NG Boilers: Condensing vs. Traditional

[Wet1]

After reading Webmaster’s gloating about NG for the past couple of months, I finally decided to track down my local NG company last week. As it turns out, their gas line comes down my street (despite the fact that nobody on my street uses NG) to feed additional streets and areas. I talked to their sales rep and he said they would run the line and install the meter to my house on their dime, plus offer a $300 rebate towards the boiler purchase (the boiler distributor matches this $200 rebate as well). The price for NG in my area looks like it’s around $1.75 to $1.80 per CCL with all the fees, and these prices are regulated since it’s a utility… but there could (most likely will) be a price increase in Nov.

Based on today’s prices and my low fuel usage, the cheap $1000 boilers ($600 after the rebates) would probably be my best bet, but if NG prices go up a lot in the future, the more expensive/efficient boilers might be worth considering. OTOH, I’m not sure I really want to deal with a condensing boiler. Where can I learn more about how these work and what’s involved with installing them?

So what are the general thoughts on the condensing vs. traditional boilers? I got prices on three models of boilers. The first two are traditional boilers rated at 80% and 83.3% and cost $1050 and $1250 respectfully (w/o rebates). The higher efficiency of the two would be a direct vent so I figure this model will cost about an additional $300 for the venting materials. The condensing boilers are rated at 93%+, but the cost for the boiler itself was going to be around $2500 to $3000. This doesn’t include parts or installation. I can install the traditional boilers myself, but I know nothing about the condensing models and what it takes to install one and what additional parts are needed to complete the system (any ideas?). I was also told I could not install the condensing boilers myself (is this true?) and that installation isn’t cheap on these models (any idea what this cost?). I need to get a better feel for what’s involved with completing the condensing boiler systems and the total cost of doing so before I can determine the payback… but, my guess is gas prices would have to be pretty high for it to be worth my while to go with the more efficient condensing boilers…

Lastly, I thought I've read in the past that the efficiency of the condensing boilers (93%+) tends to be overstated in real world use. Is this true, and if so, what is a realistic number for these systems?

 

[Chris S]

Most condensing boilers start at 90% eff. thats when they're not condensing. Most have outdoor reset , so the eff. and savings is when this is working (shoulder seasons). Look online for installation instructions on the boiler your considering ( to see if you can handle it) and find out if you need a professional installation to get warranty.
Finally- is condensing a good fit for you? If you have hydrocoils the answer is probably not. If you have bb, or radiators, or better yet radiant Mod- con is a great fit.
As for overstated efficiency, do you believe the numbers on the atmospheric boilers?
combustion eff. and system eff are 2 different things. If you can get the heat to your house efficiently, then yes you will have a highly efficient heating system.

In my situation the utility wants $6000 to bring the gas line to my house ( with me digging, sanding the trench & backfilling) so I'm going with 87% eff oil as my back up to my wood unit, a wall hung mod con would be my first choice. Do some math & figure the difference between your 83% atmospheric & 95% mod con, and you'll see a short payback

Chris

 

[heaterman]

Way to much to explain this late at night but the difference is far, far greater than the AFUE rating when comparing a on/off cast iron boiler to a modulating condensing type. In every install where we have taken an iron boiler out and replaced it with a M/C boiler the reduction has been from 25-45%. The AFUE rating bears virtually no resemblance to real world operating conditions and was originally intended to be used only as a comparison between boilers of the same type. Don't go for the cheapo deal from the gas company. Get a good boiler mechanic in there to do a heat loss on your house and make a recommendation on the right M/C boiler for your application.

To me, a person buying a fuel burning appliance from the company that sells the fuel is about like a chicken buying stock in KFC. It might sound like a good deal going in but you'll likely get cooked in the end.

 

[Willman]

To me, a person buying a fuel burning appliance from the company that sells the fuel is about like a chicken buying stock in KFC. It might sound like a good deal going in but you’ll likely get cooked in the end.

Great analogy !! What interest does the fuel company have in selling a gas sipper versus a gas guzzler ? Not much at all.

Will

 

[Wet1]

Most condensing boilers start at 90% eff. thats when they're not condensing. Most have outdoor reset , so the eff. and savings is when this is working (shoulder seasons).

What is this and how is it an advantage?

Look online for installation instructions on the boiler your considering ( to see if you can handle it) and find out if you need a professional installation to get warranty.

I'm considering the Slant Fin B-120 (x2), mostly because they are the only ones I've seen that will fit my needs and can be had below $2500 each. I've also installed a couple other Slant Fin boilers in the past and have had no issues with them. I looked at the instructions online, I fail to see what's so much more difficult about installing these... other than being direct vent (which is appealing to me), it basically looks like it's the same as hooking up any traditional boiler with the addition of the condensate line. I'd also have to install a condensate pump, but that's obviously not overly difficult either. The warranty is another issue which I'm also concerned about, I'll have to look into this. I know a couple of plumbers and a HVAC guy so I'm sure I could get someone to sign off on the work if need be.

