Condenser boiler?

[philip dhrymes]

Hi everyone,
I am having my boiler replaced b/c its very old. From what I had read online etc, I was looking at the condenser type of boiler because its so much more efficient then a traditional boiler.
I had two contractors come look at it to give me an estimate. One said the condenser boiler is a good option, however the other one said it isn't because I have a smaller home and the condenser boiler work better with a larger volume of water because it creates more vapor and then more to condense. I am confused at this point because I don't want to get the wrong type of boiler.
any help would be appreciated.

Phil

 

[Dana]

Since you have a heating history on the place, use the old boiler as the measuring instrument for determining the whole-house heat load using this methodology. If you're willing to share the wintertime fuel billing dates and fuel amounts I can run those numbers for you here.

Then measure up the radiation (baseboards, convectors, radiators, whatever) zone by zone. The type and amount of radiation per zone makes a significant difference on which modulating-condensing boilers can work effectively at condensing temperatures, and which are going to short-cycle. The napkin-math version of how that works lives here.

The contractor who claims it won't work will in smaller houses is either ignorant, or perhaps you misunderstood what he was actually saying. (It "...creates more vapor...", did he really say that? If yes, lose his number!) He might have been referring to not have sufficient radiation to run at condensing water temperatures without short-cycling the boiler, but that would be an extreme outlier case. As little as 30' of cheap fin-tube baseboard will work with some of the smallest modulating condensing boilers (but more is better.)

With the whole house heat load and the radiation numbers and your ZIP code (OK, Yonkers, I get it) it's possible to calculate roughly how much of the heating season the boiler can operate in condensing mode. It has nothing to do with the size of the house, and everything to do with the ratio of the heat load/radiation.

With ultra-low heat loads and correspondingly small radiation output it's sometimes better to use a condensing tank type water heater for both space heating and hot water, but I won't go into that until we have the fuel-use calculated load and radiation sizing figured out.

 

[Sponsor]

 

[philip dhrymes]

Since you have a heating history on the place, use the old boiler as the measuring instrument for determining the whole-house heat load using this methodology. If you're willing to share the wintertime fuel billing dates and fuel amounts I can run those numbers for you here.

Then measure up the radiation (baseboards, convectors, radiators, whatever) zone by zone. The type and amount of radiation per zone makes a significant difference on which modulating-condensing boilers can work effectively at condensing temperatures, and which are going to short-cycle. The napkin-math version of how that works lives here.

The contractor who claims it won't work will in smaller houses is either ignorant, or perhaps you misunderstood what he was actually saying. (It "...creates more vapor...", did he really say that? If yes, lose his number!) He might have been referring to not have sufficient radiation to run at condensing water temperatures without short-cycling the boiler, but that would be an extreme outlier case. As little as 30' of cheap fin-tube baseboard will work with some of the smallest modulating condensing boilers (but more is better.)

With the whole house heat load and the radiation numbers and your ZIP code (OK, Yonkers, I get it) it's possible to calculate roughly how much of the heating season the boiler can operate in condensing mode. It has nothing to do with the size of the house, and everything to do with the ratio of the heat load/radiation.

With ultra-low heat loads and correspondingly small radiation output it's sometimes better to use a condensing tank type water heater for both space heating and hot water, but I won't go into that until we have the fuel-use calculated load and radiation sizing figured out.

hi thank you so much for the reply. My gas bills were very high 300 to 400 even though I kept the house cold (thermostat at 50) I didn't save the bills b/c they were so bad. January this year I just turned the heat off all together

It didn't sound right what I heard either, and yeah those were his exact words. thanks again I really appreciate all the information and input.

Phil

 

[NY_Rob]

A new boiler will save you maybe 20% or so on your gas consumption... but you have more important issues to address first if you have $300-$400 heating bills in a "smaller" 50 deg house in Yonkers!

Something isn't adding up here.....

 

[Dana]

If you've been keeping the place at 50F you'll need to use base 50 heating degree-days for the calculations.

Is the "...300 to 400..." dollars, therms, CCF or... ???

If you have no real fuel use to work with, run a better-than-average online load calculator, using the most optimistic assumptions possible on R-values and air tightness. (That tool will oversize reality by more than 50% if you assume leaky rather than tight construction, and R9 in the walls instead of R13, etc.) Even with aggressive assumptions the heating load numbers for that tool are padded, so the boiler's DOE output doesn't need to be

As much as I really hate rules of thumb, at +15F outdoors (your approximate 99% outside design temp-yes I know it gets colder than that, but only 1% of all hours in a 25 year period) and 68F indoors (the code minimum for HVAC design purposes), most 2x4 framed insulated houses with at least 6" of fluff in the attic and clear glass storms over single-pane double-hungs will come in between 13BTU/hr per square foot of conditioned space (tight house, insulated foundation), and 25 BTU/hr (leaky house, no foundation insulation.). So a 1200' house over a 1200' basement that isn't directly heated would come in somewhere between 14,000 - 30,000 BTU/hr. ASHRAE recommends no more than 1.4x oversizing as the best compromise for comfort and efficiency with non-modulating boilers (cast iron), so if that describes your house the optimal cast-iron boiler would have a DOE output of about (30K x 1.4=) 42,000 BTU/hr, which is enough to cover the load during Polar Vortex coolth, and has a reasonable recovery time from overnight setbacks.

For modulating condensing boilers the more important number is the minimum-modulated output relative to both the radiation sizing and design load. If your calculated heat load is 30,000BTU/hr it will be more comfortable and run more efficiently if the minimum modulation is less than half that, so anything that can't fire under 15,000 BTU/hr would be sub-optimal. As a rule smaller is always better, as long as the max firing rate still covers your design load.

 

[Dana]

A new boiler will save you maybe 20% or so on your gas consumption... but you have more important issues to address first if you have $300-$400 heating bills in a "smaller" 50 deg house in Yonkers!

Something isn't adding up here.....

The description of the boiler was "...very old...", which could easily mean combustion efficiencies under 70%, and if it's 3x+ oversized and in an uninsulated basement literally half of the losses could be going to over-heating the boiler room from jacket losses & standby. Replacing such a beast with a right-sized, insulated condensing boiler can easily result in cutting fuel use by 40% or more.

Of course the very old boiler would be in a very old house, which could have quite a few thermal deficiencies in the building envelope too. eg: An uninsulated balloon framed house with a leaky basement would have huge stack effect infiltration issues in winter that could be fairly cheaply rectified. But the only description we have of the house so far is "... a smaller home...", which is a relative term. (Smaller than what?) There are many homes in Yonkers in the 1000-1500' range (I've been in several), but there's no saying that philip's house is one of those, or something smaller (or larger).

 

[philip dhrymes]

my house is 1580 sq feet.
I had an old refrigerator in the basement that I got rid of, which has lowered my Con Ed bill about 150 to 20 a month already
the window are all new and the roof is too

the house was built in the 1920's, the boiler is about 30 to 35 years old

 

[Dana]

With new code-min windows and 6" or more of fluff in the attic your load is almost certainly under 50,000 BTU/hr, even if there is no insulation in the wall cavities and no foundation insulation.

Measure up your radiation.