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End of Season Report - How's my boiler doing?
fentonc
Member Posts: 270
I'm just an enthusiastic home owner, but I set about trying to understand and characterize the heating system in my house this winter.
The footprint of the house is about 820 ft^2, no attic, built in 1987. The zones have the following radiation:
High-level summary of data from this heating season (November - April):
A typical cycle begins with one long-ish burn until the aquastat hits the 160F high-limit, followed by just the circulator running for several minutes until the boiler hits 140F, and then an 80-90s burn to raise the temp back to 160F, repeating until all zones stop calling for heat.
Here is the basement (smallest) zone calling for heat from cold-start:
And here is all three zones calling for heat starting at the same time:
Measuring based on 'peak to peak' temperatures from the supply/return lines, it looks like I get the following delta-T values for each zone:
I think my biggest takeaway is just that my boiler is way too big. I've been reading through the Idronics series, and I did notice the following:
Comments/thoughts/critiques/suggestions welcome - I was mostly interested in understanding how it worked, characterizing it and seeing if there were any obvious improvements that could be made. It turns out hydronic heating is super interesting! Our main complaint is that there is too much baseboard in the two smaller upstairs bedrooms, so they get much warmer than the larger bedroom that contains the thermostat, and the larger bedroom has enough baseboard to make it uncomfortably warm whenever the heat is running.
- Boiler Model: Weil-Mclain Gold CGa-PIDN-5 installed in 2016
- Rating: 140K BTU/hr input, 117K DOE output, 102K IBR
- Location: North of NYC, 4250 Heating-degree-days@65F for this season
- Setup: 3-zone system (1 per floor for basement, 1st floor, 2nd floor) with Taco 007-F5 circulator and honeywell zone valves connected to 7-day programmable thermostats for each floor.
The footprint of the house is about 820 ft^2, no attic, built in 1987. The zones have the following radiation:
- Basement Zone: 22 ft of nominally 790 BTU/ft (at 180F) fin-tube baseboard
- 1st floor: 54 ft of nominally 550 BTU/ft (at 180F) fin-tube baseboard
- 2nd floor: 72.5 ft of 550 BTU/ft (at 180F) fin-tube baseboard
High-level summary of data from this heating season (November - April):
- The boiler ran 2,777 heating cycles with an average heating call time of 950 seconds/cycle
- The boiler fired 7,050 times with an average duration of 162 seconds, and an average of 2.5 burns/heating cycle
- The boiler burned an estimated total of 381.4 therms of gas over the entire season (about 4250 HDD)
- On the coldest day of the season (right around the design temp of 12F), the boiler fired for 24% of the day and the circulator ran (so at least one zone was calling for heat) for 66% of the day
A typical cycle begins with one long-ish burn until the aquastat hits the 160F high-limit, followed by just the circulator running for several minutes until the boiler hits 140F, and then an 80-90s burn to raise the temp back to 160F, repeating until all zones stop calling for heat.
Here is the basement (smallest) zone calling for heat from cold-start:
And here is all three zones calling for heat starting at the same time:
Measuring based on 'peak to peak' temperatures from the supply/return lines, it looks like I get the following delta-T values for each zone:
- Basement: 8F (by itself or with both other zones calling)
- 1st Floor: 10.6F
- 2nd Floor: 12.4F
I think my biggest takeaway is just that my boiler is way too big. I've been reading through the Idronics series, and I did notice the following:
- There is no differential bypass valve, even though it has a fixed-speed circulator
- The circulator is on the return side right before the boiler, but the expansion tank is on the supply side. Pressure at the boiler seems to gradually increase from ~10 psi to ~15 psi over the course of the heating cycle, not sure if it's an issue or not.
- When the 2nd floor stops calling for heat while another zone is still calling, there is a big 'thunk' noise I assume is some kind of water-hammer effect from closing the valve while the circulator is running.
