Long-term project to update old hydronic -- let's start with the Mercoid switch!

Bernie_the_Brewer

We bought a large 1925 duplex in Milwaukee, WI, about a year ago, and I am setting a long-term course to update the old boiler.  It is a fairly ancient beast (see pics). The guts of it are a U.S. Radiator Corp. unit (Model 25), but I cannot tell how old it is.  It was originally oil-burning, but the burner was retrofitted at some point to natural gas with a Cox Mfg. Co. Wondaire fan/burner.  I assume the original 1925 heating system did not originally have a circulator, but it does now.

Everything in the house, including the heating system, has suffered from a lot of deferred maintenance.  Aided by Dan’s “Classic Hydronics,” I did a lot of maintenance last summer which I was happy to have done before winter set in.  It is performing well so far.

My long-term plan is to get the building envelope tightened up significantly, and then aim to switch to a more efficient heating system.  The previous owners must have been burning $100 bills to heat the joint: There was minimal attic insulation, no wall insulation, and drafts everywhere.  So far I have had the attic insulated and air sealing performed (by pros).  I have also done a lot of weatherstripping, etc.  Next up is wall insulation, replace some drafty old double-hungs, etc.  I am striving to get the heating losses down, and then get a handle on what my heating load will eventually be.

I harbor hopes of switching to an air-to-water heat pump system.  Obviously, that is challenging in this zone (5b/6a), and will require a tight house.  But, as I understand it, ANY highly efficient heating system will require lower water temperatures, and therefore a tighter building envelope.

Soooo, I thought about just lowering the water temperature now, on the present furnace, just to see how the house responds.  The boiler has an old mercury Mercoid temperature switch bolted to the outlet pipe from the boiler.  Incidentally, the system was wired to have the Mercoid cut off all power to the furnace (switching relays, 24VAC, circulators, blower motor, gas valve, etc.) if it trips due to an over-temperature condition.  Largely because I now have a WiFi thermostat that needs the 24VAC, I rewired the system so that the Mercoid will only cut off power to the gas valve and blower motor, keep the 24VAC, and let the circulator run as long as the thermostat is calling for heat.  This seems more sensible to me, but does anyone here see a problem with that?

The Mercoid had been set to 170F.  Last night I lowered the Mercoid set point to 140F.  The Mercoid did indeed trip and shut the system down during the morning heating cycle.  I also have a thermocouple taped to the pipe, and it maxed out at 162F.  Despite tripping the Mercoid, the house did achieve the target temperature of 68F.  I haven’t checked the return water temperature yet, but will do so soon.

If I wanted better control, I could, fairly easily, set up a cheap, modern temperature controller to take the place of the Mercoid, and turn the boiler on and off depending on the outlet water temperature.  This would allow me to set a deadband of, say, 10F or so.  Thoughts?

I suppose my real question boils down to: Is it feasible to try to run an old boiler at a low temperature?  Any gotchas, like short-cycling, or flue drafting concerns?  I wouldn’t think short-cycling would bother a simple, old furnace with a standing pilot light, but I am no expert.  I am vaguely aware of the concern about condensation in the flue, but I don’t think I am anywhere near that with the inferno I have going up the chimney now.

I don’t think there will be any inherent energy savings just from running the old boiler at a low temperature. Again, my goal is to check the feasibility of heating this house with lower temperature water, in anticipation of replacing the old beast down the road.  Any insights are appreciated!

Jamie Hall

Two very different groups of actions here.

As you note, the number one priority if tightening up the house. That will determine how much fuel you use — although a newer boiler will, of course, make a difference too. But the envelope is the biggy.

The boiler… well, you probably need a new one. Until you do that, though, there isn't much point in worrying about the control. The old Mercoid may be out of adjustment — but it is reliable. You do want to worry about return temperature with the old boiler, though. They were never intended to condense, and so you want to make sure the return temperatures get above 140 for most of each heating cycle.

bburd

If that pressure gauge is correct, the water pressure in your system is too high. 30 psig should trip the pressure relief valve and put water on your floor. This is especially notable since the temperature gauge shows the boiler was cold when the photo was taken; water expands when heated and that will raise the pressure.