Finally- is condensing a good fit for you? If you have hydrocoils the answer is probably not. If you have bb, or radiators, or better yet radiant Mod- con is a great fit.

I have bb and may install some radiant at some point when I redo our kitchen, so it sounds like this is a fit.

As for overstated efficiency, do you believe the numbers on the atmospheric boilers?
combustion eff. and system eff are 2 different things. If you can get the heat to your house efficiently, then yes you will have a highly efficient heating system.

I honestly don't know enough about how these ratings are done to make any opinionated comment. I guess that's part of why I'm asking you guys about this.

In my situation the utility wants $6000 to bring the gas line to my house ( with me digging, sanding the trench & backfilling) so I'm going with 87% eff oil as my back up to my wood unit, a wall hung mod con would be my first choice.

It sounds like I should probably jump while my company is still willing to run the line for free... I expected to be faced with your scenario before I actually talked to my gas company. Adding $6k to the project would certainly kill the deal for me as well.

Do some math & figure the difference between your 83% atmospheric & 95% mod con, and you'll see a short payback

Surprisingly, the payback isn't that short for me. I created a quick spreadsheet based on my historic energy usage (in oil), different gas prices, boiler efficiencies, and total system costs, and the results were not really what I expected. Compared to the low price basic traditional 80% efficiency gas boiler, the payback on both the direct vent traditional boiler (83%) and the more expensive condensing boiler (I used 93%) were both around 14.3 years at today's gas price of $1.80. Now I doubt gas prices will stay at this level so I looked at it at $2.50, $3.00, and $3.50 as well, the payback drops to 10.3, 8.6, and 7.3 years respectfully... and this is with me doing all the work myself. Obviously the higher gas prices are, the more attractive the higher efficiency boilers look. In the end I think I'm still leaning toward going with the condensing models, but I expected the payback to be a lot quicker... unfortunately (or fortunately), we just don't use enough energy to really make it a clear cut case.

 

[Wet1]

Way to much to explain this late at night but the difference is far, far greater than the AFUE rating when comparing a on/off cast iron boiler to a modulating condensing type. In every install where we have taken an iron boiler out and replaced it with a M/C boiler the reduction has been from 25-45%. The AFUE rating bears virtually no resemblance to real world operating conditions and was originally intended to be used only as a comparison between boilers of the same type.

Great, that's the kind of info I'm looking for.

Don't go for the cheapo deal from the gas company. Get a good boiler mechanic in there to do a heat loss on your house and make a recommendation on the right M/C boiler for your application.

I'd like to point out my gas company does not sell boilers. They did give me a couple names of supply houses at my request, but they certainly didn't try selling me anything.

 

[heaterman]

OK on the gas company. There are a lot of areas where the gas utility offers a "free" boiler if you take your oil fired unit out and go with them. That's where the KFC analogy comes from. The gas company has no interest in selling you an efficient appliance because they would in effect be hurting themselves in doing so.

As to your installation and whether a condensing boiler is right for you, the very first thing that needs to be done is an accurate heat loss calculatoin of your home. That is the basis on which all of your decisions regarding equipment need to be made It will tell you if you have enough BB in the house to be effective at reduced water temps. It will tell your pipefitter what size tubing is required, what circulators to use, how much flow is needed and all the other things that make it a functioning system instead of a collection of parts that should work but don't.

Now, a little about AFUE ratings and why they bear virtually no relationship to real world conditions. The AFUE rating test goes like this.
The boiler is fired at it's name plate rating continuously. What we in the trade call "steady state" firing. Then water is circulated through the boiler at a rate which provides 120* water in and 140* out. These would be absolutely ideal conditions for efficiency but in reality are never achieved in the field. Out of literally hundreds of installations I have looked at, I can honestly say I have never seen a boiler operating under anything even remotely close to these parameters. Back to the test; using the flow rate required to maintain the 120-140 spread the BTU's actually transferred to the water are calculated and the efficiency as a percentage of maximum firing rate is established. I'll say it again. These test conditoins are never duplicated in the field.

A modulating/condensing boiler can however, come much closer to re-creating these conditions because it can, based on a number of factors, turn it's burner up and down to match the load. This comes much closer to the "ideal" steady state firing mentioned above. Along with that, many M/C boilers have onboard controls which can be adjusted to provide a given water temperature at a corresponding outdoor air temperature. That is called outdoor reset. What this does is eliminate water temperatures that are actually higher than needed to heat the space in your home. You'll find that during 95% of the winter, the typical 180-200* water temp a boiler usually runs at is far higher than it needs to be. My gas fired Viessmann rarely see's 160* let alone 180 or higher. Shoulder seasons like October through November and then again starting in mid March my system temperature is in the 125-140 range. This is where OD reset and a modulating burner really cut fuel consumption. Now on top of that add the fact that at water temps below 140-145* a M/C boiler can collapse the flue gas and extract the latent heat from the condensation and you have a very smart and efficient heating package.

