Gas Combination tankless boilers

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CRAZYBOBDS
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Post By: CRAZYBOBDS » Sun. Feb. 11, 2018 10:50 pm

Hey everyone, a little off topic but was looking for some insight. I was looking to replace my 1940’s oil boiler, which feeds a water to air heat exchanger in the forced air ducting. The guy who owned the house was a commercial heating person and the system is crazy.. and also uses oil like it’s a commercial building I’m heating.

I was wondering if anyone has experience with condensing gas combination boilers that i can use for hot domestic water as well as heat. I have about a 2400 sq ft house.

I was looking at a Navian NCB-240 . Before i got too far along i was looking for advice. I do have a new gas line installed in the yard. Has not been brought inside yet.

Thanks. Bob

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Post By: Rob R. » Mon. Feb. 12, 2018 5:53 am

Many heating pros do not recommend them because it leads to oversizing in most homes. Depending on the heat load of building, that may not be an issue for you.

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lsayre
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Post By: lsayre » Mon. Feb. 12, 2018 6:19 am

If we had natural gas on my street, my house and its DHW would be heated by a Navien NCB-150E.

I'm heating 1,680 sq-ft, plus a "walk out" basement at 840 sq-ft, and a garage that is also 840 sq-ft. The garage T-Stat is set at 45 degrees. House built in 1964. My locations historical measured 'HDD's' are somewhere between 6,350 and 6,500.

My bet is that the model 150 or model 180 would suffice for your needs. Your old oil boiler is probably on the order of only about 55% efficient, and it is only output BTUH that matters. The NCB-240E is a monster.

My house was heated for many years (10 of them by me) with only a 13.5 KW resistance boiler (that is now my back up heat source). 13.5 KW = 46,062 BTUH input/output.

The NCB-150E is rated at 60,000 input BTUH for home heating.

60,000 x 0.92 = 55,200 BTUH (where 0.92 is the efficiency of the boiler [Navien says 95%, but I'm playing it safe here])

55,200/1.15 = 48,000 BTUH (where 1.15 is the current industry standard hot water gross/net "pick-up factor")

* Navien actually rates their 150 model at 49,000 net home heating BTUH, so my method as seen above is pretty close *

In addition to this, the NCB-150E's separate DHW heater is rated at an input of 120,000 BTUH. Whole house on demand DHW requires way more BTUH than does typical home heating. Per Navien's figures, their 150 model can raise a DHW flow of 3.1 GPM by 77 degrees F.

Almost all older homes existing heating units are grossly over-sized. Energy was cheap back then, efficiency was terrible, and over-sizing to assure sufficient heat was the easy way out.

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Post By: franpipeman » Mon. Feb. 12, 2018 8:07 am

Gas condensing boilers have high efficiency(95% when the return water is 130 degrees or less.) 130 degrees can heat a lot of house depending on how they exchange heat to the living space. If were surrounded by radiators and the water was constantly 130 degrees you would be quite warm or radiant floors The gas condensing boilers usually have computer operated system and can at the touch of finger, when you learn the system, change the operating temperatures quite easily. You could have the water setpoint be 180 degree which you may need for a coil inside a air handler as yours now proper duct sizing is on the table also ., but the efficiency will change as it leaves condensing mode. then when ti operates to heat a indirect hot water heater will have a different temperature range and a separate pump . I use it for radiant floor heat where the boiler senses the outs door air and changes the heating water temperature to ahigh 130 if its 0 to lower temperature as the out door air is in the forties and 50 till it turns off at 53 . All of which you can change.
Sizing as mentioned is important oversizing is not good as you want the system to have long run times. Planning is essential for proper operation. If you look at homes with pvc exhausts you will notice many seem to run non stop but the volume of vapor plumage decreases based on temperature of operation . Accurate heat load assessment is essential. My condensing propane/gas boiler can run as low as 26 percent to 100 percent . Designers must alway plan for highest demand yet that is not needed most of the time. They have so many different modes of operation its too much to mention here. Some homes are not suited for it as due to insulation level and methods of heat exchange, there is a lot of technology involved and some folk dont like that either, as they are mysterious some time and a control board could cost 400. bucks for replacement .

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Post By: McGiever » Tue. Feb. 13, 2018 5:22 pm

$400.00 can buy a lot of NG/LP for a not-so-efficient heater w/o a control board, no?
What a 'racket'!!!

