Want to move from oil to gas boiler. Best way forward?

Widdershins

I live in Lancaster, PA. We have an old, inefficient oil boiler. For a number of reasons, we would like to replace the boiler with a high efficiency gas combination (combi) tankless boiler / heater. I was / am interested in Navien, and was settled on the NCB-240. Its specs seem excessive for our ~2-story, 1200 sq ft (7 hot water radiators on a single zone), 1 bath home, but I thought better to oversize. My assumption is that an oversized unit doesn't affect operating efficiency, or certainly not to the degree that an oversized wood-burning appliance does...

Anyway, I contacted our gas utility to inquire about gas pressure. The response said, 'your home is located on a street that has LOW PRESSURE MAIN - you will need to purchase standard equipment with a minimum input rating of 3.5" of water column. If he (my contractor) can provide you with 2" water column that would be best.'

I'm not the sharpest tool in the shed, but I think my utility rep meant that I should purchase equipment with a maximum input rating of 3.5" w.c. and preferably a boiler that doesn't require more than 2" w.c.?! At first I thought great, the Navien NCB-240 would work. After all, the NCB-240 brochure (<a href="http://www.navienamerica.com/__DATA/ProductDocument/2013/11/12/13-1108%20NCB%20brochure%20Flip%20book.pdf">http://www.navienamerica.com/__DATA/ProductDocument/2013/11/12/13-1108%20NCB%20brochure%20Flip%20book.pdf</a>) on page 3 states, "Our negative pressure gas valve ensures maximum performance all the way down to a minimum gas pressure of 3.5" of water column." I passed this information on to my contractor. He responded saying that the pressure supplied by the gas main would be insufficient for the boiler. I'm a bit frustrated because a) my contractor isn't being very responsive with suggesting alternative systems and b) just today I got a flyer from my gas utility, urging me to switch to natural gas heat and save up to $1500/yr. The only currently installed gas appliance is a gas range/oven.

Does my utility and/or contractor know of what they speak? Any chance the NCB-240 (or a competitive unit) would work for me? Any other ideas / suggestions?



Thank you!

Tim McElwain

Those are very low

pressures that you are being offered in fact most of your equipment (gas range) will not even work with 2" W.C. or even 3.5" W.C. The 3.5" W.C. number is what most conventional gas valves give for an outlet pressure and the inlet needs to be around 6 to 8 " W.C. I would have someone check the gas pressure you are getting now.



You should have a heat loss done and a good contractor will do that for you. They will also typically be able to match you up with a boiler that will pretty closely meet your heat loss. The output of the boiler should equal the heat loss of your house so that you will be sized pretty close to what you need. Oversizing can cause short cycling.

Snowmelt

Navien

If your in pa navien is having a class next week on there units you might want to go.

As far as gas pressure is concerned most residential gas is ,comming at 1/2 psi, don't confuse that with water column. 12 inch on water = 1/2 psi (I may be wrong but that's a safe number to go by, you have to look it up.

Then there's charts on what you have in psi and how many btu, vs how many feet your going. All you have to know as a h.o. Is increase the size of the pipe for more volume.

As far as units are nothing wrong with the nav 240. Just have to be piped right..

As far as doing a heat loss, these units are a different animal, just because it's 199,000 btu doesn't mean it heats up to 199,000 for space heart. It dose 100,000 btu and it modulates down. Also make sure you get outdoor reset . The 240 has it but it's an extra.

SWEI

Where do we start?

Minimum modulation on the NCB-240 is 18,000 BTU per hour.  You have a 1,200 square foot house.  It s NOT "better to oversize"and it WILL affect efficiency (as well as longevity and comfort.)  You must start with a heat loss analysis of the building.  Unless the house is very old and has leaky windows, I suspect you will find that your house needs less heat on a design day than the smallest gas-fired boilers you can buy in in North America.



Among many other issues with your plan is the fact that all of your gas pipe must be sized to the maximum firing rate of the appliances.  3.5" WC is _very_ low pressure.  Your your contractor or designer needs dig out the tables and "do the math from scratch" for the whole house.  If I were asked to do this by a client I would have a frank conversation with one of the utility engineers about real-world probability of low pressure conditions during peak load times before I specified either the appliance or the piping.