Comments/thoughts/critiques/suggestions welcome - I was mostly interested in understanding how it worked, characterizing it and seeing if there were any obvious improvements that could be made. It turns out hydronic heating is super interesting! Our main complaint is that there is too much baseboard in the two smaller upstairs bedrooms, so they get much warmer than the larger bedroom that contains the thermostat, and the larger bedroom has enough baseboard to make it uncomfortably warm whenever the heat is running.
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Comments
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What program/controls are you using?
There was an error rendering this rich post.
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Are you talking about the actual control hardware for the boiler? It’s just the controller board that came with it from W-M - it says “Model 1135-605” on it.0
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If you set the high limit to 180, the boiler might run longer during the circulators running in the winter. I like your boilertron. I use a sound activated recorder on top of my boiler to see how many times it runs a day. It doesn't record the duration of the runs though.I'm not a plumber or hvac man and my thoughts in comments are purely for conversation.1
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At 180F, we were waking up sweating every time the heat kicked on at night (even though we had the thermostat set at 62F) - possibly from radiating heat vs convection? Lowering it to 160F seemed to help at least a little with that (although I didn't try to quantify it in any way).
The boilertron worked out great - it actually reports back to a computer every 1 second over bluetooth, and the computer estimates delivered heat for each zone using the average supply/return temp (and linear interpolation based on the baseboard datasheets), so that way I can actually try to track 'efficiency' (burned fuel vs delivered heat) - on cold days it seems to approach 60-70% when estimated that way.
After reading another thread on this forum, I also clocked the gas meter while it was running, and I was surprised how 'low' it was. For the nominally 140K btu boiler, it was burning almost exactly 120 cubic feet/hr on 3 separate days when I measured it. When I properly derated my measurements based on that calibration, they agreed pretty well with my actual utility bills.0 -
4x oversized! That’s very common, especially for houses <1,000sqft but a bit surprising to see oversized like that with a gas boiler, which come in sizes down to ~30,000 btu. Baseboard is well oversized too, you could run much lower temperatures if you wanted, while still protecting the boiler from condensing.
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I’m really interested in switching to an air-to-water heat pump, as it looks like I could meet my heat loss at design temp with anything above 110F. To run lower with my current boiler, I would need to re-pipe things to have some kind of primary/secondary scheme, right?1
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your boiler is way oversized,
i would do a heat loss for the house, Use the Slant fin app.
your burner times are short causing inefficiency and wasted fuel.
A buffer tank could smooth things out.
Changing the piping will not help. you have more output than the radiation can use.
you could have a defective thermostat or zone valve
You shouldn't be waking up sweating at 180 boiler water temp any more than you do at 160.
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Air to water would be fine with this amount of radiation. It’s 872 sqft including the basement?There’s different ways to do it, but yes primary/secondary would work.0
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do you have any flow checks on your returns as it looks like the zone valves are on the supply? your probably getting gravity flow in your returns if your getting better comfort by lowering the loop temperatures. once a fin tube system is off there is not enough Btu's left in the piping to make you uncomfortable. tens minute after the zone is off you should be able touch the baseboard piping in the room and most of its heat will be gone.0
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I may have not been clear - the footprint of the house is like 29.5' x 27.5' or something, so each floor is a little over 800 square feet (so ~2500 total square feet of conditioned space if you include the basement, which is mostly finished except for the 5'x15' mechanical room). The 2nd floor with the bedrooms has a gambrel roof, so there's no attic - just 10" of insulation between the ceiling and the roof.
On the worst day of this heating season, the boiler burned 6.77 therms, which would work out to about 24K BTU/hr of heat if it were actually 83.5% efficient. Based on the delivered heat estimates from the boilertron, it was about 66% efficient with approximately 18.6K BTU/hr actually being delivered to the three zones (so probably somewhere between 18-24K). The Slant/Fin app estimated it at about 30K BTU/hr at 12F, which is also reasonable, as it has a number of south facing windows that get a reasonable amount of light.
