95% Efficient Condensing Replacement Boiler

Unknown

Need to replace 30 year old cast iron boiler. I've read about new condensing boilers which achieve 95% efficiency and have rebates from local utility. One contractor told me that these boilers will not condense in my old system. (180 deg ci radiators). Is this true? Is there anyway I can change my system to get these efficiencies?

Aaron_in_Maine

Cast iron rads

If your system has cast iron rads and not fin tube copper a condensing boiler will condense in the spring and fall maybe most of the winter too. First step would be to do a heat loss of the home. Then next step would be to calculate how much radiation you have. If you have too much radiation you can lower the temp on the boiler to bring up the efficiency of the boiler. The lower the water temp the higher the efficiency of the condensing boilers. You can also add more rads to archive this.

Paul48

Not True

Cast-iron radiators are perfect for a high efficiency mod/con boiler. Scratch that contractor off the list.Also, scratch off any contractor that does not do a heat loss.Sizing is critical with these new boilers.

Jamie Hall

One presumes...

that we are talking about a hot water system here.



As Aaron notes, the key to achieving something near the advertised efficiency of condensing boilers (you'll never achieve the advertised -- any more than you can get the advertised gas mileage out of a car) is to keep the circulating water temperature down.  Will you be able to keep the temperature down in your system?  Good question.  Again, as Aaron notes, the first step is to figure the heat loss of the various spaces.  Then figure the effective amount of radiation in those spaces.  From that you can figure out what the temperature and flow rate in each of the radiators will need to be to heat the space.



It is quite possible, particularly if you (or someone) has done a fair amount of work on insulation and tightening up the building, that you can run the radiators at a low enough temperature to condense and still get enough heat -- but it will take some analysis, which your contractor should do.



Of course, once you have found that it is theoretically possible, the controls on the boiler will have to be set to do that in practice.  And then, you will need to maintain that boiler to keep it operating properly...

Unknown

Sizing & Heat Loss?

The utility rebate is greater than the price difference in the condensing and non condensing boiler.  In my case is there any reason not to buy the condensing boiler?  Also as far as accurate sizing, this boiler modulates from 40,000 btu's to 199,000 btu's and supposedly matches the real time heat loss.  Is sizing and heat loss still critical? 



Thank you







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Unknown

Sizing & Heat Loss?

The utility rebate is greater than the price difference in the condensing and non condensing boiler.  In my case is there any reason not to buy the condensing boiler?  Also as far as accurate sizing, this boiler modulates from 40,000 btu's to 199,000 btu's and supposedly matches the real time heat loss.  Is sizing and heat loss still critical? 



Thank you







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Paul48

Sizing

It is absolutely critical. If, for instance, the heat loss of your home is 100000 btus, you would want to find a boiler rated  around that. OK, so you find a mod/con with rated output from 25 -100 btus, which means it has a 4 to 1 turn-down ratio. You might only need 100000 btus a couple days a winter, if at all. The rest of the time you would need to take advantage of that turn-down ratio. Mod/cons need to run long, at low temps. Do not let a contractor tell you that he can put any size mod/con in because it modulates.

Unknown

Thanks!! Sizing ?

Thank you.  My existing boiler is rated at 200,000 and is probably significantly over-sized.

If the boiler modulates from 40K to 200K what difference does it make if my heat loss is precisely 127,000 or 178,500?  Won't the boiler fire to match the actual heat loss as it changes throughout the season?



Thank you.







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Jean-David Beyer

My old boiler was rated at 70,000 BTU/hour input.

My installing contractor recommended a 105,000 to be safe.



I calculated the heat loss of my building with 0F outside and 70F inside and got 30,000 BTU/hour. And design temp around here is 14F, so even that is too high. He said it would modulate and run just fine. But it would go down only to 21,000 BTU/hour. Now the old 70,000 BTU/hr boiler was oversized and cycled 20x an hour on high limit even when it was quite cold out. So I refused that one and he reluctantly put in the smallest one in the product line; 80,000 BTU/hr and it would modulate down to 16,000 BTU/hr. Now remember that the 30,000 BTU/hour I calculated would be for only a few hours on one or two days year, if it went down to 14F below the design temperature. Safe from what? Mainly from the fact that he did not even do a heat loss. If I could have gotten a 40,000 BTU/hour mod-con, I would have done that.



As it is, I have two heating zones, and to have a chance of using the minimum output, both zones would need to call for heat at the same dime, and they rarely do.



I advise getting the smallest boiler you can get (that meets quality and spare parts availability requirements, and has competent technicians in your neighborhood). If you have 6 teenage daughters you might need a little bit more capacity for domestic hot water.



