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Gas water boiler recommendation
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Steve Garson_2
Member Posts: 712
I own a WM GV5 with an IBR capacity of 106 MBH that I am preparing to replace next summer. It supports five hydronic heating zones and one indirect hot water heater. I am thinking that cast iron might not be the best option in light of summer hot water heating. Would appreciate some suggestions from pros on boilers to consider. I am located in Denver.
Steve
Steve
Steve from Denver, CO
0
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Sizing is important, that’s a pretty large boiler. How much gas did you use last year?0

883 therms for the year:
JAN: 152 Average temp: 35
FEB: 117 Average temp: 35
MAR: 152 Average temp: 33
APR: 135 Average temp: 43
MAY: 91 Average temp: 46
JUN: 73 Average temp: 61
JUL: 31 Average temp: 72
AUG:22 Average temp: 76
SEP: 12 Average temp: 74
OCT: 27 Average temp: 65
NOV: 80 Average temp: 51
DEC: 116 Average temp: 47
My home has two floor with 2,120 square feet combined, plus the basement which does not have a door separating it from the first floor. Will be adding a room with 250 sq feet. The house is well insulated with double pane windows, only south windows are lowE. Second floor is all cathedral ceilings. I can put the time into a ManualJ calculation if that is the right thing to do. In the winter, Denver has cold nights and warm days. At 6AM this morning, it was 17, but by noon, it will be 60.Steve from Denver, CO0 
Nah a manual J is a waste of time if you have the fuel usage. You need a much smaller boiler than you have now.0

Without the manual J calculation, what’s the right boiler size? Linear feet of baseboard? I always thought that proper calculations are needed.Steve from Denver, CO0


@Steve Garson_2 it’s a simple calculation: take your 831(12 months x 12 therms (this is roughly DHW)) = 687 for heating. 20% of those were wasted, so 550 therms output. Divide that by Heating degree days for Denver, using a base of 65 to start. I’d guess that’s probably around 5000 HDD, but you’ll find the right answer. Then find the design day  I’m guessing 0 degrees average temp, which is 65 HDD. So you take the 550 therms x 100,000btu/therm / 5000HDD = 11,000 btu/HDD. Then multiply that by 65, your design HDD and divide by 24 to get hourly load. That’s 30,000 btu/h. The smallest modulating boiler is much larger than that (around 55,000) but an 80,000 btu boiler can modulate down just as much as a 55,000 can, so you could just go with that. If cast iron, try something closer to 30kbtu.0

how how can that calculation account for a two day span of zero degrees?Steve from Denver, CO0

That’s the 65 HDD day. Heating degree day base temp 65 = 65 average temperature. Number of days at that temperate are irrelevant. You have an efficient house!0

The heat load calc is important in that it tells you the room by room load and if there is enough fin tube in each room to cover the load.
If you upgraded insulation or added rooms you should have an accurate load calc performed
If you go with a mod con, it would be good to know how low of a supply temperature you could run to the fin tube and still cover the load. The plus side of mod cons is the can run low temperatures efficiently, but also modulate to the changing loads.
I’d take the time to do the load calc so you can maximize your system and better decide between a cast or mod con.Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream2 
He should know whether or not there is enough radiation since he lives there so I don’t really see the value in the heat loss. If it were closer, maybe, but using only 830 therms with an existing indirect implies a load that a 80kbtu modcon will have no issue with.How many linear feet of baseboard is there?0

Could be it is over radiated if the structure has seen any energy upgrades. So that could allow lower swt. Sounds like he is willing and able to do a manual J.Hot_water_fan said:He should know whether or not there is enough radiation since he lives there so I don’t really see the value in the heat loss. If it were closer, maybe, but using only 830 therms with an existing indirect implies a load that a 80kbtu modcon will have no issue with.
How many linear feet of baseboard is there?
What is the downside of spending a few hours to get current load data? It will also show where upgrades could be made to lower the loads.I’d even consider a blower door test to see where infiltration loads could be lowered.
A load calc is the roadmap to a good design, and efficient operating conditions.
https://www.caleffi.com/sites/default/files/file/idronics_25_na.pdf
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 
I think it’s just spinning wheels. I want the lowest possible water temperature used, but effort should be directed towards valuable, actionable activities. I suspect Steve’s over radiated but if by some chance there’s a room that truly needs high temp water, he’ll know when the swt of 150 or whatever doesn’t keep up on the coldest day in a particular room. Maybe that prevents condensing 100% of the winter, but 90% is good too when total gas usage is so low. A manual J would possibly find the lagging room, but doesn’t add radiation and/or reduce heat loss, so it provides the same info as glancing at the thermostat on a cold day.0

You raise a good point, my living room has inadequate radiation, being 3 degrees cooler than the room with thermostat, I’m planning to add more, so I’ll be the Manual J calculations.
Steve from Denver, CO0 
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