@Hot_water_fan - I haven't actually found any contractors in Westchester, NY that advertise A2W heat pumps, although I haven't started calling around yet.0 -
@EBEBRATT-Ed - The thermostat and zone valve are fine as far as I can tell (I also have temperature logging for every room). The issue is the 26' feet or so of baseboard around the bed, which puts off a lot of heat. The two smaller bedrooms have a similar amount of baseboard (they basically just ran it around the external perimeter of the rooms), so with all of the doors closed they'll end a call for heat 8-10F above the larger bedroom where the thermostat is.0
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@pedmec - there are no check valves on the returns, but from temperature monitoring on the supply and return pipes (mounted several feet from the boiler), I don't see more than a 10F or so rise on the return lines of inactive zones, so I assumed any kind of gravity flow must be pretty minimal. The radiators just put off a lot of heat whenever they're active (I assumed the radiant heat was making us uncomfortable, while it was taking convection a while to actually heat the air enough to de-activate the thermostat, although that's just a guess).0
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couple of things.
you stated "you don't see more than a ten degree rise on the return when the zone is satisfied". if the zone is satisfied you should be reducing in temperature. you shouldn't have any rise at all. that return line should be cooling off as there is no forced circulation. if its still rising then you are getting gravity circulation. where did you measure the temperature/ and how long did you measure it for? those honeywell zone valves are notorious for not closing 100%. they use a rubber ball for closure and it fails. so i recommend checking temps at beginning and end of loop on that floor level. if the zone valve fails when another zone calls you will still get flow in the failed zone.
so the room that has the thermostat is the room that is overshooting by 8-10 degrees? or is it 8-10 degress difference between the rooms.0 -
@fentonc - Are all the bedrooms on the 2nd floor? What if you relocate that thermostat to the hottest location?
the two smaller upstairs bedrooms, so they get much warmer than the larger bedroom that contains the thermostat
High-level summary of data from this heating season (November - April):
• The boiler ran 2,777 heating cycles with an average heating call time of 950 seconds/cycle
• The boiler fired 7,050 times with an average duration of 162 seconds, and an average of 2.5 burns/heating cycle
By your data, the burner fired 1175 times per month, 39 times per day, and for less than 3 minutes each time. That's ridiculous! IMO, you could benefit from a Hydrostat controller that will attempt to match the boiler's water temp output to your heating load demands. https://www.supplyhouse.com/Hydrolevel-48-3200-Model-3200-Plus-Fuel-Smart-Hydrostat-for-Gas-Boilers-Temperature-Limit-LWCO-Boiler-Reset-Control0 -
Here is a (bad) floorplan of the 2nd floor and baseboard elements with what I think is how they're plumbed. The baseboards have what I think is the return piped as a bare copper tube above the fin tube.
Edit: The smaller bedrooms are actually 10.5' wide, not 12.5' wide.
@pedmec - The smaller bedrooms are typically 8-10F warmer than the large bedroom at the end of a heating cycle. The thermostat is in the large room, and it controls the air temperature in the room as expected.
I will retract my statement about the return temperature rising somewhat on the inactive zones, as I was just guessing from memory rather than referring to actual measurements (which I can take). My impression was that I am not getting significant leakage through the zone valves, rather that all of the near boiler piping gradually heats up via 'heat creep' during a long call for heat. I have never noticed any significant amount of heat coming from the baseboards in inactive zones. I can try to get exact numbers though.
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@MikeAmann - It's worse than that! If you look at the histogram of burn times I posted, fully half of them are 90 seconds or less. That hydrostat looks somewhat interesting.0
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If you are planning on an A2WHP some day, that will need a buffer tank. So you could connect to the buffer. Add a buffer now to help that boiler cycling Connect the HP some day and have a dual fuel system
How many feet of fin tube. No need to have more boiler than the radiation connected to it.Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
It will help. The real problem is that the boiler is 4X oversized.fentonc said:@MikeAmann - It's worse than that! If you look at the histogram of burn times I posted, fully half of them are 90 seconds or less. That hydrostat looks somewhat interesting.
