Understanding heat flows in high mass heating systems in old houses

jesmed1 · October 2023

@EdTheHeaterMan and others here have been very helpful and patient in helping me understand the heating system in our 100-year-old 4-unit condo building heated with two Weil-McLain WGO-5 hot water boilers and old cast iron radiators. I've also found a number of articles by @RonBeck very helpful.

Some of the most illuminating points I've found were made by Ron Beck are in this article he wrote about outdoor reset:

"cast iron radiation, contrary to popular belief, is low-temperature heating, especially in modern homes...Back in the day, they may have needed 170 degree water. With single pane, wooden frame windows, and zero insulation in the walls and ceilings, homes had HUGE heat loads. Now, insulate that same house and install new windows. What do you have? About half the heat load, at most. Nonetheless, the cast iron radiators haven't shrunk, so you'll be overheating the home or short-cycling the boiler....

I've been asked many times about what water temperature cast iron systems require. My answer is always the same, "it depends." There is a lot of mass and high water volume in cast iron systems. Therefore, if it's and insulated home, the water temperature doesn't need to be very high...

In cast iron applications where the home has some insulation, I like to see low boiler temperature, minimum water temperature, and possibly the high boiler temperature reduced. Do the homework. Do a heat loss, calculate the water temperature required at design temperature with the amount of installed radiation. Once you have your design water temp, change the outdoor design temp to 60F on the heat loss and calculate the minimum water temperature. You may be surprised."

https://www.usboiler.net/outdoor-reset-doesnt-work.html

Sorry for the long quote, but it's all so relevant to our heating system. It's like Ron wrote that after visiting here and analyzing our building.

Just as he said, we have a massive amount of radiation, roughly twice what we need for our 45,000 BTU/hr heat load at zero degrees. But because we have so much radiation area (EDR=480 sq ft) and so much thermal mass (over 1,000 lbs water and 4,000 lbs cast iron), the thermal mass takes a long time to heat up. So boiler installers look at all the radiation and thermal mass, and oversize our boilers based on those factors, and not on our modern, vastly reduced heat load, just like Ron said.

As a result, we can heat a 4,800 sq ft 100-year old building with water that rarely gets above 140 degrees. The average radiator surface temp at boiler shutdown is 120 degrees. And there's so much radiator surface and boiler capacity that the boilers run only about 1/3 of the time during a zero-degree day. The rest of the time, the system is slowly radiating all the BTU's that were stored in all that thermal mass.

So I began to wonder, at the end of a typical 45-minute boiler cycle, how many BTU's have been radiated, how many end up stored in the water, how many end up stored in the cast iron of the radiators, etc? Most/all of them eventually end up radiating into the house, but I found it instructive to see how they were distributed throughout the thermal mass at the end of a 45-minute boiler cycle.

So here are some relevant numbers:

Gross boiler output in 45 minutes: 105,000 BTU

At the end of the 45 minute cycle, those BTU's have either been radiated away or stored in the water, cast iron radiators, or the boiler mass itself. (I've lumped the piping mass in with the water mass for simplicity).

Here's our thermal mass breakdown:

System water volume: 135 gallons (85 in rads, 40 in pipes, 10 in boiler)
Water weight: 1125 lbs
Cast iron radiator weight: 4000 lbs
Cast iron boiler weight: 600 lbs

Using our typical water and radiator temperatures, in which the radiators and water end up around 125 degrees, from a cold start of 68 degrees, I made a little spreadsheet to see where those BTU's ended up, based on masses, delta T's, and specific heat capacities of water and cast iron:

9,000 BTU (9% of total) radiated (based on avg water temp during 45 minute cycle)
9,000 BTU (9% of total) stored in boiler cast iron
23,000 BTU (22% of total) stored in radiator cast iron
64,000 BTU (60% of total) stored in water

105,000 BTU output total

So what surprised me is the finding that, based on our average water temps over the 45-minute cycle, less than 10% of the heat produced has been radiated into the building. Over 90% of the heat remains stored in the thermal masses of the water, piping, radiators, and boiler itself. And while some of it gets convected up the flue, most of it slowly radiates into the building after the boiler shuts down.