Note that I am not a heating contractor, but just the owner of a mod-con boiler with an attached indirect fired domestic hot water heater.,

Paul48

Do The Heat Loss

You may find out the heat loss comes to 50 or 60000 btus. Mod/cons only are efficient when they are condensing. They condense when the return temps are cool. Sizing the boiler correctly, gives you the proper control across the widest temp variables. If the heat loss is 60000 btus, your heating needs are from zero to 60000, not zero to 200000. There is also quite a difference in price between a 60000 btu output mod/con, and a 200000 btu one.

gennady

Ci radiators

Cast iron radiators are perfect for high efficiency system and condensing boilers. As a rule radiators are oversized and will work on much lower temperatures of water 95% of the time. Because high efficiency systems are based on constant flow and variable temperatures of the water you do not need 180 F water temperature anymore, find good contractor with extensive experience in designing and building high efficiency systems. Scrap this contractor from the list.

gennady

What heat loss has to do with application of cast iron radiators in high efficiency system?

Heat loss here heat loss there. How can one say that cast iron radiator based system will condense only in mild weather? Heating emitters sized for deign temperature and this temperature happens to be only few days a year. Also over sizing of the radiators rather a rule than exemption.

gennady

199.000 btuhr ???

Are you heating a building? Where did you get those numbers?

gennady

Rebates

Rebates will never cover difference between high efficiency and standard system. You have an issue with contractors and their sizing and pricing.

Rogger

Furnace replacement for boiler (hydronic system)

I have a 100,000 Btu's Furnace and I lke to replace this sytem for a hydronic systema, and a multiple mni-split AC system.

What are your recomendations.

Snowmelt

I suggest

The navian combo unit 240 Ncb and a fijitsu flex system.

RobG

Why?

Snowmelt, why would you sugest that boiler? Did I miss where the poster gave their heat loss numbers? Did I miss where the hot water needs were stated? I'm just wondering how you came up with a Navian 240? Do you go on sales calls and just look at the existing equipment and size based on that? Please tell me the secret? Is the Navian unit that boiler that I've been looking for? One size fits all?





Rob

Snowmelt

Sure

He said he had 100,000 btu furnace and wanted to replace it with a hydronic system.

Why not kill 2 birds with one stone.

The costumer really didn't say if he wanted to use a hydronic coil, go baseboard or radiant floor.

If he has 100,000 btu furnace chances are it could be oversized but if it's not the navian is rated to do 120,000 btu max, but it will down fire for what's needed, so even if it's 1/2 the btu's the machine will respone to what's needed to het the home.



The water part is just a bonus, the venting can be of 2 inch venting, the gas line can be 1/2 for first 15 feet and safely operate.



But I will say this, your right the customer didn't have a heat loss done, didn't say anything about hot water. I thought I would give hind something for a bonus.



So with very little information that he gave us, I suggested the navian, I could suggest a peerless or a weilmclain, with the max of the 100,000 btu or less.



There would be nothing wrong with what a suggested and would have the ability to have the same machine do hot water also, can always suggest a storage tank or indirect water heater if he has a quick dump.



What do u suggest he uses with the info givin?

Snowmelt

Bc

Your right , I like to give the max domestic water.....

Zman

One post wonder...

This is a one post wonder who tacked a question onto an unrelated old post.

I'm with Rob on this one. How in the world could you come up with any recommendation with the info that was given?

Heat loss?

DHW needs?

Must be a slow day...

Carl

RobG

Gas piping?

Can you post the gas pipe chart that you are using Snowmelt?

Rich_49

Rob

Do you really use those charts ?  Depending on where you are and the regulator the utility uses those charts will have your gas pipe oversized most of the time .

Just like when we do room by room heat losses , in my opinion we should also do the math for gas pipe sizing if meeting the INTENT of the code is important .  The intent is "to minimize the hazards associated with , within the building "     Basis for this method can be found in 402.5 IFGC if that is your model code . Barring the fact that unless you and others are in an atmospheric bubble where physics acts differently than where others are located it works there too and is an approved engineering method , the same method that all the CSST manufacturers used when they figured out their spin to make everybody believe that CSST was magic . Can you tell me why you can run smaller sized CSST to an appliance / appurtenance when in fact CSST has a greater pressure drop (" wc) per foot than Iron pipe ?   Not really defending what Snowmelt said but pointing out that there are other methods that all may not be aware of  and that if there is something some may learn from it should not be discouraged  .   Just my thought on gas pipe , as for the Navien references at every turn I ask this , George , are you trying to get a job with Navien ?