Move your thermostat out of the big bedroom and across the hallway nearer the 2 smaller bedrooms.0 -
As has been said get a buffer tank. You are way oversized. I'm surprised that boiler is still alive. For comparison my easily 2x oversized boiler only runs about 200 times a month in the dead of winter. With average run times in the ball park of 15-20 minutes. Granted I have CI rads so there is a lot of thermal mass with my system that you do not have with baseboard.0
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@MikeAmann - The bedroom doors are always closed at night, so if the thermostat was in the hallway the only baseboard it would 'see' was in the bathroom, and I think it would lead to generally worse temperature regulation. I did consider getting a thermostat that would let me put a temp sensor in the smaller bedroom, and just let the larger one be cooler on average, but didn't follow through with it.
@hot_rod - it has the following baseboard:- Basement Zone: 22 ft of nominally 790 BTU/ft (at 180F) fin-tube baseboard
- 1st floor: 54 ft of nominally 550 BTU/ft (at 180F) fin-tube baseboard
- 2nd floor: 72.5 ft of 550 BTU/ft (at 180F) fin-tube baseboard
I'll make a floorplan of the mechanical room later to see about the feasibility of a buffer tank.
0 - Basement Zone: 22 ft of nominally 790 BTU/ft (at 180F) fin-tube baseboard
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Okay, here is a quick layout of the basement
It would be possible to remove the wall into the workshop area, although not ideal. I've also been considering putting in a heat pump water heater to replace the gas heater currently in the corner.
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This is another reason i push for mod/con boilers. Although you might not get condensing temperatures when your baseboard return temperatures are high, you still get better load matching. Even at a 5-1 downturn you would have a better chance of matching the heat loss of the structure. Most manufacturers are offering 10-1 downturn with load rate limiting. Its not perfect but with what your seeing from your boilertron I think its a better investment. with a buffer tank you not only have to purchase the buffer tank but pay to have it installed. that's how i look at it.
Adding a buffer tank is a good option but wont give you a return on your investment like a mod/con. your heating a stand alone tank to handle the load. not trying to be negative here but i thought that what we are trying to avoid. stand bye losses. its the reason on demand water heaters are so popular. your not having a tank of water standing around to be used. and i know the buffer tank is insulated but still its a tank full of water at a higher temperature than ambient.
back to the overheating problem. it looks like you need to do a heat loss calculation for the second floor. you obviously have more elements in the small bedrooms than you need, which would explain the temperature differences. or vice versa. you don't have enough enough in your master bedroom. i would do a heat load calc and see what it tells you. without knowing actual heat loss calc's if it was my house i would add more baseboard or anything that would add more heat to the master bedroom that would bring the rooms closer in temperature. adding more heat to the master bedroom would shorten the run time thereby not giving up as much heat to the two smaller rooms. this is providing that your absolutely sure that gravity flow is not happening.
Oh, and one other question. Does the supply start in the smaller bedrooms?
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@pedmec - In retrospect, my home would be well-suited to a much smaller modcon boiler, and it's unfortunate I didn't go for it when I replaced it in 2016 (although the installer would have surely way oversized it). I don't think I would ever need to exceed condensing temperatures, even at the design temp.
Re: Buffer tank - the only 'return' would come from getting improved run-time efficiency at the expense of (probably) increased standing losses. I would guess that my actual standing losses are quite high given that the boiler itself contains around 3 gallons of water, and the area it is in has poor air sealing and no insulation - on many cycles the boiler heated the water from ~65F to 160F, satisfied the call after a few minutes, and then cooled back down to room temp before getting another call. I think much of my boiler's heat goes to slightly warming the surrounding cinder blocks and concrete floor (and the air outside), so at least storing more of it in an insulated tank might help, but the prospects of a financial return do seem unlikely.