These numbers made me realize that the thermal mass in our 100-year-old system is like an aircraft carrier. It's a huge mass, but because our building heat loss is so low, we don't need the aircraft carrier to go very fast. Only a few miles an hour. So we can put a small engine on that aircraft carrier, run it for 45 minutes, and at the end of that time the aircraft carrier is moving only a few miles an hour. But that's all we need, and now we can shut off the engine, and let that massive aircraft carrier coast for several hours until we have to restart the engine.

Sorry for the long post that isn't really a question, but I found it interesting that Ron Beck's quote above describes our heating situation perfectly. I had come into this house assuming that our old cast iron radiators were high-temperature devices, and with the help of Ron Beck and EdTheHeaterMan have come to understand that, in fact, they are now low-temperature devices because of how our 100-year-old house has evolved and modernized over the years, and that sizing new boilers based on installed radiation and/or water volume instead of the "modernized" heat load will lead to oversized equipment.

PeteA · October 2023

@jesmed1 so did you end up also lowering your boilers max temp at some point? What is the final number you left off with if the rads never got over 120 degrees, what temp water did the boiler reach to get that heat at the radiators?
I will be testing something similar this winter in my house but my setup is different since I kept the radiators but I've removed a lot of the thermal mass of water out of the system when I repiped everything to pex al pex.

jesmed1 · October 2023

@PeteA, I forgot to mention that our boilers start cold, and run until the thermostats satisfy. That typical takes around 45 minutes, during which time the water temp goes from 68 to maybe 130-140 max in the boiler, and maybe 125 in the radiators.

PeteA said:

I will be testing something similar this winter in my house but my setup is different since I kept the radiators but I've removed a lot of the thermal mass of water out of the system when I repiped everything to pex al pex.

You may be surprised to find that most of the water volume is in the radiators themselves, not the piping. In our case, we have about 80 gallons in the cast iron rads, about 45 in the piping, and about 10 gallons in the boiler itself. So you can see that even if we replaced all the high-volume piping with Pex, we'd still have over 90 gallons of water, or about 750 pounds. That's still a lot of thermal mass.

Then our radiators themselves average around 300 lbs each, times 13 rads, for about 4000 pounds of cast iron. Even more thermal mass. (Since the specific heat capacity of cast iron is only 1/9th that of water, this is like 450 lbs of water or so in terms of heat capacity. So now we have an equivalent heat capacity of 750+450=1200 lbs water.)

So add up the masses of your cast iron rads and the water they contain. You may be surprised.

PeteA · October 2023

Thanks @jesmed1 great info

Hot_water_fan · October 2023

There’s so much misunderstanding concerning “mass”, but you have it figured out!

jesmed1 · October 2023

Hot_water_fan said:
There’s so much misunderstanding concerning “mass”, but you have it figured out!

Thank you. I certainly don't have it all figured out yet, but Ed and Ron have put me on the right track.

EdTheHeaterMan · October 2023

I try to put @jesmed1 on some tracks, but that train keeps bringing him back here.

Reminds me of the time I was singing on the job and a gentleman asked me "if I ever wanted to be on the Stage".

I replied "YES"

He told me "There is a stage leaving at Noon, and he would appreciate it if I was on it."

Watching too many old westerns

jesmed1 · October 2023

EdTheHeaterMan said:
I try to put @jesmed1 on some tracks, but that train keeps bringing him back here.

Watching too many old westerns

I think the train you want to put me on is the 3:10 to Yuma! I'll go buy my ticket now.

hot_rod · October 2023

The key to high mass is a finely tuned reset control. You would like to run constant circulation and just pulse heat from the boiler into the loop to match the loss of the building

The on/ off boiler with a single temperature could present wide swings in temperature. Keep the mass in motion.

EdTheHeaterMan · October 2023

I would not be surprised if that building was heated by one hand fired boiler that was in the center of the basement with half of the radiators served by one set of pipes on the left side of the boiler and the other half served by the right side of the boiler.

