Inrirect and sizing

bill_105

So I do a heat loss and I call my supplier.the parts Woman tells me their out of that size but They have a 140,000 BTU. I then explain my Calc.came to 60,000 and that is a bit steep. Then comes the, "But when the indirct turns on it will need every bit of that 140,000" I disagreed. She was a litttle irked. Now in sizing a boiler how much consideration should the indirct get? Information or lieratue on this would be nice Now I've got to replace every boiler I did with an injdirect!! OH NO!! THX

Ironman

Depends...

On what the domestic heating load is. The larger indirect does not require 140k input to operate. It only requires 140k to produce at its rated capacity. 60k input would give it a 1.7gal per minute recovery rate @ 70deg. rise. 140k would give it a 4gal. per minute rate @ 70deg. rise. How many Gpm. do you need?

Brad White

Depends are what your supplier wears

Ironman is right, nail down your domestic load or at least your anticipated first hour rate, starting with a full tank.



That 140 MBH input beastie vs. your 60 MBH heat loss has the tail wagging the dog, if you are using an indirect.



(If you are using a flash heater, heat as you go, then re-think the system! I know a very "green" house near me, where I was brought in as a post-construction consultant. Building heat loss was about 24 MBH but it had an NTI of 200 MBH input as an instantaneous DHW source. Broke my heart because even on the coldest days, that boiler cycles, well, like Lance Armstrong going back to Sheryl Crow for make-up sex.)



Remember too, that the boiler capacity vs. indirect capacity only becomes an issue on the coldest design day and chances are your AM shower load in your DWH tank was charged during night setback.



The general logic I use is this, when sizing a typical single-family home with DHW via an indirect off the boiler:



1. Calculate the heat loss- you got that.



2. Run the DHW first hour load. Now, typically an average single-family 3 BR house does well with a conventional 40 gallon DHWH, maybe a 50. An indirect of similar storage capacity potentially could triple this production with the right boiler, but would at least double it. Absent better information, any 40 or even a 30 gallon indirect (accepting 80-100 MBH input), would easily outpace a typical 40 or 50 gallon fired heater with 33 to 45 MBH input.



3. Compare your typical boiler increments AND their turn-down ratios.



There are 50 and 60 MBH input units ideal for smaller or tighter homes, then 80, 90-94, 110 MBH and on up, depending on manufacturer.

With a 60 MBH heat load, I would be OK with an 80 MBH input boiler with a few cautions, such as timing your AM tank charge before morning warm-up. If in doubt, I would go up a size increment, but no more.  But if you DO go up to say that 90-110 increment range, will the boiler turn-down be low enough to minimize cycling in milder weather?



If your heat load were 40 or 45 MBH, all the better vs. that 80 MBH input boiler. Just be aware of your recovery time at 100% capacity to heating DWH.



Another factor in my decision tree: If your heating emitters are old cast iron, maybe on a large-pipe gravity system, that thermal mass will carry through a mid-day tank warm-up even on the coldest day, in a way you will not notice.