Burnham, Lennox to replace busted Weil Mclain boiler — efficiency and durability?
Our 349000 BTU Weil McLain boiler pooped out after 12 years–corrosion in the heat exchanger.
Looking to replace with a Burnham 309 (280000 BTU input 84%) or a Lennox GWB8-299E (299000 BTU input 83.7%).
First are Burnham and Lennox good brands? Will the Lennox titanium burner help prevent corrosion that we experienced?
Second, I am told by both dealers that (as I had suspected and discussed with. y’all before), 349000 was way to big for the house. Anything to be aware of when I downsize the boiler?
Third, two other plumbers quoted prices twice as high as the big boys. They basically said they do a better job and will make all sorts of little improvements making everything neater in the boiler area. Any thoughts?
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It’s leaking. Something about corrosion and a broken nipple in the heat exchanger. No one has offered to fix it.
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Tom: slateandall says it's leaking, so I'm assuming liquid water is evident.
Boilers assembled with push nipples on the water passages between heat exchanger plate sections do sometimes fail and suffer that type of leak (and it's sometimes even temperature dependent!) but usually much later in a boiler's life. Repair is sometimes possible, but requires disassembling and re-assembling the cast iron heat exchanger plates. If the boiler is showing signs of corrosion at the tender age of 12 it's probably not a boiler that's worth investing in that level of repair, and most boiler installers (rightly) wouldn't touch it.
The fact that the boiler is insanely oversized for the heat load of the house is reason enough to pass on any extensive repair. A right sized mod-con could cost less than rebuilding at the plate-assembly level.
I am having a hard time convincing plumbers to install a smaller boiler.
The most reasonable one so far calculated 215,000 btu output based on house size, which gives 256,000 input at 84% efficiency, and then he said due to the large pipes (this was converted from gravity or steam), recommended a 280000 Burnham unit. He also said Burnham makes a nearly identical 85% 280btu unit if I’m willing to install 8” stainless chimney liner (which by itself is $2,800 for 40 feet).
>"The most reasonable one so far calculated 215,000 btu output based on house size..."
Oh really? How does that work?
Was it something like "Lessee, 4000 square feet times 40 BTU per foot comes to 160K, so let's make it 200K just to be sure" ??'
Some contractors measure up the existing radiation and spec a boiler big enough to deliver an average water temp of 170F-180F with all radiators hot. That's just as-bad, often worse.
Can you describe this house? (Size, condition, construction, R-values, window types, ZIP code, etc.)
You can probably get a professional Manual-J load calculation for about 1/3 the cost of that steel liner, at which you can put out a request for proposals based on the actual load numbers, with a "Not to exceed 1.4x the calculated load" clause. That alone usually saves well over the cost of the calculation when dealing with a contractor culture of rule-of-thumb types. Many are reluctant to install anything smaller than what went before, even when (as is often the case) the existing boiler is 2-3x oversized. They know the big one at least worked.
If you're going to punt, you can also buy an easy to retrofit 199,000 BTU/hr stainless steel fire-tube modulating condensing boiler for LESS than the cost of just that liner, and go with $100 of PVC venting instead:
https://www.afsupply.com/htp-uft-199w-ng-lp-uft-boiler-199-000-btu.html?gclid=EAIaIQobChMIyqCr4MWH4AIVzUoNCh0ysQSVEAQYAyABEgLrHPD_BwE
But even a mod-con that size is likely to be overkill for almost any normal sized house. The average house in NY state has a design heat load less than 50,000 BTU/hr, and even fairly large leaky houses are rarely more than 100,000 BTU/hr, but it's common to find heating systems more than 2x or even 3x oversized for those loads. AFUE is tested at a presumptive 1.7x oversize factor, ASHRAE recommends an oversize factor of 1.4x the load at the 99th percentile temperature bin (aka "99% design temperature") for the location.
With non-modulating boilers oversizing has real costs, both in operating efficiency and in longevity, not just the higher up-front cost. That silly-oversized 199K boiler can modulate down to under 20,000 BTU/hr, and could even heat my house efficiently despite being ~5x- 6x oversized for my load, but a right-sized boiler would do it better, and for a lot less money.