Then at some point in time over the last 70 years, the building was split into two separate dwellings a left and a right side. At that time the two different owners put in their own heater when the common one broke. This way the bickering about what temperature the home should be was resolved. Then at some time in the last 50 years someone decided that they could make more rent if there were 4 units but that owner included the heat and piped the two oil tanks together. Changing to 4 separate heaters was out of the question. Too expensive. At some point in the last 40 years the building was changed over to condo type ownership so each individual could own the property and share the utilities and common areas. Who shovels the snow and who cuts the grass? ...and we will split the fuel oil 4 ways

Teemok · October 2023

Lowering your set point with a good outdoor reset curve till your on cycle is as long as is comfortable for inhabitants. Efficiency is a cruel game.

mattmia2 · October 2023

you're really only heating that mass on the first firing if you have your thermostat set up right, subsequent firings are just running long enough to replace the heat that was transferred to the structure, the thermostat firs while the system is still hot and the anticipator shuts off the burner before the thermostat feels much of that heat.

jesmed1 · October 2023

mattmia2 said:
you're really only heating that mass on the first firing if you have your thermostat set up right, subsequent firings are just running long enough to replace the heat that was transferred to the structure, the thermostat firs while the system is still hot and the anticipator shuts off the burner before the thermostat feels much of that heat.

In theory yes, but in practice that would be difficult. As I said in my post, we have almost 3x the boiler capacity we need. So even on a zero-degree design day, the boilers only run about 1/3 of the time. So they run 45 minutes, then shut down for 90 minutes or more. By the time they fire up again, the water is cold. So they're alway starting cold. And 99% of the time, the interval is much longer than 90 minutes, typically 3-6 hours of cooldown time.

A low-output modulating, condensing boiler would be ideal where we could run it, say, 10 minutes on, and 20 minutes or longer off, to maintain 110 degree water or less. But that's not what we have. We have big old cast-iron boilers that don't like low water temperatures, and don't like short-cycling, so we're forced to run them for longer periods in order to get the water temperature higher.

jesmed1 · October 2023

EdTheHeaterMan said:
I would not be surprised if that building was heated by one hand fired boiler that was in the center of the basement with half of the radiators served by one set of pipes on the left side of the boiler and the other half served by the right side of the boiler.

Ed, I think you're right. I just read Classic Hydronics and saw a similar diagram on page 13, and had the same thought. Our piping looks exactly like that, except near the boilers where it was recently adapted to two boilers versus the original one.

A neighbor who knew the guy who built this house told me the owner was an older man who built it for himself and his three adult children, so they could all have their own separate apartment. So it's unusual in that it was built originally as a 4-unit, not subdivided later. Which also explains the one boiler for four units--the occupants were all of the same family, so no haggling over heating bills!

The building was turned into condos around 2000 and sold off one by one. We know there were heating problems even then, because the new owners began writing letters complaining of insufficient heat to the lawyer who had owned the building previously and sold it off as condos. We have a copies of her letters and the lawyer's replies ("not my problem") on file. I've since fixed the problems, which had to do with improperly located thermostats, improper swing settings, and probably air in the radiators.

So now the system runs as well as it can, and now we're just planning for the future when one boiler, or both, need replacing.

jesmed1 · October 2023

hot_rod said:
The key to high mass is a finely tuned reset control. You would like to run constant circulation and just pulse heat from the boiler into the loop to match the loss of the building

The on/ off boiler with a single temperature could present wide swings in temperature. Keep the mass in motion.

Yes, exactly. Too bad the boilers we have are totally unsuitable for that!

Jamie Hall · October 2023

This is an interesting analysis by @jesmed1 . Unhappily, it is also seriously flawed. I have no intention of going into the details here, and doing so is more suited to private messaging, which I will do with him when I have time.

jesmed1 · October 2023

Jamie Hall said:
This is an interesting analysis by @jesmed1 . Unhappily, it is also seriously flawed. I have no intention of going into the details here, and doing so is more suited to private messaging, which I will do with him when I have time.

@Jamie Hall OK, thank you Jamie. I am used to being told that I'm wrong here

, so I look forward to learning where I went astray when you have more time.

One thing I see I neglected was radiation loss from the piping during the 45-minute boiler cycle. I've measured the pipe runs and diameters, so I can calculate the total pipe surface area. When I add that pipe radiation in, it bumps the total BTU's radiated up to about 12,000 BTU, or about 12% of the gross BTU's output during the cycle. That still leaves 88% of the BTU's stored in the thermal mass of water, radiators, etc.

Teemok · October 2023

@jesmed1 I support your open spit ball effort to understand where heat is going and when. Not many people make the effort. Being 100% correct about it all may or may not change your big picture. Enjoy your privet lesson. Maybe you can share where you were seriously off after you finalize/refine/correct your understanding.