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Post By: 2biz » Wed. Feb. 14, 2018 7:44 pm

I don't have direct experience with a combination boiler, didn't know they made such a thing until reading your post. But I do have experience designing and installing a condensing demand Takagi water heater for hydronic heating at the carwash I own. 3-4 years ago they didn't offer a dual purpose boiler for both DHW and heating....The one I installed is 199K btu demand that replaced a 350K btu boiler...The amount of $$$ in NG bills I saved in the first year more than paid for the whole project. This heater heats about 2000 sq ft of concrete floors, in the open, with no doors on entrance/exit to 4 bays. I have one bay with no roof, completely in the open for Semi's. All the bay floors are heated.

I ended up using the Primary/Secondary plumbing system and fire the 2 pumps supplying the boiler using a 15° differential Aqua stat located in the return plumbing. The entire system is energized at 32°, the zone circulator runs 24-7 till temps rise above 32°. Because of the differential, the boiler kicks on for roughly 15 minutes, then off 15 minutes eliminating short cycling. The colder it gets, the on time is more and off time is less. Even down to -16° tested, the floors stay ice free and on time is like 20 minutes, off time is 10 minutes...That's a 46° delta out in open air at -16° keeping the slabs ice free! I never have to adjust the return temp t-stat, its all self regulating to take on heat load. BTW, I do this with the heater set at 105°. The two boiler pumps energize at 55° return temp and off at 70° to keep bays ice free. I couldn't have set it up any better. Its saved me a fortune over the old 70% efficient system (When new)....Probably down around 30% since it was ran in a condensing state for too many years before I bought the wash.

So yea, if I had NG available and knowing what I know now about hydronics, I wouldn't hesitate installing one at home. What I have at the Car Wash works awesome. Here in Southern Ohio, the winters are getting brutal. The 199k Takagi sips NG compared to the old 350K boiler and does the job with ease. Having the additional DHW exchanger at home would be a bonus... BTW, I installed a 199K BTU 55 gallon Phoenix Condensing Modulating water heater for hot water at the carwash...It can heat 400 gallons an hour. Needless to say I never run out of hot water and its super cheap t operate. Condensing/modulating heaters is where its at!

BTW, if you do install one, on the heating side, check out the primary/secondary way to plumb. It works great! ..Some pics of my install...

Image

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Post By: 2biz » Wed. Feb. 14, 2018 8:01 pm

Just for proof that it works!

Some stats:

Date Range: Feb. 11th thru March 12th (29 days in Billing Cycle)

Temperatures:
21 days Max. temp was below 32°
29 days Min. temp was below 32°
19 days Min. Temp was below 15°
7 days Min. temp was below zero
Coldest night was -15°

NG usage in this billing cycle was 449ccf for a total of $440. The floor heat ran 100% of the time below 32°. This usage includes the floor heat, 199K btu WH, 40K btu ceiling furnace, 2 burner wall heater that I run 1 burner below freezing and 2 burners below 15°…

Image

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Post By: David... » Thu. Feb. 15, 2018 9:14 am

The min/max input ratings for heat are 18000 - 120,000 btu. I don't think this is over sized since it can modulate down quite low. These things are marvels of engineering.
As has been mentioned, the savings will not be as great when 180° water is used. I would not have one of these in my home without a backup source of heat. Every part in these is proprietary. That usually means expensive and can mean not immediately available.
A cast iron gas fired boiler, while not as efficient, is going to be cheaper to repair, parts readily available, and should last much longer.

David

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Post By: 2biz » Thu. Feb. 15, 2018 8:48 pm

David... wrote:
Thu. Feb. 15, 2018 9:14 am

A cast iron gas fired boiler, while not as efficient, is going to be cheaper to repair, parts readily available, and should last much longer.
While I don't completely disagree, I wonder what the cost difference to replace is? The 199K BTU Takagi I have is only $1034. And its a drop in....Can be replaced in less than an hour with all the shutoff valves I have in place. At that price, you could afford to have one on the shelf! And you know what happens when you have a spare on the shelf!? LOL...

https://www.supplyhouse.com/Takagi-T-H3-DV-N-T-H3 ... -Heater-NG

It more than paid for itself in just a few months the first year. I've ran it for 4 years now, so I am way ahead in case something goes wrong with it! NG wasn't the only savings...The original circulator pump had a 12a 220v motor....I do the same thing only a lot more efficient with a 2a 120 volt taco pump....That reinforces a previous comment on how installers oversized everything, just in case!

When I installed this, the "Carwash Boiler Pro's" said it would never work. They wanted to sell me a $10,000 boiler and charge me another $5,000 to install. Its no wonder the "Pro's" try to intimidate you into making you believe they know best!

I agree I would not want something like this to be my only heating source even though it works so well, just as the new coal furnace isn't my only heating source. I don't think anyone should trust just one source...That only makes good sense.

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Post By: David... » Fri. Feb. 16, 2018 8:38 am

Any properly sized and installed heating system will perform better than any improperly sized and/or installed system.

David

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