Re: Overheating - you're correct, there's just a fundamental mismatch between heat loss/volume and heater output in the rooms. The smaller rooms have 23' of baseboard each, while the large room has 28'. Using the slant/fin app, I estimate the smaller bedrooms at around 2K BTU/hr and the larger room at ~6K BTU/hr. I am not sure what order the baseboards are connected in, although it occurs to me that I might be able to determine it with my thermal camera by looking at the fin-tube pipe and the bare copper pipe in the smaller bedroom located directly above the boiler room, since there is a 10F delta-T.
So the bigger room has 1.2x the heat output and something like 3x the heat loss, which probably explains why raising the temp 1-2F in the bigger room raises the smaller rooms by 8-10F. There is not room for 41' of additional baseboard in the room, although swapping it for 'high output' baseboard like the basement has might actually work. Alternatively, I could try to modify/remove the baseboards in the small rooms to only have ~9' instead of 23', to be 1/3 that of the larger room (which we've kind of done, but in a haphazard, ineffective way).
It does seem like the excessive amount of baseboard makes it a good candidate for an A2WHP retrofit, which my utility currently offers attractive incentives for, although as I mentioned I have yet to find a contractor advertising them around here.0 -
Whoever installed those bedroom baseboards just covered the outside walls with fins, rather than calculating the heat loss and selecting the correct length of element.
If I were you, I would calculate the heat loss of each bedroom and figure out the correct element length; then wrap the rest of the finned elements with heavy duty aluminum foil to block airflow through the fins. You will still have some overheating, but a great deal less and since this costs very little, it’s worth a try.
If that’s not satisfactory, I would simply take the results of that same calculation and replace the excess finned element with plain copper pipe. Problem solved.—
Bburd1 -
regarding the air to water heat pump its not a simple installation? maybe that's why nobody does them in your area residential. And they are real expensive. my company has installed and service them. no return on investment if your trying to be green.0
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@bburd - Trying to properly wrap the baseboards in the smaller bedrooms does seem like a nice cheap way to try to balance out the heat, although presumably this will actually make the short cycling even worse. I have wireless temperature sensors in each room, so I think I should be able to wait until there is no significant solar influx and the air temps are pretty even, close the doors and try to raise the temperature 2F or something - and just keep covering up more baseboard in the smaller rooms until all three rooms have about the same temperature rise.
@pedmec - I assumed it was mostly lack of familiarity, as they're pretty new in the US. Do you know what models you have tried? I was interested in something like the Enertech Advantage AV030. My electricity prices have been going up like 3-4%/year for the last few years, while my gas prices have been going up ~20%/yr (and that was before NG prices doubled in the last month or so!). This area is fairly constrained due to pipeline capacity, so there is an NG moratorium for new customers already in most of the county, and ConEd has decent incentives for people to switch. This season I would have only needed to average a COP of 2.4 to be cheaper than gas, assuming I was actually getting 83.5% efficiency (when I was probably averaging significantly lower than that), and it would let me run at lower and hopefully more comfortable water temperatures. Given that nobody seems to know how to setup gas-fired hydronic heating properly, I guess I shouldn't be surprised that nobody knows how to do A2WHPs either.0 -
@fentonc People here tend to obsess about short cycling, but there are far worse problems in heating systems. Lack of temperature balance is certainly one of them. I would not worry about the short cycling until it’s time to replace the boiler, when you should have a proper heat loss calculation done to size the new one.
Slant Fin has a free room by room heat loss calculator that will give you an idea of how much element you should have in those rooms. It’s on their website.—
Bburd0 -
Even with an over sized boiler, a 90 second or less run time sounds like something is malfunctioning like the circulator or control board. It can't possibly increase the temperature 20 degrees (or 40 degrees if set to 180) in 90 seconds, can it?I'm not a plumber or hvac man and my thoughts in comments are purely for conversation.0
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Which does not rule out a circulator problem, but the control logic/temp sensor seems to be working fine, as confirmed by the temp sensors I have on each supply/return pipe. I seem to have an 8-12F delta-T, depending on zone, so it doesn't seem like there is something obviously terrible about the circulator.0
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@fentonc
^^^ You just made the case FOR a buffer tank. ^^^fentonc said:Re: Buffer tank - the only 'return' would come from getting improved run-time efficiency at the expense of (probably) increased standing losses. I would guess that my actual standing losses are quite high given that the boiler itself contains around 3 gallons of water, and the area it is in has poor air sealing and no insulation - on many cycles the boiler heated the water from ~65F to 160F, satisfied the call after a few minutes, and then cooled back down to room temp before getting another call. I think much of my boiler's heat goes to slightly warming the surrounding cinder blocks and concrete floor (and the air outside), so at least storing more of it in an insulated tank might help, but the prospects of a financial return do seem unlikely.