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After a cold start with jacket on, it runs for an hour or so before leaking. About a quart dropped out.
I took the jacket off during a period of inactivity and did not see any cracks. Boiler was very dry.
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> “Akos is right, 27 lbs is way high. The relief valve on hot water systems is only 30 psi and will open slightly if near that pressure and fully if it gets any higher. This is probably where the water was coming from.”
Very interesting point. The relief valve opening is not near the floor on my system. It looks like somebody skimped on the piping—the opening is near the top of the boiler at the left side.
Could water from there have travelled somehow to the right side where at one point it was going straight into the right most burner and shutting it down?
Also, the Watts 1156F regulator has a range of 10-25.
No leakage since Friday, I think.
You guys have been great.
All very curious. Where is honest knowledgeable service person in my neck of the woods.
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The temperature shutoff is 170 and the psi is around 27.
Hasn’t leaked in a couple days and I haven’t had a chance to take off the jacket.
Thanks for your help.
Unless your house is 50' or 60' tall, the pressure should not be that high.
Either:
- the water feed regulator is not adjusted properly
- feed regulator is leaking
- busted expansion tanks (full of water)
Getting the pressure back to reasonable number (~15psi) might stop your leaks.
Akos is right, 27 lbs is way high. The relief valve on hot water systems is only 30 psi and will open slightly if near that pressure and fully if it gets any higher. This is probably where the water was coming from. You may not have noticed it since it comes out of the relief valve as steam and condenses in the dip tube going down to the floor then dripping out. Is there green corrosion on the bottom of tube? (The tube screwed into the relief valve to prevent hot water from spraying out if it opens.)
There should be a pressure regulator valve (water feed) on the cold water supply to the boiler and it usually factory set at 15 lbs. It can be adjusted by unscrewing the cap with the lever, loosening the nut and turning the threaded stem up or down, then hand tighten the nut, and screw the lever cap back on. (lifting the lever will bypass the pressure setting to fill faster) (Opening the drain and letting out some water will lower pressure, and it probably wouldn't hurt to lift the lever on the relief valve and flush that too and make sure it seats and shuts off)
Definitely flush and purge the system. Any trapped air will heat up and create the high pressure. After purging, check the fill valve for operation, (you'll hear water coming in until it reaches set pressure). If it goes beyond 15-17lbs, shut off the main water feed, and adjust or replace the fill valve.(No problem if you just shut off the main water feed and keep it off for now in case it is leaking by)
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So you can see the water feed valve is factory "set" at 12-15 lbs. It can be adjusted between 10 to 25 lbs depending on the system. Most hot water systems run at 15 lbs. If my tire is rated at 35 lbs, I can put 50 lbs in it, but it may cause some problems. The green corrosion on the 6" piece of copper tubing coming out of the 30 lb PRESSURE RELIEF valve verifies that the relief valve has been leaking, due to HIGH PRESSURE.
Just open the drain or relief valve and bring the pressure down. Very simple to solve your problem and save yourself $32,000. If you leave it like it is, at 27 + lbs, when the boiler heats up, the pressure will rise and the valve will leak. Then if everything is okay and no more leaks, at some point get someone over there to flush and purge the system if you cannot do it yourself. If there are still cast iron radiators, there should be an air vent that you can open to bleed each radiator.....bill's in the mail.
> "I will explain why the previous owner probably put in such a large boiler later"
I am all ears!
If that system was converted from an old steam or gravity system, then they probably left all the old piping in place. That means 1-1/4" - 2" pipes in your basement and walls. All those pipes hold a lot more water than if it was just 3/4 " copper pipe, probably around 10 x or more. So the time it would take to heat up all that water with a normal sized boiler, especially from a cold start, could take hours. So a much larger system would heat the water up faster. Possibly the previous homeowner wanted that faster operation.
Best to leave your thermostat set at one setting so all that water in the system stays warm and will take less time to heat up and satisfy the thermostat. If you let it get cold, you will use much more energy to raise the temperature of all that water back to where it needs to be.
> "If that system was converted from an old steam or gravity system, then they probably left all the old piping in place. That means 1-1/4" - 2" pipes in your basement and walls."
Maybe I should convert back to steam?
> "Best to leave your thermostat set at one setting so all that water in the system stays warm."
