baseboard or other radiation? Can you use a smaller boiler because the water volume is lower? Can you use a smaller boiler because the coils are in close proximity to the boiler and therefore less loss through piping and a quicker recovery time due to short distance piping?

GroundUp

A BTU is a BTU regardless of the emitter, boiler, or water volume- do a heat loss analysis on the home. Your assertion of each coil being "X" BTU is nothing but a guess unless you have the exact flow rate and delta T for each one. Obviously those 5 coils are not emitting 80k each, because there's only half that much boiler available.

hot_rod

The best way to know without a WAG is to do a room by room load calc. Use the SlantFin calculator or any other of the online hydronic calculators.
A 10,000 sq ft home would need to be a load well below 20 btu' ft to heat with only one 190,000 input boiler. Possible but unlikely, at design conditions.

The structure dictates the load, not the boiler, not the air handlers.

On a multi zoned, multi emitter application a modulating boiler is a nice match, although you may be running out of condensing mod of a mod con with AHs.

mikeInMA

it's oil so the only wiggle is a nozzle change

clammy

Aside from agree w the above posts ,have you checked the noz size in the existing burners to see If they have been downsized that should give you a clue . Also have you been there while there what’re calls for heat ,if rampant short cycling then u know there oversized . On more then one large home I ve seen them just install 2 properly sized boiler just for redundancies not that the load is that big but they never want to be without in which case w a h9me that large I highly doubt there worried about fuel consumption it’s about fail safe and Redundancy and comfort with no emergency no heat calls being they have a secondary boiler Until that ones off on safety also.
Peace and good luck clammy

mikeInMA

So this particular 10,000 sq foot house uses the boilers to heat the rather large outdoor in-ground pool which is why i believe they did the 2 boilers. That being said though, they are not heating the pool at the same time as the house. I have not seen the heat exchanger for the pool so I do not know what the capacity is on that.