Snowmelt

It is

What it is, also it's one of the advertisement on the right hand side of this web sight.



I seen it work,.



What you have to do is size the gas pipe correctly from the gas meter up to the last 15 feet of the gas valve on the water heater. Then you can cut it down to 1/2 or if your doing a retro it usually works.



Rob just answer me this, this other post when I said go with fijitsu and navian for b.b., he did originally wanted to install b.b. but it w out of his price budget. I really don't like forced hot air but either did this renter. But when it came down to it he did pick the figitsugeneral heat pump.

I could tell everyone that post just hi with Geo-thermal, it's the best bang for your buck if installed correctly but I don't.

For a home owner to tell me that I am going with fijitsu just because it has a heat pump then I don't have to put b.b, in, is just being cheap in the north east United States. If you live in warm climate wheather well that's a different story.



So why only suggest the navian, I can say the same with the triangler tube also.



So if you want I will take the navian back , if the owner wants he can go with the triangler tube , weil McClain or peerless.



For me anything under 100,000 btu I suggest the navian Ncb and for a bonus you get hot water , venting if close can be 2 inch and the gas piping can be 1/2, but only 15 linier feet of piping from the machine .

Snowmelt

Yes

Yes rich I am, I go on a few more service calls then I should, dam people don't like to put the P/S piping in,

RobG

I agree Rich

I agree Rich. Gas pipe sizing is based on pressure drop, specific gravity and supply pressure. That said, it is not fair to tell someone that they can automatically use 15' of 1/2" piping for 180,000 btu's. That is just as reckless as not recommending a heatloss and just pulling a boiler selection out of your *** because you're to lazy or inexperienced to do the heatloss. Manufacturers can state whatever they want, but I don't believe everything that I read either.

Rich_49

Seems to me

that depending on system piping and requirement of emitters that P / S piping is not always necessary .  If the on board circulator can move enough fluid at design through the emitters and the HX why would you require P/S piping ? I see that no P/S is shown for use with an air handler George , is this boiler smart enough that it can determine what it is supplying ?

  As far as gas piping this thing I see that it will operate at a 3.5" w.c pressure at the inlet . This is a good thing and the fact that it works with 1/2" pipe is not surprising . Think about this now , NJNG uses an 8" w.c regulator in most areas of service where I am located .  With a 1/2" P.D through the meter you still have 7.5" available , if you sized a header properly for all fixtures calling and used the allowable pressure drop you could pull off the following ,

  7.5" after meter 425,000 total load , 1 1/2" x 10' Iron pipe header would have a pressure drop of .03" with all appliances calling leaving 7.47" available at the farthest tee on header .  7.47" minus 3.6" to NCB gives an allowable pressure drop of 3.87" w.c .  3.87" / .039" p.d per foot of 1/2" I.P @ 150,000 CFH = 99.231 feet .    The point I am trying make is that you can't call somebody cheap and you have to read between the BS and not just spout off specs from a book and the same piece and type of equipment is not the cure all .  I shout from the hilltop about HTP but if you notice when I shout I usually refer to different models and types of units that all react differently depending on system design requirements .

   Do you have the guts to install 99 feet of 1/2" black iron to the NCB because it will work ?  I doubt you would because the book says so . George , be the guy who writes the books and quit reading  , move beyond where other guys are and be the best and for God's sake use something made here in America .

Harvey Ramer

While on the subject of Navien

I am not really familiar with the unit, but seems I remember hearing that the "space heating" heat exchanger will only allow 5 GPM at a reasonable velocity. If such is the case, one could only expect about 50,000 BTU/H from the boiler, in a high temp system designed at a 20° delta-t.



Can anybody verify?



Harvey

Rich_49

George

Do you know what circ is installed from the factory ?  Is it a 3 speed and whose pump is it ?

  Harvey , I did hear the same thing but cannot verify and interestingly enough what type of circ , HX flow characteristics are not to be found in my search for this info .

Harvey Ramer

Why?

We are talking about a condensing boiler here, right? They like low return water temps. It makes them more efficient because it lowers the stack temperature and absorbs more heat from combustion. Also, primary secondary piping is to maintain required flow rate through the heat exchanger, not to provide boiler protection.



A cast iron boiler, on the other hand, you have to be careful with your return water temps.



Harvey

Rich_49

Protection

from what ? Low EWT , this is condensing equipment where the lower the EWT the better .  By the way , when water leaves a piece of equipment at a lower temp than it returns it's called air conditioning . We are talking about leaving BTUs in the building are we not ?  I am intrigued to see the house or building that produces energy , so is the entire world for that matter . 