In short, the condensing technology is the only thing the AFUE test can detect. Hence the difference of only 8-12% on the rating. The other 20-25% of improvement is from the boiler being "smart" enough to match the load and create those ideal steady state conditions. The AFUE rating only tells a small part of the efficiency story.

 

[Wet1]

Excellent info, thank you for going through those details!

 

[Redox]

Lotsa quesions here, and my experience is somewhat limited on small boilers, but I'll weigh in...

If you can get NG installed for less than $1k, I say do it! Its cheaper (right now), cleaner, easier, lower maintenance, more flexible, yadda yadda. If I were looking at $6k, i think the oil burner might be more attractive.

The prices you are quoting seem to be wholesale and not an installed retail price. Most people can't get a boiler for $1200, but if you know someone, so much the better. FWIW, most utilities will not give a rebate to someone buying a boiler wholesale and installing it themselves. If you know someone in the biz who will work with you, I wouldn't go spreading it around...

In the commercial world, condensing boilers have developed a reputation for being finicky. Every manufacturer has had varying degrees of success, but they aren't nearly as reliable as an older atmospheric boiler. I know building owners who have spent years trying to sort out teething problems in new setups. Many of them have just gotten used to having to go reset them occasionally and are used to seeing them sitting in a puddle of rusty water. Some of these problems are site specific, but many times, the owner will take all the remedial action possible and still wind up with something that is less than satisfactory. They're great when they work, but a pain when they don't...

I will echo heaterman's comment about the heat loss calcs. The best heating system in the world will be crap if it is oversized. Are you doing new construction, or is this an existing structure? How big is the current boiler? Will the gas company put the pipe in without making you buy their equipment? What are you using for radiation? There are lots of questions that ought to be answered before anyone can make a recommendation.

I have noticed that condensing FHA is a lot less than a condensing boiler, but if you already have the radiation, I'd stick with it. Much more comfortable.

On the cheap vs. expensive question: Are you really going to use this thing, or just look at it while you pile more wood in the stove (we're all assuming you HAVE one, don't you?)? I have been trying to get as much heat as possible from my wood stove and actually kinda regret spending the bucks on the condensing furnace. I only use it occasionally when the temps dip way down into the teens (design for this area).

Is baseboard really going to work well with a M/C boiler? I thought it needed 180F to be effective. Radiant would be much more efficient in the long run.

Chris

 

[heaterman]

Chris made a comment about BB needing 180* to be effective. One has to define effective first. The fact is that BB still puts out heat even at temps of 110-120*. It's just less per foot than at 180*. A person has to sit down with his heatloss and spend some time looking at it based on various outdoor temps. At design temp (call it 5*) your home may lose 65,000 btu/hr but at an outdoor temp of say 25* for example it may only require 38,000. If you are using a M/C boiler with outdoor reset, it will lower the water temp accordingly thereby lowering the output of the BB in tandem with it. Makes for a very comfortable heat when you're able to "leave it on" all the time by reducing the output proportional to the load.


Rats...........I tried to upload a file from Slant Fin showing BB output at various water temps but the site won't take it.

Basically it's like this: Entering air temp 65*, 1GPM flow, 3/4" FineLine series 15 BB, rated per active linear foot

110* = 140
120* = 190
130* = 240
140* = 290
150* = 350
160* = 420
170* = 480
180* = 550
190* = 620
200* = 680
210* = 750

 

[Wet1]

Thanks for weighing in Chris.

The prices I quoted are from retail supply houses. The sales rep from the gas company told me the rebate is valid even if I buy and install the boilers myself. They do not sell boilers or care where I buy them from. They said they will run the line/meters at no cost, as long as I am using it for heating. I'm going to call the rep again today and see about setting up an appointment with him to come out to my house, get more details, and possibly sign the deal. I do agree with you though, if the gas company is going to subsidize much of the conversion, and it's not going to cost me all that much to change over, I think the smart move is to do so.

I slept on this last night and I think I've again changed my mind. The more I think about it, the traditional boilers are probably the better move for my needs. Let me explain my situation in more detail and you guys can hopefully contribute with your more informed input...