You may have a faulty or undersized expansion tank. Old gravity systems have a much larger water volume than more modern systems with fin tube or panel radiators, and the expansion tank size is based on the water volume.

you may also have a leaking pressure reducing (automatic water feeder) valve and/or a faulty pressure relief valve which is a critical safety device.

From what I can see on the rating plate, that boiler was oil fired when installed, and given the ratings for steam and water it burned up to 3 gallons of oil per hour. It is probably not the original boiler from 1925 but dates perhaps to the 1940s or 50s. This must be a large house!

mattmia2

If it has a relief valve. I see what may be the discharge pipe for it. At 30psig it may be riding the relief valve because the precharge is wrong in the expansion tank or it is too small. Might be a higher pressure boiler too, I don't see a pressure rating on the ratings plate.

It is an old boiler but the savings in fuel from a new boiler will take decades to pay for the replacement. Looks like the mercoid aquastat has asbestos wiring in it but typically mercury switches will last forever in an application like that. The only application I know of where they have problems is in traffic light and flasher controls where they rock back and forth with a crank and clock motor and eventually after flexing once a minute or 2 for the better part of a century the wires eventually break.

The way to figure out if it will work at lower temps is to measure your radiators and figure out the EDR and convert that to output at different average water temps and compare that to a heat loss calculation. I bet under most conditions they don't get very hot before the heat call ends now but you should still keep the aquastat at 160+ or so.

Bernie_the_Brewer

Thank you all so much for your feedback. Here are a few responses in no particular order:

-Yeah, that gauge in the picture is pretty much science fiction. I am sorry for the red herring — I just was trying to show a picture of the nameplate and innards, for what it was worth. The furnace was not even running when I took that picture. I have a low-pressure tire gauge (visible in 3rd picture) and have been keeping an eye on the system pressure at the expansion tank. When I got the house, the 3rd floor was not being heated because the system pressure was too low. But that was because the backflow preventer and makeup water reducing valve were both completely clogged with crap (rusty mud). After cleaning that out, I set the cold static water pressure to about 13 psi (it's a 3-story home) and bled the radiators. Truth be told, I regularly get discharges from the pressure relief valve when the system is hot. I suspect the pressure relief valve (nominally 30 psi) lets go at a lower pressure; I have never measured anywhere close to 30 psi at the expansion tank, even when hot. I have been slowly backing off the system pressure to try to find the sweet spot. I probably will replace the pressure relief valve, but that would mean draining the system and I am not going to mess with it until springtime! :)

-I really appreciate the comments on the heat load, envelope, EDR, etc. I have been working this angle for some time, but this helps crystallize things a bit. I have been monitoring the gas consumption on a daily basis, and monitoring the outside air temperature, so also calculating my BTU/(heating degree day) each day. The BTU/HDD results have been fairly consistent, even as we varied from -2F to 37F, so that was gratifying. I have had a pro do an energy assessment and modeling. Our measured results are in line with his modeling (measured heat loss at -2F was about 15% lower than his modeled design load). I am also trying to do my own Excel-based heat loss calculation, so I don't have to rely on his black-box software. Work in progress.

-While discussing these results with my wife, and discussing whether we would be able to go to a low-temp system, it finally hit me that I needed to do a room-by-room EDR assessment. Believe it or not, I had done this for my previous home (1-pipe steam) when I was sizing a possible replacement boiler there. But it didn't occur to me to do that here.

-On a final note, I will admit that I really like having that nice, WARM, radiator next to me in the morning, and not sure how I will feel about a much cooler one (even if it keeps the house at the desired temperature). :)

delcrossv

On a final note, I will admit that I really like having that nice, WARM, radiator next to me in the morning, and not sure how I will feel about a much cooler one (even if it keeps the house at the desired temperature). :)

Then you'll probably be pretty unhappy with a heat pump.