Ouch. I would get really hot at night, and my human emotional brain won't accept that I'm not wasting energy :). Are there smart controls that can help with this?
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> "it probably wouldn't hurt to lift the lever on the relief valve and flush that too and make sure it seats and shuts off"
I lifted the lever on the relief valve twice. First time, it did not seat and kept on dribbling. Second time, it seams to have stopped dribbling.
The pressure reading is down from 29 to 20.
Max temp reads 170F. Should that ever go up to 180, like when it's 0 degrees F outside?
..or rip out all that oversized cast iron piping in the basement and replace with smaller tubing.
No, the high limit is set on the boiler control. (gray box, usually a red dial pointing towards a temperature). The temp should not pass this set point. This is the max temp of the water in the system and the boiler will turn off at this temperature while the water still circulates. As it cools the boiler will kick back on at whatever differential it is set at, usually 10-25 degrees lower. There may be a second dial to set this differential in the same gray box.
Glad you finally found the nerve to open the relief valve and lower the pressure, do it again and lower it down to 15lbs. But definitely purge all cast iron radiators and the system in general to get all air out. If you have been losing water, air has gotten in.
When you purge the radiators , system pressure will drop, hopefully your water feed will compensate if working correctly, otherwise lift the lever on the water feed to increase the system pressure and bleed/purge again, then check system pressure and adjust by letting more water in or out.
Hasn’t leaked in 10 days. Today the pressure was back up to 25. I released the relief valve again.
At what point should I feel relatively comfortable that the cast iron boiler is performing as intended despite the prior leak and the broken pins and
What do I tell the repair person to get them to fix the pressure stuff?
Thanks!
If it's running 25 psi and not dripping I'd call it fine right now, but pay attention to it for the next few weeks.
Tell the repair person that the system is gaining pressure slowly over time, and that you suspect the auto-fill pressure reducing valve and possibly the isolating valve (assuming you've turned it off) are seeping.
If they seem like a competent outfit, ask them for a budgetary quote for installing an Intellicon HW+ or Hydrostat 3250 Plus or other heat-purging economizer control to help manage the oversizing issue.
Tell him to purge all the radiators and the system in general. The expansion tank has about a ft^3 of air and will have no real effect on a system as large as yours. Air in every radiator will.
https://www.youtube.com/watch?v=rry1jCWu-Zs
The system volume here is unknown.
The expansion tank size is also unknown.
Assuming facts not in evidence doesn't always lead down the right path...
Well, the system volume is irrelevant and the job of an "Expansion Tank" is just that, to absorb any minimal expansion or pressure surges that may occur to the water, which is basically a non compressible fluid. It has nothing or very little to do with balancing a systems pressure. That is the job of the pressure regulator valve, (water feed). I am sure if he has a 30 lb pressure relief, the expansion tank is more than likely a standard 30 lb tank. The volume of air in each radiator is more likely the main factor. Once the air is trapped and heated, pressure increases throughout the system, PV= nRT, increase the temperature of a gas, the pressure rises. He mentioned earlier some radiators don't get hot, this is why.
At those sizes this doesn't sound like a residential boiler, even an uinsulated 10,000 square foot house doesn't need a boiler that big. (Unless it'a an uninsulated 10,000' house in Fairbanks AK.) In a normal sized house those boilers have enough output to theoretically heat the house to 70F even when it's absolute zero outdoors.
If you have a heating history on the place, run these numbers, and be prepared for a HUGE downsizing:
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
For most homes with 300K+ boilers the radiation is too much for a right-sized cast iron boiler to work without protection from return water temperatures that are too low, causing condensation on the boiler plates. That's a perfect setup for a modulating condensing boiler though, and mod-cons can even be cheaper to install, since they use cheap plastic venting, don't cost significantly more (and sometimes less) than moderate sized cast iron. The napkin math on zone radiation and mod-con sizing lives here:
https://www.greenbuildingadvisor.com/article/sizing-a-modulating-condensing-boiler
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$32K sounds like gouge unless there was extensive re-zoning or other complications/revisions, but those expenses would be the same if cast-iron. Some condensing boilers are expensive, some less so, but the installation time is not dramatically different than correctly installing a cast iron boiler. Fire tube condensing boilers are pretty forgiving & easy to install as replacements for cast iron.