My house is a fairly large cape with a finished basement and three floors. The house was built in the 1930's, so it's not a new construction. Since my wife and I don't need nearly all the space we have, I've set the house up so it can be a single or a two family dwelling. We currently occupy the basement/first floor and have been renting out the second and third floor to a nice single occupant for many, many years now (great tenant, never home, always pays, and very quiet!) This is why I'll need two boilers. The house has all separate utilities, with the exception of water/DHW. I cover the water and the DWH is taken care of via indirect off the tenants boiler... I know this really sounds like the tenant is getting the shaft on the water, but we use very little DHW. Anyway, the finished basement and first floor are just under 1000 sq ft each, the 1st floor has 9' ceilings, basement is 8'. The basement is plumbed with bb, but I've never bothered hooking it up to the boiler since it seems to stay fairly warm down there w/o any direct heating. The first floor is bb. The second and third floors are about 950 and 350 sq ft respectfully. The second floor is bb and the third floor has electric heat. The current downstairs oil boiler is an old dog and I believe it's 85k btu. The oil boiler for upstairs (and the indirect) is only about 7 years old, it's either 105k or 115k btu... clearly oversized but it seems to cover all the current needs w/o any issues and obviously the "wasted" energy doesn't come out of my pocket. All the exterior walls in the house are insulated and the windows are fairly new with filled glass. The ceiling between the first and second floor is insulated with R15 and R19. The ceiling between the second and third floors I do not believe is insulated (although I'm not sure about this) and the insulation above the third floor is only fair (if I was living up there I'd improve it, but it hasn't been much of a priority for me for obvious reasons).

I don't plan on heating the basement at any point with the first floor boiler (more on that latter), so I could probably get away with a 35k to 50k btu unit downstairs (50k btu to play it safe). Given the relatively small heated area and the fact that we keep the temp near 60-62*F in the winter, we obviously don't use much energy... I don't really see how the cost of a condensing boiler will pay off in any reasonable time period in this application. And as Chris questioned, I do have a pellet stove I can, and do, use as well when it's cheaper to do so.

Upstairs (which we don't plan on living in within the next 15 years) is a little trickier. First there's not all that much area to heat (950 sq ft), but much of this heat probably ends up at the third floor through the ceiling and oven star case, plus the insulation above the third floor isn't all that great. I suspect a 50k to 75k btu unit would probably cover those needs, but then there's the indirect DHW heating. I could be wrong about this, but I seem to recall these systems need about 100k btu to work effectively, please correct me if I'm off base on this. This seems like a better application for a mod con boiler as they can vary their output as needed, but since I'm not paying for the energy, I don't really reap the payback myself... plus I think I'm hearing the condensing boilers require more maintenance and upkeep, which I certainly don't want either. What I think I might do is just go with another oversized boiler for this application.

I know the next step is to sit down with a specialist and get their recommendations, but since I'm not the most trusting person in the world, I don't want to go into this blind or uninformed either.

At this point it's starting to become clear to me that two condensing boilers are probably not a good fit for my application. Regardless, I would like to convert to direct vent as this would allow me at some point to put a wood stove in the basement (using the flue the boilers currently occupy). I burned wood for a long time before I bought this house, and I don't miss it, but I do want to have yet another heating option going forward so a stove in the basement is probably going to happen.

Doing a little looking around, I see a company by the name of Lochinvar makes a line of boilers that are 84% efficient, can be direct vented, come with a two stage gas valve, lo/high/low temp controller, and you can get an optional outdoor reset energy management add-on. I talked to a local dealer and he said these are well under $1500 each (w/o rebates) so these might be a very good option... plus I really like the fact that they have the lo/high/low temp controller which would not only save some money, but I would also feel a little better about intentionally over-sizing the one boiler. Here's a link for these boilers: (broken link removed to http://www.lochinvar.com/pdf/RSB.pdf)

Sorry to be so long winded, but at least you have all the details now. Again, thoughts, opinions, input, and ideas are always welcome!

 

[Wet1]

Again, thoughts, opinions, input, and ideas are always welcome!

Anyone???

 

[chrisfallis]

Kind of off topic, but related to the future of natural gas prices, is a story in our local paper. The utility company wants to retire two coal plants and put a "clean burning" natural gas power plant on line. The economics of the project look good, until you examine their assumption that the price of gas will not rise in the next 30 years. This from a utility company that has already told consumers that 2008-2009 natural gas heating bills will be 30-50% higher than last winter.

I was looking at the rising cost of a gasifier like the Garn. Every time I turn around lately, the prices seem to inch up $500-$1000. Last year we burned $2500 worth of natural gas for heating. A $15K Garn would take me 6 years to break even. With natural gas prices skyrocketing, now I am looking at a 4 year payback. Both of these examples assume that I don't pay for wood and that my "scrounging" and processing time have no value. A close approximation in my case.

 

[Chris S]

re: new boiler

get a mod con- it's an excellent choice for your situation since it can match your heating load. This is a great way to keep your efficiency up. As stated previously AFUE does not have much to do with overall system efficiency. As prices rise, this becomes more & more affordable, unless you believe that fuel is going to go down