I'm also thinking your expansion tank is too small. Those gravity systems have a LOT of water.

mattmia2

The pressure of the expansion tank has to be set with no pressure on the system. even te 10ft of water above it really should be removed when setting the pressure of the airside of the tank. it should be set to whatever the cold fill pressure of your system will be. to measure the system pressure, you need a gauge on the water side. the air pressure in the tank may not exactly be the system pressure and you are loosing an not insignificant amount of your precharge every time you test the pressure.

You should get a pressure gayge with or adapted to hose thread and test the system pressure at one of the drains.

the outdoor temp is only part of the heat loss. some factor for increased infiltration due to wind is also added so unless you have whatever wind speed was used you don't have the design conditions. the infiltration is just a guess unless you've done a blower door test to measure it.

Bernie_the_Brewer

https://forum.heatinghelp.com/discussion/comment/1840345#Comment_1840345

Thanks for the tips on system pressure. Very helpful. I will rig up a gauge on GHT.

Yes, we certainly have infiltration! I have fixed a lot of obvious air bypasses, but there are many more to tackle, I am sure. There is some obvious stack-effect infiltration going on on the first floor. I put some film insulation on a few of the obviously bad windows, and they are blown up like a beach ball on cold days.

We didn't do a blower test due to asbestos. There is no asbestos on the pipes in the basement. However, there is one set of exposed pipes (out and return) that run through a closet in the first floor to the 2nd floor that appear to be asbestos wrapped. I wasn't all that worried about it, but the energy assessor declined to do the blower test. I am debating between having that asbestos removed, or just box it in with a small section of wall in the closet.

In his modeling, he is assuming 0.96 NACH or 13.5 ACH50. I assume this is a SWAG. And yes, you are correct: I did not take the wind on those days into account so it is just a ballpark. (I did save the hour-by-hour weather, including wind speed and direction, for those days, so I suppose I could review that, but I doubt it will be anything important to note.)

Thanks again!

EBEBRATT-Ed

I would guess the EX tank is too small as well.

delcrossv

If it were me, I'd pitch the bladder and go with an old school compression tank

https://qualitytanksinc.com/4574-2/

and add an Airtrol fitting to the tank.

https://www.supplyhouse.com/AirTrol-Tank-Fittings-1238000?

hot_rod

you might do a drain down come spring and get an actual volume if water in the system. That is the best way to size an expansion tank

If you improve the envelop that much, Id guess that boiler will be grossly over-sized

A mod con would work great in a home where the radiation may be greatly oversized after the improvements

Bernie_the_Brewer

Well, it will surprise exactly no one to learn that, on a number of points raised above, you were right and I was wrong! :)

I put a gauge with GHT on a drain located close to the pressure relief valve. During the afternoon heating cycle, it got up to ~30 psi. Also, that pressure gauge that is integral to the boiler is operational, and also read ~30 psi. (It seems to read a pound or two higher than the external gauge.) Meanwhile, the top of the expansion tank was reading ~16 psi. I apologize for stating that it didn't work.

I "burped" the PRV briefly, and the on the two gauges pressure dropped to about 20 psi. They then slowly climbed back up to ~30. So, yes, it seems to be riding the PRV at 30, relieving a little from time to time, then, I imagine, refilling when the system is cold.

I realized that the expansion tank (16 psi) is at the inlet side of the circulator, and the boiler (30 psi) is on the outlet side. I thought that would be the whole explanation. However, when the circulator cut out, neither of these pressures changed, at least for a good while. Still thinking abou that one.

I came to realize that the previous owners had a large expansion tank: It was called the 3rd-floor radiators that didn't have water in them. :)

Back to my experiments with lowering the Mercoid temperature: At least for the past couple of days, the Mercoid does cut out during the morning heating cycle, but the house gets to the desired temp (or even a little hotter) anyway. During the afternoon burn (which is starting from a warmer house), the Mercoid does not cut out. Oh, and I did measure the return water temperature by taping a thermocouple tightly to the return pipe near the boiler. During the afternoon burn, it (slowly) got up to 142F before the thermostat stopped calling for heat. It got up to more than 130F fairly quickly, but then took an hour or so to creep up to 142. I don't know how high it gets on the morning burn (presumably the same or higher), because I was sleeping. :) So, at least at this setting, the Mercoid is not limiting the return temperature to less than 140F (but a short heating cycle might, regardless of Mercoid setting).