CMadatMe

The Blind

The btu/hr output ratings are for DHW not heating period! The on board boiler pump will move 5gpm across the HX that's it period...Now how many btu/hr can you get out of that unit Snowmelt? After that the question is at what supply temperature..



And as a FYI they are not a different animal and cannot change physics..



To the original poster I'd take Tim's and Kurt's advice. They both are correct.

Widdershins

More info

Where in PA is the Navien class? I checked their website and didn't see schedule information or really any information for consumer training/seminars.



Yesterday I found out the following about my NG supply line (between the street main and our house):



Installed: 5-2-1985

Pipe: 1" plastic

Pipe Length: 59 feet

Pressure: 7" w.c.



Heat loss calculations and other considerations aside, I think this at least means the NCB-240 is more of a possibility.



More information gathered from Navien's specs on the NCB-240:



Gas Input

Space: 18,000-120,000 BTU/Hr

DHW: 18,000-199,900 BTU/Hr



Natural gas supply pressure from source

3.5" w.c. - 10.5" w.c.



Natural gas manifold pressure, min to max (don't understand this one at all):

-.06" w.c. to -.98" w.c.



The NCB-240 utilizes existing 1/2" gas line. Negative pressure gas valve and fan with dual venturi operation allows for better adaptability to low gas pressure applications. This makes it possible for the NPE (what is this?) to be installed 1/2" gas lines up to 24'.



Can anyone make more sense of this?



Thanks for the responses so far!

SWEI

More important info

"Heat loss calculations" should never be put aside.  I would be quite surprised if your design day heat loss comes in higher than 40,000 BTU per hour.  Installing a boiler with more the three times the required capacity is highly unlikely to produce a happy ending.

RobG

What SWEI Said

What SWEI said is absolutely correct. As well, see the post in this forum titled gas piping.



Rob

Widdershins

Not an exact science

You did see where I posted that the unit's advertised space-heating range is 18,000 BTU/Hr to 120,000 BTU/Hr, yes? By the way, my old oil boiler's net output is rated at 140,000 BTU/Hr.

Much as we'd like it to be, home heat loss analysis isn't a precise science and I'm beginning to wonder whether your insistence is disinterested. Do you perform energy audits or do something similar as part of your job?

SWEI

Indeed, not an exact science

yet still better than no science.  Sounds like your old boiler was indeed massively oversized, but that hardly makes oversizing the new one a good idea.  The old boiler was a high mass design, both in iron and in water.  The new boiler is a low mass design, with very little water content.



The second sentence in your initial post demonstrated an apparent lack of understanding of the proper selection and application of heating appliances.  I made an effort to remedy that by providing you with correct information.  You have chosen not to discuss the merits of your proposed plan, but instead to question my intentions and my integrity.



I do not perform energy audits, though I do employ many of the same tools and techniques which energy auditors use.  I am located more than 2,000 miles away from PA and do not perform work there.  I have nothing to gain or lose from this other than my time.  Good luck with your project.  Please consider employing the services of a qualified designer or installer before you decide where and how to spend your money.

CMadatMe

Here's Your Science Widdershins

Yes they publish a 120,000 btu/hr output. But what they don't tell you is at what water temperature. The on board circulator can only move 5gpm across the heat exchanger so if your heat loss is let's say 100,000 btu/hr and you need 10gpm across your system do you know where the other 5gpm comes from and its effect on the heating systems water supply temp? Snowmelt who claims is a Navien installer can't answer it so I wouldn't expect a homeowner to know it but I know SWEI does..

Steamhead

I would say

that at least 70% of boilers we encounter are oversized, some by ridiculous amounts. This is what happens when people guess, or size boilers based on what was there, etc.



Why would you want to buy a boiler that's bigger than you need? It will cost you more to buy, will need bigger gas piping, and cost more to operate. Makes no sense to me.