The plumbing is about the same- any additional set up time would be about programming the parameters to optimize boiler operation to the system and load, which takes more time and thought than cranking a few aquastats. Some installers just set mod-cons to run at a fixed temperature (usually the maximum) and walk away, leaving the tweaking as an exercise for the homeowner. Better installers will at least take a napkin-math stab at optimizing it then check back with at least one site visit for fine tuning it. It's worth taking your time to set it up for outdoor reset control and adjusting the temperature response curve to the lowest temperatures that can still heat the place without short-cycling on zone calls. For more on that see:
https://www.greenbuildingadvisor.com/question/sizing-a-modulating-condensing-boiler
Since it's YOUR comfort and YOUR energy bills it's often better if you do those tweaks and understand the system's limits. This does not take rocket science degree, but it doesn't take a plumbing license either. Plumbers who learned hydronic boiler installation on the job may not have the time, skills, or interest in optimizing operation of a mod-con, but it's not really all that hard.
>"Isn't it crazy to ship a 600 pound hunk of cast iron half-way across the globe to a country that has plenty of iron capacity? "
It's not about the iron capacity, it's the foundry capacity / cost. Shipping is cheap.
It IS crazy to have a 600lb boiler heating your house though.
A leak of a quart per hour won't necessarily show up as a drip with a burner that big- it's just as likely to evaporate and go up the flue unless the leak is near the bottom.
Big news, seems that boiler is still under warranty. Hopefully that will offset the cost of new SMALLER boiler.
How much smaller?? Finding someone to calculate hard.
Slate,
Here is a link to an article that might help you: "Who Can Perform My Load Calculations?"
Just to be clear on boiler size, when you guys quote BTUs are you saying the input BTU or the net AHRI?
If working from a competently executed Manual-J, it would be net AHRI from a boiler spec point of view.
If working from fuel-use based load calculation it would be the D.O.E. output, not the net number:
https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new
A fuel use load calculation doesn't take hard math, and is a measurement, not an estimate based on construction assumptions. It includes all distribution losses, standby losses, etc. Indeed, it's not possible to disaggregate the other losses from the number. IBR & AHRI deratings are a fudge-factor estimate of those other losses, but not very legitimate if the boiler is within fully conditioned space, compared to say, in a garage on the far side of an insulated wall.
Use the fuel-use numbers as a sanity check on the Manual-J. If going with another cast-iron boiler, upsize by no more than 1.4x from whichever number is LOWER. It's rare that the Manual-J comes in significantly lower than a fuel-use load calculation on wintertime-only fuel use, but when grotesque oversizing is in play that can sometimes happen. In almost all cases upsizing by 1.4x from a fuel-use calculation would still be higher than the Manual-J. If the numbers work out differently report back.
While it's silly to run the boiler at 27 psi that shouldn't be enough to cause the boiler itself to leak, though it doesn't take much pressure fluctuation from there to get a pressure relief valve to open up or dribble. Most cast iron boilers are rated for up to 50 psi operating pressures, though the pressure relief valves shipped for residential boilers are usually 30 psi. Boilers that size serving taller multi-story buildings need more pressure than that, and can be ordered with 50 psi relief valves. It seems like your boiler "leak" may actually be a piddling pressure relief valve(?) due to the operating pressure.
The primary thing keeping the boiler pressure in range is the expansion tank, which needs to be pre-charged with air pressure to the target system pressure -0/+3 psi. The target system pressure needed is at least 0.433 x the vertical distance (in feet) from the pressure gauge to the highest point on the system, plus 3 psi to accommodate fluctuations induced by the pumps, but not less than 12 psi (to limit the potential for flash-boil.)
The pressure regulator valve is a nuisance, often a "solution-problem" creating more problems than they solve since they are prone to seeping. Setting the pressure manually with the flip lever on the regulator valve while eyeballing the pressure gauge then closing the valve isolating it from the potable supply is a reasonable thing to do. If the system leaks enough to cause the pressure to drop it'll be making all kinds of flash-boil banging long before it's a hazard.