A house that my uncle bought needed a new heating system. He showed me a 30-40 gallon fiberglass bottle (it was yellow/beige and looked like a giant thermos). He claimed that the hot water stored in that tank only lost 1 degree per day!
You say your boiler only contains about 3 gallons. That's nothing, so it will cool quickly. So what.
Let the boiler make 40 gallons of hot water and the only thing that will be running is your circulators.
You still will have to address the problem of too much radiation in the small bedrooms.0 -
There isn't a ROI on modcons either... Much shorter life expectancy, and much higher yearly PM costs. And what happens when that proprietary part fails in the middle of the coldest night of a holiday weekend 10 years down the road?
I'd also argue that standby losses can be reduced with a buffer tank. Sans buffer every time there is a call for heat the boiler fires. And then the call is satisfied and the boiler shuts off. How much of the heat is still lost up the stack after it shuts down? With a buffer that heat is slowly lost, but lost to the structure and only the structure. If you have a large enough buffer that the boiler only needs to fire only once every couple hours vs multiple times an hour your losses up the stack are reduced.
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The best estimates I have suggest a mod/con, properly sized and installed, will save 10% on fuel usage over a proper CI boiler installation.JakeCK said:There isn't a ROI on modcons either... Much shorter life expectancy, and much higher yearly PM costs. And what happens when that proprietary part fails in the middle of the coldest night of a holiday weekend 10 years down the road?
At today's current oil prices (May 2022- $5.60), a mod/con will save me $375 dollars per year over my CI oil fired boiler, or $3750 over 10 years. That savings will go a long way to pay for a new mod/con assuming a 10 - 12 year lifespan, from which I gathered talking to a lot of professionals locally, is when I would be on bonus time.
But, the mod/con also requires expensive maintenance beyond what a CI boiler does, is far more complicated in design and requires expensive and proprietary parts should it break which offset the fuel savings. A CI boiler running a Carlin or Beckett can be repaired for a lot less and the parts are generally available locally including big box stores for when they quit on nights and weekends. Add that a CI boiler will last 2X as long as a mod/con and there is no ROI, even at today's ridiculously expensive fuel costs.
Now, if oil hits $10 a gallon there may be a ROI, assuming I don't switch over to an electric boiler..
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Right, both a modcon (properly sized!) or a buffer tank would get their 'return' by saving on fuel, so actual ROI (positive or negative) is going to depend on equipment, installation and maintenance costs, along with fuel costs over time. @MaxMercy, note that you were comparing to a 'proper CI boiler' - My tracker estimated the actual achieved efficiency for my setup at about 50% averaged across the season rather than the nominal 83% of my boiler, approaching high 60s/low 70s in efficiency only on the coldest days, and my all-in natural gas prices have gone up 20%/yr for the last three years (and that was before prices doubled recently!). I suspect I could decrease gas usage by ~20% or more by doing any of the following: 1) adding a properly installed buffer tank, 2) putting in a properly installed and sized modcon boiler, 3) putting in a properly sized CI boiler, but my gas usage probably isn't high enough (<400 therms for the whole season) to make any of those a positive financial investment given how high labor costs are around here.