Jamie Hall · October 2023

The points I had to make are relatively minor, after all, and I have addressed them in a PM to the OP.

jesmed1 · October 2023

Thank you Jamie for your review and comments. Jamie's main point was that the number I calculated for the BTU's radiated during the boiler cycle made sense only if the system is starting cold, which it is. We have so much radiator square footage and so much boiler capacity that the system runs only 1/3 of the time even on a design day of zero degrees, with the result that the water and radiators have time between cycles to reject all their heat and start cold. And 99% of the time, the boilers are idle even longer, typically 3-6 hours between cycles.

Jamie also made a good comment about the overall mismatch between the amount of installed radiation and the excess boiler capacity, and our relatively low heat load. This mismatch forces us into relatively short boiler burns followed by long off-times, instead of allowing a slower, more steady heat output from the boiler that would ultimately be more efficient.

After adding the pipe radiation and tweaking the numbers slightly, the BTU breakdown is as follows:

BTU's stored in water: 58%
BTU's stored in radiator cast iron: 22%
BTU's stored in boiler cast iron: 8%
BTU's radiated by radiators: 9%
BTU's radiated by pipes: 3%

Again, that's the breakdown for the 105,000 BTU output of the boiler during a 45-minute cycle, starting cold and ending with water temps around 120 degrees average in the radiators.

So during the boiler cycle, only about 12% of the heat gets radiated away while the boiler runs. The remaining 88% is stored in the thermal mass at the end of the cycle and gets released over time into the building while the boiler is off.

Teemok · October 2023

I'm wondering how your boiler is piped. I assume the boiler is set up so that it reaches 140F fast and then feeds heat off to the distribution. Or, does it just go through a condensation period every cold start and never reaches 140F. There are hundreds of high mass radiant homes near me that had cast iron and copper fin tube boilers installed with primary only piping, cold slab water direct. They lasted multiple decades despite the misuse, not all with zero problems, but some.

mattmia2 · October 2023

The thermostat should be cutting off before it gets as hot and should be calling again before it cools down so the system stays near an average temp that matches the heat loss. You could do even better with outdoor reset on even a conventional boiler but having return temp protection is even more important here. That is point 1.

Point 2 is a mod con would be selected with a turndown ratio that matched the heat loss on mild days at the low end and would have an outdoor reset curve set such that it more or less matched the heat loss of the building so it would run more or less continuously matching the temp of the system to the heat loss of the building.

PeteA · October 2023

@mattmia2 can you point me to somewhere that explains the outdoor reset functions so I can have a better understanding of how it installs and works. I see it mentioned a lot on this site but I can't find someone that really dumbs it down for the average joe (or Pete in this case) to understand.

Thanks in advance

EdTheHeaterMan · October 2023

jesmed1 said:

Ed, I think you're right. ( you can't say that too much) just read Classic Hydronics and saw a similar diagram on page 13, and had the same thought. Our piping looks exactly like that, except near the boilers where it was recently adapted to two boilers versus the original one.

A neighbor who knew the guy who built this house told me the owner was an older man who built it for himself and his three adult children, so they could all have their own separate apartment. So it's unusual in that it was built originally as a 4-unit, not subdivided later. Which also explains the one boiler for four units--the occupants were all of the same family, so no haggling over heating bills!

The building was turned into condos around 2000 and sold off one by one. We know there were heating problems even then, because the new owners began writing letters complaining of insufficient heat to the lawyer who had owned the building previously and sold it off as condos. We have a copies of her letters and the lawyer's replies ("not my problem") on file. I've since fixed the problems, which had to do with improperly located thermostats, improper swing settings, and probably air in the radiators.

So now the system runs as well as it can, and now we're just planning for the future when one boiler, or both, need replacing.

Just an Idea... While you are all on the same page. Perhaps at a meeting you can all agree that if one of the boilers goes down, that all 4 owners are responsible for the replacement boiler, regardless of which owners are affected. Then make it part of the Condo Docs. This way if someone sells and there are new owners, then the agreement is already in place. That way the new boiler can be installed for all owners' benefit. And it will be cheaper than buying 2 boilers over several years.

Just a thought to run by the other owners. See if you get them to agree now, then the problem is solved. The remaining boiler can be left connected as a backup in the event there is a failure of the new system. Just exercise the old boiler once a month for an hour.