Thanks!

mattmia2

The precharge on the expansion tank has to be set with no pressure in the system. If it is at 16 psig with 30 psig in the system then the bladder is expanded as far as it can go, the tank is waterlogged likely because of insufficient precharge.

If it keeps dribbling and replacing water that fresh water will quickly corrode everything in the system, especially the boiler.

Bernie_the_Brewer

https://forum.heatinghelp.com/discussion/comment/1840720#Comment_1840720

As you said, the expansion tank was indeed waterlogged. I had valves that allowed me to isolate it from the system, but had no way to drain it other than to unscrew it. (Of course, water shot out of it like an oversized supersoaker, but I was expecting that.)

I drained it, set the precharge to 13 psi, and reinstalled it. But first I put a tee and boiler drain valve in so it will be easier in the future. (Why didn't they do that to begin with?) Hopefully, I set the inlet water supply at about the same pressure. (I prefilled the pipes, then increased the pressure until I just heard some water flow and the pressure in the tank increased a bit, then I backed it off a hair.)

We'll see what the next heating cycle brings.

mattmia2

Installing them with something like this is common so that they can be isolated and drained:

https://www.supplyhouse.com/Webstone-40612-1-2-Threaded-Pro-Pal-Full-Port-Ball-Valve-w-Hose-Drain

Big Ed_4

I don't see any safeties on the unit but the strap-on Mercoid …. I would recommend an upgrade ….Just saying …

Bernie_the_Brewer

https://forum.heatinghelp.com/discussion/comment/1840858#Comment_1840858

I'm not disagreeing, but I want to make sure I understand you.

I will definitely be replacing this unit, probably this summer, maybe summer 2026. Are you saying there are safety measures I should take with this current boiler in the meantime, or just that it is time to upgrade the boiler in general?

PeteA

@Big Ed_4

It's really hard to see but I think this is a pressure relief valve hidden between the pipes.

PeteA

@Bernie_the_Brewer

The one thing I noticed you said was you "burped" the PRV and both gauges dropped but it went back up to 30psi. This means that the OTHER PRV the (pressure reducing valve) seems to have an issue. This is the one that's on the cold water pipe that feeds the boiler water. This should stop feeding water when it hits 12-15 pounds of pressure. You may need to close the ball valve on your cold water feed (the valve is above the pressure reducer valve and backflow preventer) and then burp out some of the pressure at the pressure relief valve until your cold system is around 12-15psi. Then you should be able to safely run the system. But I would take a close look at your pressure relief valve and make sure that it is not one that is rated for over 30 ponds because it does seem weird that you haven't been spraying water all over the place while the boiler has been running at 30 PSI.

The photo below is your pressure reducing valve which should not allow 30PSI of water pressure to flow into the boiler system. Close the valve that is above these 2 components to stop the water from refilling when you burp off the pressure.

PeteA

here is the fresh water fill valve handle I mentioned above that you should close before relieving some of the pressure

Bernie_the_Brewer

Update: As described, yesterday I emptied the expansion tank and set the precharge to 13 psi. I went to bed with the shutoff valve to the makeup water valve (pressure reducing valve) open. Despite PeteA's observation, I do not believe there is a problem with that valve; when cold, the system pressure sat nicely at 13 psi, i.e., no additional make-up water was coming in.

I woke up at 4:30 to check on the furnace. It had been firing for ~90 minutes, I think. ("Smart" thermostat makes it hard to know.) At that point, it had, in fact, just burped the pressure relief valve, and the pressure was reading ~30 psi. Note: this pressure was indicated at the gauge built into the boiler itself, and the one hooked up to a drain valve, AND at the top of the expansion tank. So, fixing the waterlogged expansion tank at least has made the pressure in the tank reflect the pressure in the rest of the system.

Nonetheless, it is still riding the PRV (relief valve), although it seems to have lost significantly less water than previous cycles.