Suddenly this leaking isn't so scary anymore--but tinkering with the pressure DIY with Polar Vertex coming is beyond me. Here is what I'm thinking:
(1) I will take off the right jacket before the Polar Vertex arrives tomorrow.
(2) Camp out in the basement and observe the pressure relief valve and pressure dial for a few hours, and look for any signs of water, while the thermostat is set at a temperature to make the boiler work hard for a few hours.
(3) If no signs of water leaking from within the boiler, I will put a longer pipe on the pressure relief valve. Done.
Thoughts?
While you are down there, a quick check of sizing during this cold weather is the run time of your boiler (provided it is not a modulating unit). With a stop watch, measure the time the boiler is on and than again the time that is off.
If your boiler is on for 10min and off for 20min, than it is about 3x oversized.
After the cold snap is over, have a plumber check your water feed regulator and your expansion tanks. One of the two is not working properly.
Doing short term duty-cycle measurements are fraught with error- there is a LOT of noise in the signal. With a datalogger with 1-second accuracy running for a week, maybe, but sanding there with a stopwatch for an hour or two, fuggedaboudit.
A better approach is to run fuel use load calculation, from which the average duty cycle at any arbitrary outdoor temperature can be estimated with reasonable accuracy, as well as the load at any outdoor temperature.
Doing the fuel calcs is the way to go. The stopwatch method is a quick and dirty way to check if the unit is grossly over-sized (I measured mine to be about 2x, fuel calcs came in at 2.5x, ), I would never trust it for sizing.
Even if it were a converted high volume gravity feed or 2-pipe steam system slapping a ludicrously oversized cast iron boiler on it just to allow overnight setbacks and have reasonable recovery ramps would be an extremely lousy solution.
Installing right-sized (for the heat load) cast iron boiler using NO setbacks operating the radiation temperature under outdoor reset control mixing would be more comfortable and more efficient than a beastie boiler capable of ramping the systems thermal mass from an idling 70F to 170F in under 20 minutes.
A right sized modulating condensing boiler under outdoor reset control would be even MORE efficient.
Since you have some winter months gas bills to work with, run a fuel-use based load calculation, or post the meter reading dates and volumes for the past couple of months here along the first three digits of your ZIP code and I'll run those numbers for you. But I encourage you to run that exercise yourself- it's not very time consuming, and it gives you a better handle on what you have vs. what it actually takes to heat the place even when temps drop 20F below your 99% outside design temp during a Polar Vortex event. (It takes a far smaller oversize factor than most people think.)
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
Keeping track of the load numbers via fuel use can also tell you when something is wildly off with the heating system, or how well any building envelope upgrades are working, etc.
It doesn't matter what size the boiler is, bigger or smaller. The amount of BTU's needed to heat up the system is the same. The only change will be boiler run time. I would rather have a bigger boiler that runs minimally than a smaller one that runs all day and can't keep up.
It absolutely DOES matter how big the boiler is! The BTUs needed to raise the temperature of the system the same, but the efficiency is not.
AFUE testing is done at a presumptive 1.7x oversize factor & duty cycle. At 2x and up the standby losses start eating substantially into the as-used AFUE. Take a look at where the knees in the regression curves are in the Appendices on any of the systems tested in this bit of research (eg Appendix 1 – page 3, p. 19 in PDF pagination- there are others) :
https://www.bnl.gov/isd/documents/41399.pdf
At 5x oversizing even during the best 87 hours of the year you're only at the 20% mark on the curve. The other 99% of the time it's to the left of that, on the steeply declining efficiency cliff. With heat purging controls set up correctly the knee can move to the left, but at 5x it's still not enough to overcome the oversizing effects.
A right sized CONDENSING boiler will run pretty much all the time AND it will keep up, AND it will use 30% less fuel than a 5x oversized cast iron beast.
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"It absolutely DOES matter how big the boiler is! The BTUs needed to raise the temperature of the system the same, but the efficiency is not. "
Actual boiler efficiency has nothing to do with how well it heats your house, it is a function of the design related to how efficiently it transfers the energy coming out of the burners to the water within and the amount of excess energy that is lost up the flue in the process at a given temp and pressure. this value cannot be changed.