A heat-loss calculation is not a big deal, now that we have computer programs to do them. Do yourself a favor and have a calculation performed.

Holowinko1

domestic hot water

I am a certified by Navien for service and installation. 

When chosing a combi boiler you want to size the unit to handle your domestic hot water. The unit will modulate and fire itself according to the load for the heat side of it. I highly suggest using a circulator like the taco bumblebee or equivalent. The bumblebee has a sensor for the supply and return of the secondary heat loop and will adjust its pumping volume accordingly. Without a variable circulator you will never get the most out of your heating system.

The NCB-240 will be just fine for 1 bathroom and heating of a small home. 

It's the domestic you need to worry about.  In an area like PA. where it is very cold in the winter I would suggest the 240.

As far as the gas supply... have the gas company answer your questions. The contractor is only hooking the Navien up to what they supply you with.

Good Luck

Rich_49

Combis

The only combis you hear about are tankless . Why is that ?  Because the guys installing them don't know or only care about the wrong thing , first cost . After they install them and add P&S piping and do it wrong and come back to your house 5 times and your still unhappy then you go to the wall for a 300 post discussion you still have garbage .  Here , this is made right here and serves your purpose better . With a 1 bath house it is plenty and your efficiencies will be just fine with the right control strategy . This is a combi , just the right kind .  http://www.htproducts.com/versaflame.html

  Keep your money here at home and have it done right .  All you need extra with this is a nice Taco I series outdoor reset valve . Tankless looks sexy but when you get to the party she's really just an ugly fat chick .

Rich_49

Holowinko

Are you empl;oyed by Navien or did they certify you to install their product ?

Rich_49

Fairness

Chris and Snowmelt ,

   Chris , in all fairness the Navien would be able to provide 100k for space heating at a 40* Delta T . Lest we never forget the Universal Hydronics Formula . 

George the Navien will never be able to provide 199K for space heat . Again ,Lest we never forget the Universal Hydronics Formula .

Zman

Something for nothing...

Rich,

I completely agree with you about the need for more buffer and storage in these systems.

Companies like Navien will always sell product to their customer base.

There are people out there that insist that you can get something for nothing. The navien is irresistible to folks with that mindset. Just don't try to confuse them with all this talkabout quality, mass, buffer, short cycling of gas line sizing. The Navien costs less, has a high efficiency sticker on it and makes the house and hot water warm (usually) end of story.

Carl

Rich_49

Absolutely

correct Carl . I apologize to any who I may have offended with actual theory which makes sense .

 Funny thing happened last June 2013.  I was in Cranston and there were a bunch of guys there that work for Navien . Some were from the West coast and some from Cherry Hill , All tech support guys though . What was amazing is the fact that while overhearing their conversations and listening to their questions it occurred to me and many others that these guys knew nothing about hydronics , they only had manufacturer product specific training and knew NO SCIENCE .  Really was illuminating to me . Just sayin

jonny88

navien

why are so many people stuck on Navien.I am looking forward to five years when the problems arise

Rich_49

Something for nothing Syndrome

They all are enthused with the idea that they are gonna get what we offered them in our bid for much less .   Space is the other thing , you mean I can have an extra 25 sq ft for all my s$%t that I don't need .  And then there's the Walmart theory , we can make it somewhere else cheaper .  Go ahead everyone support S Korea all the while you **** about the economy here . 

jonny88

navien

how many more freaking posts are to be hijacked by Navien.Christ I can go tomorrow and get certified.What does that mean?I actually went to a class and was shall we say unimpressed with the teachers knowledge.Dont want to derail this but when Swei and Chris voice opinions I for one have the good sense to sit back and listen.Navien in my opinion is like a water heater =a throw away item.

jonny88

navien

how many more freaking posts are to be hijacked by Navien.Christ I can go tomorrow and get certified.What does that mean?I actually went to a class and was shall we say unimpressed with the teachers knowledge.Dont want to derail this but when Swei and Chris voice opinions I for one have the good sense to sit back and listen.Navien in my opinion is like a water heater =a throw away item.