Later in this heating cycle (~7 am), I decided to try @PeteA 's suggestion: I closed off the valve to the makeup water supply. Now that the system has cooled, the pressure on the boiler itself and at the drain valve read 0 psi. The expansion tank reads 10 psi.

Is this an acceptable way to operate? When the system cools and the water contracts, is there anything bad that can happen? What can make up that volume? Air coming out of solution, perhaps?

Longer term, it seems clear that the bladder tank is undersized for this system, as many of you suggested upthread. I am working on estimating the volume, but it won't be easy. But I was just playing around using the estimator on the Amtrol website. My furnace burns about 225,000 BTU/h. (That is, I measure that it consumes that much gas when firing; I don't know the efficiency.) When I tell the Amtrol tool that I have a 200k boiler, it estimates that I should have 80 gallons of water. It calculates that the EX-60 that I have would be 97% utilized, and maybe I should consider the next size up. But it doesn't even know that I have a gravity system! So I think it is clear that everyone's correct that this tank is too small for this system.

I am somewhat tempted to just slap a larger tank on there (but I hear Ed Youngs sig line "After you make that expensive repair and you still have the same problem, What will you check next?" in the back of my mind!). The EX-60 has 6.7 gallons. I can get a EX-90 (14 gallons) for $160 or a SX-40 (20 gallons) for $250. If that solved my problem, it would be money well spent.

But first I am going to try a bit harder on estimating the volume.

https://forum.heatinghelp.com/discussion/comment/1840919#Comment_1840919

delcrossv

Be careful sizing by BTU, your gravity system has a lot more water volume than a more modern system designed around pumps.

hot_rod

https://forum.heatinghelp.com/discussion/comment/1841031#Comment_1841031

you could add a second tank, maybe another EX 90

mattmia2

where does the expansion tank "read 10 psi"? is there a gauge on the water side connection to the expansion tank?

mattmia2

you have to size the tank to the volume of water in the system.

water expands as it is heated and contracts as it cools. this change in volume compresses the air in the expansion tank. if there isn't enough air to compress to keep the pressure under 30 psig then the relief valve opens and releases some volume of water. when the system cools there is less water in the system because the relief valve opened. this lower amount at its cold volume doesn't compress the air as much so there is less pressure in the system.

since you have a converted gravity system that has much larger pipes than a modern system you either need to roughly measure the length of all of the pipes and note their sizes plus the rough size of the radiators and estimate the volume from those measurements or drain the system in to a bucket of a known volume and count how many buckets it fills.

Bernie_the_Brewer

https://forum.heatinghelp.com/discussion/comment/1841130#Comment_1841130

I meant by applying a tire-pressure gauge to the Schrader valve at the top of the tank. (I purchased a special low-pressure, 0-20 psi one for this purpose.)

mattmia2

If that has gone down from the precharge you set it to with no pressure on the water side than either you have let the precharge out of it buy checking it multiple times or both the schrader and the cap are leaking. it usually leaks out the cap and schrader eventually but that is usually over several months to years.

you need to start over setting the precharge, put the cap tightly on the schrader, then set the cold fill of the system and see if it still gets to the relief valve setting.

if anything err on setting the precharge a little higher rather than a little lower on the tank.

mattmia2

the boiler itself is probably 50+ gallons too

Martial_7erran

Quote: [since you have a converted gravity system that has much larger pipes than a modern system you either need to roughly measure the length of all of the pipes and note their sizes plus the rough size of the radiators and estimate the volume from those measurements or drain the system in to a bucket of a known volume and count how many buckets it fills.]

Since your 30PSI TP relief valve is reliably working, and is letting hot water out at 30PSI, you can measure the expanded-volume of the water that will be required in your total of expansion tanks by removing the existing expansion tank (capping its hole), running the boiler to full temperature, and collecting and measuring the volume of water that is expelled out of the 30PSI valve. Do you even have a down pipe on the outlet of the 30PSI relief valve?

