Boiler selection in an old home - To be efficient or not to be?

[MrBasseyPants]

Hi all,

Thanks for taking the time to read this post....I am new and quickly learning about HVAC issues dealing with our home. My reason for posting is to get a better feel for the decision I need to make on a new boiler for our house. I have an installer who I really like and trust his opinion, but at this point I have to make a decision, and as you will see, to me, any of my three options are valid.

Here is some background. I currently live in a 130 year old house near Chicago, IL. We have a 40 year old gas boiler (I want to say 260,000 BTU - roughly 60% efficient we guess) that is chugging along just fine and a 3 year old 40000 BTU gas water heater. Both vent up the chimney via a 9in diameter exhaust.
(Assume my BTU numbers are correct here)

Our heating system is hot water through standing radiators.

Now here is where it gets tricky, the clay tile liner in the chimney is failing. To the best of our knowledge the chimney is original to the house...so obviously we are most worried with saving the chimney...all 37.5 feet of it. We cannot put a liner in the chimney that will appropriately vent the existing boiler and water heater so here are our three options:

1) Install chimney liner and vent new cast iron ~ 80% efficient boiler and existing water heater up chimney.

2) Install new 95% efficient condensing boiler (yes we know we won't get near that efficiency) and power vent water heater and vent to the outside.

3) Install new 95% efficient condensing boiler with indirect water heater and vent it outside.

As far as upfront costs go options 1 and 2 are a wash with the condensing boiler actually coming in about $500 cheaper (we were surprised about that). Option 3 would be about $1500 more than option 2. The prices aren't the deciding factor though, but just used as a reference (and show that there is no clear "cost" winner here).

I guess I am curious to find others opinions on which boiler type we should go with and if we go with a
condensing boiler, should we go with a power vent or indirect hot water system.

I understand that that the cast-iron boiler will last much longer than the condensing, but I am having issues with paying so much money to repair our chimney just to go with a cast iron boiler.

If anyone has any comments; your input is appreciated. My installer has given me the pluses and minus for both systems, but ultimately it is my choice...he only added that he liked the condensing because it was easier for him to get to our basement :)

Also for reference the cast-iron boiler is a Dunkirk PWX-6E and the condensing unit would be a Dunkirk Q95. I assume the water heating systems would be Dunkirk as well.

Thanks for your info.

jc

[DesMech]

What is the actual heat loss of your home?

[MrBasseyPants]

What is the actual heat loss of your home?

I'm not sure...you've bested my HVAC knowledge....but does it really matter in my case as I need a new boiler anyway? I would assume that the high efficiency boiler would be able to get at least the same efficiency as the cast-iron. ...which then I would assume would translate to both boilers being able to heat the house with the same economy as that of the cast-iron. ...and with the prices being a wash...

...well you know what assuming does....I am really out of my league here :)

jc

[Big]

boiler I like . BUT, they work best with radiators or if you have baseboard lining the room. They will heat your house at a lower temperature . They do save you money, they do get 95% efficiency. You need to know the turn down of the boiler. Which is how the boiler modulates , does it do it by,1000 btu's or by 10,000 btu's or run x minutes they go to the max. The best is 1000btu's ,that will save you the most if it goes to the max btu's after x minutes , the saving will be less.The difference in standard boiler and a mod-con boiler . You pay for the 90plus one time in the initial investment , you pay the difference of efficiency each time the boiler runs FOREVER

[beenthere]

With cast iron rads, a mod/con boiler with outdoor reset is an ideal choice. And will provide you with a lower heating bill(Or at least use less gas).

A heat loss calc so you don't buy more boiler then you need.
Over sized boilers tend to short cycle, and use more fuel then it needed to heat the house.

An indirect is more efficient then a gas fired water heater.

Even though teh Dunkirk install instrctions say it doesn't require primary secondary piping. Its better for the boiler if its done that way.
Most contractors don't verify GPM flow through the boiler. And if its low, you lose alot of its heat transfer efficiency.

[DesMech]

I'm not sure...you've bested my HVAC knowledge....but does it really matter in my case as I need a new boiler anyway? I would assume that the high efficiency boiler would be able to get at least the same efficiency as the cast-iron. ...which then I would assume would translate to both boilers being able to heat the house with the same economy as that of the cast-iron. ...and with the prices being a wash...