  This is kinda the basis for the recent comments on this discussion . Trying to solve problems that arise when the wrong piece of equipment is specified . If you need to figure out how to make something work to a certain standard it is most likely the wrong equipment and should not have been brought up to begin with . 

Snowmelt

Harvey question

During a webinar for navien the instructor referred to a john Barba webinar, on primary secondary. The primary doesn't have to have as much gpm as the heating side. On that note let me tell you about a snow melt at my own house.



2100 square foot driveway

199,000 btu tankless water heater

1 inch primary loop with air separator with a 009 pump

1-1/2 secondary with 0011 pump

3/4 pex tubing with 11 loops about 275 feet each.



The tankless has a gpm read out, it reads between 4 & 6 gpm as the return temp gets hotter the gpm goes up.

Filled with glycol, roughly 70%

Using 90 btu per sq ft , that's 189,000 btu.

With that being said the secondary loop is keeping up with what we will assume is 18 gpm, the primary doesn't have to be the same as the heating load.

I only know this because I physically see this with my own eyes.

I might not be a 20 year vet, but I do have 10 good years of real field experience where I go to jobs where other people can't figure out.

I seen it with my own two eyes therefor, I have to believe that 5 gpm pump can push 120,000 btu as long ad you have the pipe sized correctly, I just wish I had a pump on the heating side to show the gpm of that zone.

Harvey Ramer

Don't have the time now but.

Topics you should study.



The Universal Hydronic Formula Gpm = but/h ÷ (▲t × 500)

Injection mixing



We will continue this conversation later when I have more time. Your current way of thinking will get you in a whole lot of trouble sooner or later. Definitely worth your time to dig a little deeper.



Harvey

Snowmelt

I know

What you mean, let's just hope it's later then sooner, you asked for some type of explanation.

I think it may do with the delta tee part of the equation, I do watch a lot of john B. Webinar, the snow melt is a math problem all of its self.

I go to tons of jobs where I'm the 2nd or 3rd guy at the job ( notice I didn't say plumber) because some plumbers don't really do boiler work. What happened to me is that I stayed with these tankless water heaters, was called to look at a few of these tankless used as space heating applications, added a different pump , change pipe diameter, add servicing valves, pumped away from expansion tank , then bam the customer had heat.



I never never ever got myself in trouble I do homework on every job. Sometimes my typing gets misread or I don't say it in mechanical terms, I do go to the pump sizing and btu classes when I can go, I never had a heating job fail on me.



So let's go back to the original question.

If your home has a 100,000 btu boiler at 80 % i am going to say a navien unit will be able to be used and at same time supply you with hot water. Also with possible 2 inch venting.



That's all I said on original post. And you get hot water, add a bumble bee pump to the heating side & you have a very efficient system.

CMadatMe

OMG, SMH, Are You Serious

The on board pump can only move 5gpm across the Navien HX. So what has to change to get the btu/hr out of the boiler when not piped pri'/sec.....THE DELTA-T.. How many fin-tube and even cast iron systems you see out there running on a



120,000/500/5 = 48 Degree Delta-T... Zero, zippo, nodda!



Its not the boiler that is delivering the 120,000 btu/hr when piped primary secondary and you can take that to the bank!

RobG

The Math

The math doesn't lie! That is why delta t is entered into the equation.

gennady

Delta T

With 48F delta t you run into the situation of average water temperature in radiator going way down, heat rejection per square foot of radiation drops. To compensate this drop you need larger radiators. It is a matter of calculation, but increase in radiator size will be significant.

Rogger

100,000Btu's Furnace replacement

Thank You guys for all your good information regarding of my project for this year.

My plan is to get the best efficently Boiler for hydronic system.Using a baseboards radiant heat.Will be nice to have a type of Heater Exchanger to raise the temperature from  45'f  -  65'f or 70'f , or a 40 gals.Steel Tank to Tempere the incoming cold potable before to get into Domestic Hot water Heater; the idea is to consume less Natural Gas.Is a 5 adults and two teenagers living in this 55 years old house.

Thank You.

Rogelio Garcia

Zman

Boiler replacement

If you wants really good advise, start a new thread and post all the pertinent info.

Here is a good example http://www.heatinghelp.com/forum-thread/150864/How-to-choose-suitable-size-of-furnace



This post has almost nothing to do with your project. It has turned into a debate about at least a half dozen unrelated subjects..

Carl