Jamie Hall · October 2023

PeteA said:
@mattmia2 can you point me to somewhere that explains the outdoor reset functions so I can have a better understanding of how it installs and works. I see it mentioned a lot on this site but I can't find someone that really dumbs it down for the average joe (or Pete in this case) to understand.

Thanks in advance

I'm not @mattmia2 , but... outdoor reset is really quite simple. The underlying principle is that the heat loss from a structure is more or less directly proportional to the temperature difference between inside and outside -- so if it's warmer out you don't need as much heat. Outdoor reset simply senses the outside temperature, and adjusts the amount of heat your boiler or furnace puts out by adjusting the circulating water or air temperature to suit.

There are several ways to do this, depending on how a system is piped and particularly on the boiler or furnace. Some boilers or furnaces can modulate -- that is, they turn down the burner power to whatever is needed, just as you would turn down a gas stove to avoid scorching something. In other installations, the circulating water temperature is controlled with a thermostatic mixing valve controlled by the outdoor temperature to achieve the same purpose.

jesmed1 · October 2023

Teemok said:
I'm wondering how your boiler is piped. I assume the boiler is set up so that it reaches 140F fast and then feeds heat off to the distribution. Or, does it just go through a condensation period every cold start and never reaches 140F. There are hundreds of high mass radiant homes near me that had cast iron and copper fin tube boilers installed with primary only piping, cold slab water direct. They lasted multiple decades despite the misuse, not all with zero problems, but some.

No bypass, so with every (cold) start, the supply water rarely exceeds 125 and the return 110. Both boilers are piped this way with no bypass, and both have survived 25+ years in good shape despite that.

There's another discussion on this forum with someone who has an old Weil-McLain oil boiler (like 50 yrs old) that has always run with even lower water temps, and he says it's still OK.

So it seems that maybe these big old (oil-burning) Weil-McLains are not as prone to condensing as people think.

jesmed1 · October 2023

EdTheHeaterMan said:

Just an Idea... While you are all on the same page. Perhaps at a meeting you can all agree that if one of the boilers goes down, that all 4 owners are responsible for the replacement boiler, regardless of which owners are affected. Then make it part of the Condo Docs. This way if someone sells and there are new owners, then the agreement is already in place. That way the new boiler can be installed for all owners' benefit. And it will be cheaper than buying 2 boilers over several years.

Just a thought to run by the other owners. See if you get them to agree now, then the problem is solved. The remaining boiler can be left connected as a backup in the event there is a failure of the new system. Just exercise the old boiler once a month for an hour.

Thanks, Ed, that's a good idea. But to make it a little more attractive cost-wise, maybe I should propose that when one boiler goes, everyone chips in to plumb the remaining boiler the way you designed, with a primary loop feeding two secondaries. That way the cost of a new boiler gets deferred for as long as the remaining boiler runs, which could be another 5-10 years, the way these things are chugging along. So the near-term cost is just the cost of materials and labor for plumbing the primary/secondary loops off the one remaining boiler. I think the other owners would go for that.

I think after you heard about our low water temps, you felt that might not be a good idea, because now one boiler would be seeing twice the volume of low-temp return water. But with a primary-secondary loop design, maybe the flowrates can be adjusted so that the primary loop water temperature stays higher than the secondary loops.

Also, I'm beginning to wonder if these big old oil-burning Weil-McLains aren't as prone to condensing as everyone thinks (gas burning may be different). As I mentioned in a post above, there's another discussion thread with someone who has a 50+ year old Weil McLain oil burner that he says never gets over 130 on the supply side, and mostly gets return temps below 100. After reading something like that, and seeing that our own Weil-McLains have run low water temps for 25+ years with no apparent ill effects, maybe a primary/secondary system like you proposed would be fine with one of our existing boilers.

jesmed1 · October 2023

mattmia2 said:
The thermostat should be cutting off before it gets as hot and should be calling again before it cools down so the system stays near an average temp that matches the heat loss.

In theory that is correct. In practice, it's impossible for our system. The reason is that we have so much radiation that our average water temperature would be like 100 degrees most of the winter. And our boilers cannot be run at water temps that consistently low, at least according to the manufacturer. So the condensation concern forces us to run at least long enough to get the water in the boiler up to 130 or so.

If we had a different boiler with a lower output that could be run at low water temps, yes, we would try to run it as you say.