So, if your 30PSI TP relief valve dumps a total of 30 gallons out during one full cycle of heating (up to full maximum temperature of the water), then you know that the total expansion-volume of the set of bladder expansion tanks you need to have has to be somewhat greater than 30 gallons….. This way you do not have to measure the whole volume of the entire pipe and radiator system (which is unreliable and difficult to do)

You asked "When the system [leaks through the TP relief valve and later] cools and the water contracts [and the fill valve adds back more water], is there anything bad that can happen?". One gentleman already told you "If it keeps dribbling and replacing water that fresh water will quickly corrode everything in the system, especially the boiler." He means, that every time your fill valve adds fresh water from the main [to replace the water forced out by expansion], the new water brings OXYGEN into your Boiler, pipes and radiators, which will combine with the iron and RUST-THIN and destroy your radiators, pipes and boiler (made of iron). This is also why you already found the valves on the 3rd floor jammed full of iron oxide sludge. Your system is already compromised by years of internal oxidation, and will start to leak if it continues to corrode. And so, the sooner you install the required volume of expansion tanks the longer your radiators and pipes will survive.

Quote: "I probably will replace the pressure relief valve, but that would mean draining the system and I am not going to mess with it until springtime! :)"] You don't really need to worry about draining the system while replacing the TP relief valve. You can replace the TP relief valve with the system at zero pressure. The water generally will not drain out completely, and you can close some valves to reduce drainage. You are not sweat-soldering copper pipe there so it can be wet when you screw the new PT relief valve into the hole.

I suggest that you go ahead and immediately replace (and verify) the 30PSI TP relief valve before removing the expansion tank while measuring the expanded-volume of water in the system…. Just to be sure that your old 30PSI TP valve does not fail exactly while you are relying upon it to prevent your boiler/radiators to explode because of running with no expansion tank.

Bernie_the_Brewer

Nice! I found this post

https://forum.heatinghelp.com/discussion/183701/estimating-the-volume-of-water-in-radiators

in which Dan himself points us to this document from B&G: https://documentlibrary.xylemappliedwater.com/wp-content/blogs.dir/22/files/2013/02/TEH-1196B.pdf

This has some nice tables estimating water volume for radiators and other components.

I measured all the piping, added up all the radiators, assumed 48 gallons for the furnace (from that B&G document and in agreement with Matt), and come up with a system volume of 211 gallons. (As a bonus, I finally determined the EDR's of all my radiators!)

The B&G document then has tables for recommended tank size under different conditions, and for me it recommends a 25 gallon tank. With the same input, the Amtrol sizing tool comes up with a 20 gallon tank. I am currently rocking a 6.7 gallon tank. So, I suppose that after finishing out some of the remaining tests, like the ones Matt suggested, I will probably invest in a larger tank. As my wife pointed out, even after we replace the old furnace, we will still have a large charge of water that needs a big tank.

Bernie_the_Brewer

Sorry, I hadn't seen Martial_7erran's posts when I made my last post.

Martial said "You asked "When the system [leaks through the TP relief valve and later] cools and the water contracts [and the fill valve adds back more water], is there anything bad that can happen?". One gentleman already told you "If it keeps dribbling and replacing water that fresh water will quickly corrode everything in the system, especially the boiler." "

But that is not what I said. The parantheticals you added changed the meaning of my question. What I actually said was:

"Later in this heating cycle (~7 am), I decided to try @PeteA 's suggestion: I closed off the valve to the makeup water supply. Now that the system has cooled, the pressure on the boiler itself and at the drain valve read 0 psi. The expansion tank reads 10 psi.

Is this an acceptable way to operate? When the system cools and the water contracts, is there anything bad that can happen? What can make up that volume? Air coming out of solution, perhaps?"

That is, there is no makeup water coming into the system. I have the water supply to the fill valve shut off. I was asking if operating with only the water that was left after the 30 psi discharge could pose a problem. Obviously not a good long-term solution, but I was following PeteA's suggesion in order to AVOID adding corrosive make-up water. This is the way I have been operating for a couple of heating cycles, and have not had any more discharges.

delcrossv

You can't go wrong picking the larger tank volume.