The right boiler for the house

Will82

I have been going back-and-forth with a couple different contractors to decide which boiler is the right one to go with. Right now the decision is between a navien ncb-e combi-boiler 150E or 180E. But I have also looked at Bosch Lochinvar and Rinnai trying to figure out which system is the right system. I have attached the heat loss calculation. This house has one bathroom minute and is located in the Worcester Massachusetts supply water temperature can get between 38 and 40 in the winter. Just looking to make sure I make the right decision.

Ironman

Combi boilers are a compromise between trying to have enough btu's to heat domestic hot water and not be too over-sized for space heating. Like most compromises in life, neither side gets what they want.

You have a heat loss of 45k btus. That's what you need on the coldest night of the year; when it's 35* outside, you need half of that. So, your gonna put in a boiler a 150k+ btu boiler when most of the time you need 22k btus or less? If you system is zoned, then the problem is compounded with partial loading. The boiler will short cycle itself to an early death.

Go with a properly sized mod/con and an indirect. More$$ up front, but it will last twice long and give much better performance. And, you'll save$$ in the long run. Look at the the entire picture. Don't let the cheap gene blind you with the initial low price of a combi.

gschallert

I think the NCB-150E would be fine for you. Input is 12,000–60,000 BTU/H for space heating & 12,000–120,000 BTU/H for DHW. You've got just enough baseboard to run low enough water temps to condense on all but the coldest days and with one bathroom and those groundwater temps the combi should have no problem providing DHW. In-laws just went this route (modcon/combi) and it's working out great for them.

p.s. as Bob mentioned, zone size is important so I'd make sure all 70+ ft is one zone if it's not already.

Will82

Looks like the specs on the Navien NCB-E 150E will modulate 60,000BTU down to 12,000 BTU. Which I think will work good. But my concern is with the Hot water spec show 2.6GPM at 77 rise I know the waters coming in from the city can get very cold. So that is why some of the contractors are recommending the Navien NCB-E 180E since it will still modulate down to 14,000 BTUs. And give me Hot water at 3.4GPM at 77 rise which should be no problem. So I guess my question is would it be better to go with the bigger boiler since you're only talking 2000 BTUs?

gschallert

Will82 said:

So that is why some of the contractors are recommending the Navien NCB-E 180E since it will still modulate down to 14,000 BTUs. And give me Hot water at 3.4GPM at 77 rise which should be no problem.

What is your anticipated concurrent draw (in GPM)? You have one bathroom...

I'm going to assume you have the following fixtures:

Bathroom tub/shower
Bathroom faucet
Kitchen faucet
Kitchen dishwasher?
Laundry washer?

Of these how many do you wish to be running at the same time without compromising hot water flow? Is your shower high flow, i.e. rain or multiple heads? Bear in mind all but the dishwasher utilize mixing valves and the cold water mixed with hot will allow you to run a higher DHW setpoint with lower flow and still get endless "hot" flow out of the tap.

Will82

One bathroom with a 2GPM Showerhead. I do have all the fixtures you mentions but I do not plan on running them all at the same time. There might be a day or two we could be running 2 at the same time. Since the 180-E will modulate down the 14,000 BTUs on the Space heating does it make better sense to go with this one since it's only 2000 BTUs more vs the 150-E modulate down 12,000 BTUs. I know this is oversize the boiler but only by 2000 BTUs. I just don't want to get it installed and say why did not go with the smaller one.

gschallert

Will82 said:

One bathroom with a 2GPM Showerhead. I do have all the fixtures you mentions but I do not plan on running them all at the same time. There might be a day or two we could be running 2 at the same time.

So for the coldest days of the year when your inlet temps are lowest for that 2 gpm showerhead my calculations show you using 1.66 gpm of DHW using a SWT of 125 to deliver a 110 degree shower. If you raise the SWT to 140 you only need 1.4 gpm of DHW to deliver the same 110 shower. I honestly don't see using the edge case of "might be a day or two we could be running 2 at the same time" as a reason to oversize. If you knew you were going to put an addition on that added another bathroom at some point in the future then sure go for the higher GPM@X rise unit but from what you describe right now the 150E should meet your needs. The only edge case that I can see being an issue is showering while running a dishwasher when your inlet temps are lowest.

Will82 said:

Since the 180-E will modulate down the 14,000 BTUs on the Space heating does it make better sense to go with this one since it's only 2000 BTUs more vs the 150-E modulate down 12,000 BTUs. I know this is oversize the boiler but only by 2000 BTUs. I just don't want to get it installed and say why did not go with the smaller one.

In this case the space heating modulation shouldn't be your deciding factor, your hot water demand should. Me personally, I would go with the 150E but many others like a buffer, a "bigger than I need just in case" safety net. With only a 2k btu difference in low fire, if the higher GPM DHW gives you peace of mind, go for it.

Will82

Thank you
I am on a single zone 1 inch pipe monoflo system. Do I have to worry about any recovery?