Teemok · October 2023

I don't know if the cast iron sections are a real condensation concern but burners, bonnets and steel and type-b double wall flue would be.

hot_rod · October 2023

The vent cap is another place to look for condensing conditions in the flue system

mattmia2 · October 2023

Oil has a lot less hydrogen in it than methane. You could always add the missing bypass, i'm sure it was in the manual when it was installed.

psb75 · October 2023

We have an 1855 farmhouse in MI that we renovated using all of the existing cast iron radiators and got some more to boot, plus some panel radiators and heated towel racks. It is a high mass system of 16 or 17 radiators. We are using a Viessmann 222F mod-con boiler. No thermostats in the house. I also made a point to use 5/8 sheetrock throughout the house and it has a two-coat plaster finish. High mass. Works great.

jesmed1 · October 2023

mattmia2 said:
Oil has a lot less hydrogen in it than methane. You could always add the missing bypass, i'm sure it was in the manual when it was installed.

Yes, EdTheHeaterMan pointed that out to me when he saw the pics I posted and realized there was no bypass. We'll have one installed when we replace a boiler.

EdTheHeaterMan · October 2023

jesmed1 said:

EdTheHeaterMan said:

Just an Idea... While you are all on the same page. Perhaps at a meeting you can all agree that if one of the boilers goes down, that all 4 owners are responsible for the replacement boiler, regardless of which owners are affected. Then make it part of the Condo Docs. This way if someone sells and there are new owners, then the agreement is already in place. That way the new boiler can be installed for all owners' benefit. And it will be cheaper than buying 2 boilers over several years.

Just a thought to run by the other owners. See if you get them to agree now, then the problem is solved. The remaining boiler can be left connected as a backup in the event there is a failure of the new system. Just exercise the old boiler once a month for an hour.
Thanks, Ed, that's a good idea. But to make it a little more attractive cost-wise, maybe I should propose that when one boiler goes, everyone chips in to plumb the remaining boiler the way you designed, with a primary loop feeding two secondaries. That way the cost of a new boiler gets deferred for as long as the remaining boiler runs, which could be another 5-10 years, the way these things are chugging along. So the near-term cost is just the cost of materials and labor for plumbing the primary/secondary loops off the one remaining boiler. I think the other owners would go for that.

I think after you heard about our low water temps, you felt that might not be a good idea, because now one boiler would be seeing twice the volume of low-temp return water. But with a primary-secondary loop design, maybe the flowrates can be adjusted so that the primary loop water temperature stays higher than the secondary loops.

Also, I'm beginning to wonder if these big old oil-burning Weil-McLains aren't as prone to condensing as everyone thinks (gas burning may be different). As I mentioned in a post above, there's another discussion thread with someone who has a 50+ year old Weil McLain oil burner that he says never gets over 130 on the supply side, and mostly gets return temps below 100. After reading something like that, and seeing that our own Weil-McLains have run low water temps for 25+ years with no apparent ill effects, maybe a primary/secondary system like you proposed would be fine with one of our existing boilers.

I think that the "pipe the remaining boiler into the whole system to differ the cost of a new boiler" is a great idea. Since you are not experiencing any condensation problems now. When you have the remaining boiler piped in, to do the whole building, you can have the bypass installed and set it to maintain 140° at the boiler even if the rest of the system is only maintaining 120° in the spring and fall fringe seasons. Can't hurt to do it right after 25 years.

Teemok · October 2023

"Maybe the flowrates can be adjusted so that the primary loop water temperature stays higher than the secondary loops." This is how it works.
I was made aware of this distinction recently by smarter people.
The low temperature loop in a primary secondary arrangement may or may not be the "primary" loop.
Boiler loop /system loop? Boiler loop/distribution loop?

mattmia2 · October 2023

That is why i say boiler loop or system loop.

dcaheat · December 2023

I have been reading the posts about "Understanding Heat Flows in High Mass Heating Systems in Old Houses." I have the problem described: massive cast iron radiators that continue to radiate heat after the boiler is off.

