Sizing a boiler

In_New_England

Hello experts,

I live very near Boston, MA. We have a 150 BTU WM oil boiler we are replacing with a gas boiler.

House is 1400 sq ft, 87 ft of fin baseboard, one shower, one dishwasher, one washer.

My plumber offered me a Lochinvar Noble 199 combi or a Alpine 105 with indirect. (I didn't consider the gas gun conversion because I got the impression no contractor wants to really deal with those.)

I picked the Noble 199 but am having second thoughts re: sizing.

From my calcs the maximum output from the baseboard (180F) is 50KBTU and minimum (120F) is 18KBTU.

From my fuel consumed in Jan/Feb (150 gal) I estimate an average output of 25KBTU in the deep winter months.

Since the 199 turns down to 18K output I'm guessing I'll be OK, but what do you all think?

Thank you.

Hot_water_fan

Neither of those boilers has a particularly good turndown - I'd be looking for something that has a range like 8,000 low -80,000 high or 10,000 - 100,000. Both of those manufacturers make boilers like that. The average fuel usage isn't important: what you want to know is gallons/heating degree day, which that can be use to estimate heat loss. So say it's something like 1200 HDD for that time period and 150 gallons: That's 138,000 btu/gallon * 150 / 1200 = 17250 Btu/HDD. On a day with an average temperature of 5 degrees (that may or may not be your design temp), that's 60 HDD using a base temp of 65. So 60*17250/24 (hours/day) = 43125 Btu/hr input at design temp, about 80% of which makes it inside your house.

GGross

They sized the Noble combi based on it's domestic water production, I do not think that your plumber is under the impression you need a 199 for heating, as he offered a smaller btu boiler and tank as an option, leaving you to decide, "tank or tankless?"

In_New_England

GGross said:

They sized the Noble combi based on it's domestic water production, I do not think that your plumber is under the impression you need a 199 for heating, as he offered a smaller btu boiler and tank as an option, leaving you to decide, "tank or tankless?"

Agreed. My question is if the 18k lower end of the 199 is still too high given my back of the envelope calculations for heat.

EBEBRATT-Ed

@In_New_England

I would use a 30-gallon indirect tank and a lot smaller boiler. Installing a boiler capable of 105k or 199k makes no sense to me at all. Why install a boiler larger than the baseboard will output?

In_New_England

EBEBRATT-Ed said:

@In_New_England I would use a 30-gallon indirect tank and a lot smaller boiler. Installing a boiler capable of 105k or 199k makes no sense to me at all. Why install a boiler larger than the baseboard will output?

These are modulating boilers. The Lochinvar can modulate down to 1/10 it's maximum, to 18500 BTU, for example. The upper rate is used for the DHW.

In_New_England

EBEBRATT-Ed said:

@In_New_England I would use a 30-gallon indirect tank and a lot smaller boiler. Installing a boiler capable of 105k or 199k makes no sense to me at all. Why install a boiler larger than the baseboard will output?

Your point about not using a combi is well taken, but I would like to go tankless and save some basement space.

pedmec

I use the 199 noble combi"s and they work great. i like lochinvar. With the ability to lower the firing rate your gonna be able to match your connected radiation load pretty close and yet still be able to have plenty of domestic hot water. you get the maximum btu output for domestic hot water. its a great choice.

Hot_water_fan

Your point about not using a combi is well taken, but I would like to go tankless and save some basement space.

You can fit an indirect directly underneath many wall hung boilers if you want.

hot_rod

A 150 Nobel would give you 3 1/2 gpm at a 77 degree rise. Expect 3 gpm with cold incoming water, below 40F for example 

would 3 gpm serve your needs? The could be two showers running at the same time
We got by fine with a 110 Nobel, consistent 55 incoming however.

In_New_England

hot_rod said:

A 150 Nobel would give you 3 1/2 gpm at a 77 degree rise. Expect 3 gpm with cold incoming water, below 40F for example 

would 3 gpm serve your needs? The could be two showers running at the same time
We got by fine with a 110 Nobel, consistent 55 incoming however.

Yes, we only have one shower. Very unlikely we'd be showering, doing the dishes and laundry at the same time. A 150 would do fine, wouldn't it?

hot_rod

yep, And save some cash to put towards some heat loss upgrades. Find the infiltration points and seal them up

ScottSecor

"Since the 199 turns down to 18K output I'm guessing I'll be OK, but what do you all think?'

I think the 199 will short cycle almost every day of the heating season. The only time you should expect longer cycles is at or near design day conditions. In your area this appears to be when the outside temperature hovers around twelve degrees. I much prefer to use a smaller boiler with in indirect tank for your dhw needs.