Now there is also the additional factor in my case that the upstairs heating is not terribly comfortable, although that's possibly something I could fix myself cheaply by covering up much of the baseboard elements to even out the rooms (at the expense of likely lowering the efficiency another point or two). A buffer tank would probably only be attractive if it was much cheaper than the alternatives, and let me operate at a lower water temp for improved comfort. An A2WHP seemed like an attractive alternative to a modcon, if I was going to have a potentially finicky piece of heating equipment, as I think it would probably have (increasingly) lower operating costs, and ConEd has incentives to help offset the installation cost (I also have solar w/ net metering, but I'm currently generating somewhat less than my usage, so that doesn't really figure into this discussion unless I build out the array more).0 -
What if you set the differential/cut in of the aquastat way down to maybe 110 degrees or so. The boiler will still fire up to 160 to boil off any condensation but the system will be able to use the heat stored in the mass of the boiler much of the time and you will tun longer cycles. Might need to play with where the lower limit is to still satisfy the tstat in reasonable time but also lengthen out the cycles. Your average water temp will be less so the heat will be less harsh as well.0
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Not completely sure what you mean here. What I was comparing was a only properly sized mod/con compared to a properly sized CI boiler with respect to long term costs.fentonc said:@MaxMercy, note that you were comparing to a 'proper CI boiler' - My tracker estimated the actual achieved efficiency for my setup at about 50% averaged across the season rather than the nominal 83% of my boiler, approaching high 60s/low 70s in efficiency only on the coldest days..
If you mean that your whole system is inefficient for other reasons such as oversizing, then clearly something must be done be it replacing the system or modifying it with a buffer tank. But if I was forced to replace that boiler in order to reduced operating costs, I would still go with a quality CI boiler anyway. Assuming they're both properly sized, properly installed, and tuned properly, a mod/con will never achieve a ROI when figuring in it's short life and expensive maintenance and repairs.
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Yes, I'm agreeing with you - A 36K BTU/hr CI boiler would probably operate significantly more efficiently than my current one (and a modcon boiler would operate somewhat more efficiently than that).1
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sorry, you cast iron guys just cant let it go. you just want an easier installation lol.
I install mod/cons all the time and the mod/cons that i have installed haven't failed at 10-12 years. easily get twenty plus years out of it. a properly installed and maintained mod/con will give you comfort and savings for plenty of years. if your getting only 10-12 years then it is not installed properly. I can't tell you how many poorly installed mod/cons there was at there introduction to the market. techs installing these like the were cast iron boilers. no primary secondary piping, no combustion set up. just hit the switch and go. what could possibly go wrong.
but having said that not all mod/cons are the same. i won't install mod/cons with cast aluminum heat exchangers if i can get away with it. While they have a higher efficiency due to better heat transfer they don't handle the acidic condensate well and require a higher level of maintenance. neglect the maintenance and you will pay. Water tube boilers are easy to maintain. fire tube you can only do so much.
I don't really advocate for either type, cast iron or mod/con (I know you can't tell by my replies) as they both have a place in the market. But with all the new energy codes and all boilers being required to have reset control over boiler water temperature the maintenance, other than heat exchanger cleaning, ain't going to be that much different. cast iron is just old technology and with the price of cast iron now it is almost the same price as mod/con. (I can picture it in my head. contractors installing a cast iron boiler and receives a phone call. and wouldn't you know it he pulls out a flip phone. Just doesn't like to change with the times. I know. I grew up with that guy).
Also with cast iron boilers that they don't tell you is that for 90% of the heating season, if your boiler is properly sized, you will be oversized. Its just the nature of the beast. So throw those efficiency numbers out the window and now your back to short cycling. probably for the cost of a maintenance of a mod/con. That's my opinion and i'm sticking with it.
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The issue with modcons is that they absolutely require yearly pm to continue to operate for decades. Which eats up those fuel savings quickly. 375$ a year in savings is nothing. Now I can do my own yearly pm on my almost 40 year old boiler, the only thing I can't do is a combustion analysis.
Don't get me wrong I like new technology, but going from a ci to a modcon... Is a stop gap measure at best in my opinion. Keep the ci, install a buffer tank and go the air to water HP route. Now you have a dual energy source heating system that is 3x as efficient than the best modcon.0
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