How you use that boiler to heat your house is related to the efficiency of the user. If you have a car that's rated at 20 mpg at 65 mph and you constantly drive 90 mph you are using it inefficiently and will get less bang for your buck.
A modulating gas boiler may cover some new system over sizing errors as does a modulating gas forced air furnace. Right sizing of a new system is not that difficult with former gas bills measured against heating degree days for the period.
You get into a discussion about nightly setbacks being appropriate or not for different heating systems. Hot water heat is a whole different animal from forced air, maintaining the boiler water temperature is a big part of the calculation.
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>...maintaining the boiler water temperature is a big part of the calculation.
Make that "system water temperature" when dealing with high mass systems. That's totally not a problem with modulating condensing boilers under outdoor reset control, or right-sized (for the heat load) cast iron with radiation water temps moderated with outdoor reset control mixers.
Here is a real world example of new furnace sizing for an existing residence when gas usage and heating degree days are known for a given period.
The heating gas usage for our house for 11-18, 12-18 and 01-19 was 317 therms. The heating degree days for the period in Minneapolis was 3,869. So, 31,700,000 Btu's divided by 3,869 Hdd and by 24 hours is 341.4 Btu per degree F heat loss for our house. I will use a design temp of -20 F so 341.4 x 85 is a design temperature heat loss of 29,019 Btu's. If the allowed new sizing requirements are 1.4 then my my optimum new furnace size would be 40,627 Btu maximum output.
Current furnace efficiency is 95% and I am assuming the same efficiency for the proposed new furnace.
> “cast iron with radiation water temps moderated with outdoor reset control mixers”
Can I outdoor reset control to existing setup?
Stay away from those outdoor sensors....they just ramp up the burner like going 90 mph to get somewhere faster......your boiler is probably not compatible with them anyway.
The answer is "probably yes", but outdoor reset mixer installation is not design-by-web-forum project.
However, adding outdoor reset controls to the existing boiler is do-able, but may not be particularly useful due to the oversizing factor. A heat-purging economizer control such as the Intellicon HW+ or Hydrostat 3250 Plus is usually more appropriate, often DIY-able. Almost all Weil-McLain cast iron boilers of a dozen years ago are fairly easily retrofittable with the after-market heat purge controls, and those controls aren't very expensive, if you feel up to a bit of DIY work on it.
Many newer residential cast iron boilers already come with heat purging controls as standard equipment (some with outdoor reset options), after the Brookhaven National Laboratory testing over the past 10-15 years showed just how much of a difference it makes. Compare the steady-state thermal performance against average performance at 2x and 3x oversizing in Table 3 non page 9 (p14 in PDF) of this document:
https://www.bnl.gov/isd/documents/41399.pdf
Notice how little efficiency hit is taken even at 3x oversizing with system #3 which has integrated heat purge controls. (That was an Energy Kinetics System 2000.)
Boiler manufacturers have noticed too, which prompted a wave of products with smarter controls (eg Burnham's "-IQ Control System" boilers). But it is still possible to buy "dumb as a box-o'-rocks" cast iron with simple aquastat controls.
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You can use outdoor rest with cast iron setup. It will make the house much more comfortable as it keeps the water temperature in the rads much more stable, it will also stop the banging and clanking you probably get with your existing setup each time the system runs.
Here is the catch. It is something that needs to be set up properly and it is potentially something that might need to be fixed down the road. So, if you can't find a plumber that can set it up for you properly, then it is not worth the cost of putting it in.
If you do get one installed, make sure the system has a boiler return water temperature protection.
Kudos and Thank you!
Just wanted to say thanks to everyone who chimed in and helped me diagnose the problem!
> “..or rip out all that oversized cast iron piping in the basement and replace with smaller tubing”
What is your opinion on wrapping the large cast iron pippiping with fiberglass insulation in the very cold areas of the basement?
The "very cold area" will get even colder if you insulate, are you ok with tha? You're better off to air seal and insulate the envelope, then the heat given off the uninsulated pipes isn't wasted out the walls.
Outdoor reset, and a properly modcon are the way to go in your case.
Outdoor reset, is a proven strategy, it can get a little better with the addition of indoor reset to fine tune the setpoint even further.