...well you know what assuming does....I am really out of my league here :)

jc

The first step should be to have a heat loss calculation done. This will determine the correct boiler size. Your old boiler is probably way oversized, so you do not want to install a new one with the same capacity without doing the calculations. Once you know what size you need, you can determine how much radiation you have to work with. This will determine what water temp is needed to heat the existing radiators. Usually houses with old cast iron radiators can run water temperatures low enough for a mod/con boiler to be very efficient, but don't oversize! I am not familiar with Dunkirk, but if this is the boiler that only comes in one size, then I would only consider it if it is the proper size for your home. There are lots of high efficiency boilers on the market now, but the trick is to find someone who will install it right. Oh yeah, my opinion is to abandon the chimney, and go with a mod/con boiler and an indirect water heater. Good luck.

On edit: beenthere beat me by 12 minutes. I gotta take a typing class.:o

[flange]

I agree with the mod con and indirect setup with those old rads UNLESS your heat loss is around 95000 btuh or so, then I REALLY like the new Triangle tube excellence with the built in instantaneous water heater with prewired and piped controls. Those are hard to get right now though. too much demand for them.

[MrBasseyPants]

Thanks... you guys have been great. Yes, my installer walked the house and measured the size of my radiators and came up with my heating requirements of 175,000BTU. This is about 85000 BTUs less than my current boiler. His estimate was that the boiler would run up to 160 to 170 degrees (at times). Which should be in line for the condensing boiler.

I'm really hoping that "right-sizing" the boiler, getting a new boiler, and possibly gaining a teensy bit extra efficiency from a high efficiency unit will give us enough cost savings to render all of the costs associated with this install moot (over time...lots of time).

Our expensive heating bill in Jan/Feb usually runs about $300/month....then again my wife keeps the thermostat at roughly 61 degrees. :)

I think I am lucky in finding an installer that has really looked into my situation. I had another installer that just did a generic quote after just looking over our boiler room area....I wasn't impressed.

Again....thanks for all the info.....but if anyone else has a comment....I haven't made a decision yet...the more info...the merrier!

jc






The first step should be to have a heat loss calculation done. This will determine the correct boiler size. Your old boiler is probably way oversized, so you do not want to install a new one with the same capacity without doing the calculations. Once you know what size you need, you can determine how much radiation you have to work with. This will determine what water temp is needed to heat the existing radiators. Usually houses with old cast iron radiators can run water temperatures low enough for a mod/con boiler to be very efficient, but don't oversize! I am not familiar with Dunkirk, but if this is the boiler that only comes in one size, then I would only consider it if it is the proper size for your home. There are lots of high efficiency boilers on the market now, but the trick is to find someone who will install it right. Oh yeah, my opinion is to abandon the chimney, and go with a mod/con boiler and an indirect water heater. Good luck.

On edit: beenthere beat me by 12 minutes. I gotta take a typing class.:o

[MrBasseyPants]

I agree with the mod con and indirect setup with those old rads UNLESS your heat loss is around 95000 btuh or so, then I REALLY like the new Triangle tube excellence with the built in instantaneous water heater with prewired and piped controls. Those are hard to get right now though. too much demand for them.

Ahh didn't realize the the home heat loss was the same number as the expected boiler BTU output.....which was measured at 175000 BTUs.....like I said....I am totally green with this stuff.

jc

[beenthere]

Thanks... you guys have been great. Yes, my installer walked the house and measured the size of my radiators and came up with my heating requirements of 175,000BTU. This is about 85000 BTUs less than my current boiler. His estimate was that the boiler would run up to 160 to 170 degrees (at times). Which should be in line for the condensing boiler.

I'm really hoping that "right-sizing" the boiler, getting a new boiler, and possibly gaining a teensy bit extra efficiency from a high efficiency unit will give us enough cost savings to render all of the costs associated with this install moot (over time...lots of time).


jc

If he just measured up your rads. Then he isn't sizing the boiler to the house. He's sizing the boiler to your radiation.

I find very few houses, that don't have more radiation then they need for their heat loss.

You may want to do your own load calc, to make sure you get the right size.
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