I recently replaced a HydroTherm boiler installed in the late 1970s with a Weil-McLain CGa-4. The HydroTherm always kept the house at a constant temperature and the boiler water temperature rarely exceeded about 120 degrees. The CGa-4 seems to heat the water much faster and by the time the thermostat is satisfied the water temp is very high and the radiators are very hot. Here are the details:

When the thermostat calls for heat, the boiler water temp is 70 - 80 and the radiators are barely warm. When the thermostat is satisfied (this takes about 30 minutes), the burners and circulator turn off, the boiler water temp is 130 - 140, and all radiators are very hot (surface temp about 105). Over the next 30 - 45 minutes, the room temp increases 2 - 3 degrees. After 2 - 4 hours, the thermostat again calls for heat and the cycle continues.

The house is stucco, insulated, 2-pane windows with storms, the attic floor is insulated and the area to be heated is only 750 square feet.

I used the Weil-McLain "Boiler Replacement Guide" to calculate the heat loss of the house, which is 32k. So I need a boiler with a water rating of at least 32k. The CGa-3 would be more than sufficient.

These are the BTU/HR ratings for the CGa-3 and CGa-4:
CGa-3 Input: 67k Output: 56k Water: 48k Burners: 4 Capacity: 1.5 gal
CGa-4 Input: 100k Output: 85k Water: 73k Burners: 6 Capacity: 2.1 gal

The heating contractor has offered to replace the CGa-4 with the CGa-3 at no cost to me. Neither the contractor not I know if this will solve the problem.

This is my question: Will the CGa-3 heat the water slower than the CGa-4 thereby allowing the radiators to heat up slower giving the heated air more time to fill the room and allowing the thermostat to detect the temperature change before the radiators become too hot? In other words, might the thermostat be satisfied before the water temperature reaches 130 - 140? Thanks!

Thanks for your help!

mattmia2 · December 2023

The smaller boiler won't solve your problem and make the return water temp problem that you have not dealt with worse.

The best way to fix this is to set up the boiler with a mixing valve or injection mixing, possibly with outdoor reset to provide lower temp water to the radiators and keep the return water to the boiler hot enough to keep it from condensing. It will also need a thermostatic valve to keep the return water hot enough to prevent condensing

A thermostat with an anticipator you can set to shut off the boiler before it senses the heat can compensate for the heat in the mass of the system that will be radiated after the call ends. The boiler still needs return water temp protection to keep it from condensing which can cause corrosion of the boiler or the boiler to plug up with soot. The overshoot problem can be solved with the right thermostat configuration. The return water temp protection needs a bypass of some sort.

dcaheat · December 2023

mattmia2,

Thanks for the quick reply. I am not surprised to hear that the smaller boiler will not solve the problem. The heating contractor was skeptical and I even spoke with an HVAC instructor at Hennepin Technical College (I live in St. Paul, MN) that was skeptical.

The CGa-4 was installed with the necessary bypass as per Weil-McLain. The bypass is fully open, mixing boiler water with return water.

So, It would seem that I need to deal with the thermostat. I have lived in the house for 36 years. The old HydroTherm was controlled with the old Honeywell round mercury thermostat. The anticipator was set to 0.3 and did a great job. I started out using the Honeywell with the CGa-4 but was having the same overheating problem. The heating contractor changed the anticipator setting to 0.8 (the recommendation for the gas valve) but it did not solve the problem. So, the Honeywell was replaced with a PRO1 heat only, non-programmable, with the "heat sweep" set to 0.2. I realize that that the PRO1 "heat sweep" is not a true anticipator. Perhaps I should go back to the Honeywell and try various anticipator settings. Honeywell cautions about setting the anticipator below 0.3, but does not say why.

I am not certain what is involved with setting up to boiler to provide lower temp water to the radiators. I will ask the contractor.

mattmia2 · December 2023

You can't set the boiler to provide lower temps directly, you need to use a thermostatic mixing valve that mixes some of the return water with the hot water from the boiler that lowers the temp to the radiators or use a circulator and controller that injects water from the boiler in to the system loop at a rate based on temp. This would be a less costly fix than replacing the boiler and would solve the problem. The thermostatic valve is the lowest cost if you provide a fixed temp, the injection pump may be easier to do outdoor reset where the temp is set based on outdoor temp.

I'm not sure how the old Laars setup did it, there might have been a thermostatic valve, the bypass might have kept the loop temp down in that case too. Typically those water tube boilers are very unhappy if the return water isn't hot enough.

mattmia2 · December 2023

Setting the anticipator to a lower number on the honeywell T87 should make it end the cycle earlier.

Understanding heat flows in high mass heating systems in old houses

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