Oversizing the boiler usually ups it's minimum flow requirement, which often means a warmer (read less efficient return water temperature), and more inefficient on and off cycling and purging. You want the boiler to stay running as long as possible so you want the lowest possible min fire rate on a boiler that can still handle your design day heat loads.
Since you have a high mass system it's possible you might be able to direct pipe a medium/high mass condensing boiler with a low pressure drop.
Maybe something like this:
https://www.phcppros.com/articles/7804-when-old-tech-meets-new-tech
New technician concluded that expansion tank is faulty. Not clear if this is connected to previous water leakage.
Is it worth replacing the expansion tank? So what if it’s bad as long as I’m getting heat in the house?
It's absolutely worth replacing a failed expansion tank with one correctly sized for the system volume, and pre-charged to the target system pressure. A failed expansion tank will result in rapid rise in pressure with temperature, and is the leading suspect for why your pressure relief valve would be opening up.
The expansion tank condition & pre-charge level is the FIRST thing to check when the pressure relief valve is opening up!
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>"So what if it’s bad as long as I’m getting heat in the house?"
Are you waiting for something to break? What if the pressure relief fails in the meantime, either in the open position, flooding the boiler room with dangerously hot water, or in the closed position, allowing pressure to build until the weakest component in the system comes apart at the seams? Pressure cycling system components increase mechanical stress/fatigue. A properly sized properly charged expansion tank moderates the pressure within a fairly narrow and completely safe range across the full range of system temperatures.
My parents have hydronic heating and the expansion tank needed replacing The diaphragm had failed so the tank was completely filled with water as I understand it. This was not the first thing checked but the heating technician found the problem.
The quick & dirty expansion tank test is to tap it once on both the top and bottom. The air side should ring/ding, the water side will thud. Evaluating whether it's full of water because the diaphragm is bad can be as quick as pushing the schrader valve with a screwdriver- if water comes out the air side it's toast. If no water comes out, it could be attributable to losing the charge due to an air leak (usually the schrader valve), or an improper charge.
After eyeballing the pressure gauge and bleeding down the system (if necessary) to a reasonable pressure, this really is the first, easiest, and most suspect physical thing to check if the pressure relief has been spitting. There are other things that need checking too, but the expansion tank is really at the top of the list.
Sure, they are cheap enough, and hopefully it is isolated for a quick change. Regardless, system pressure keeps rising on you. Older expansion tanks didn't have bladders and often filled with water and systems still ran okay. Hopefully you can realize that the volume of water in the expansion tank is nothing compared to the amount of water in your system. Did you shut off the main water supply once you dropped the pressure by opening the relief valve to ensure no water is entering the system? Does the pressure increase with increasing boiler temperature? Are all the radiators getting hot? Did you watch the video on how to bleed your radiators? Who knows how long the expansion tank has been busted, the problem just started so it must be something else. When an expansion tank fails you may have a minute drop in pressure, not an increase.
More speculation on facts not in evidence, mayhaps? :-)
The very purpose of the expansion tank is to moderate the pressure changes with temperature. High volume high temperature swing systems need bigger expansion tanks than low volume low temperature swing systems, and the expansion tanks need to be sized correctly for the system to do it's job.
Older style expansion tanks didn't have bladders or diaphragms, but they still needed a charge of air to function, and are still an important part of the pressure management of the system. Part of the annual maintenance on those systems is to drain the expansion tank to guarantee a minimum amount of pre-charge.
Counting on random amounts of air in the rads to function as the expansion tank isn't great practice, and bleeding the air out of those rads increases the problem.
If the system pressure keeps rising over days/weeks even with the fill-valve isolated from the potable supply the isolation valve needs to be replaced.
"Counting on random amounts of air in the rads to function as the expansion tank isn't great practice, and bleeding the air out of those rads increases the problem."
What? You're not using the air in the radiators as an expansion tank. You need to remove the air in order to allow water to flow, otherwise the air heats up and creates pressure. You are not increasing problems you are removing them. There are countless videos on you tube showing how and why to bleed radiators, no different than any other hot water system. Air needs to be removed.
I fully understand the reasons for bleeding rads.
Perhaps I was misunderstanding the intent of ...
"The volume of air in each radiator is more likely the main factor. "
... in response #65.