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# Matching boiler size to radiator output

Member Posts: 120
My 2 1/2 story masonry house built in 1897 has an original gravity hot water system. The house is probably around 5,500 square feet. My coal burning boiler was updated in the early 70s to a cast iron WM gas boiler with an output of 196,000 BTU/hr. Even during sub-zero temperatures, the little boiler has kept the house perfectly warm. In such super cold days, the boiler runs for long periods, but still occasionally cycles off. Most of the winter, the radiators are just lukewarm, and even during the polar vortex a couple of years ago, they never became so hot that I couldn't hold on to them comfortably. I doubt they ever even reached 130 degrees.
I spoke with two contractors, and they both thought that I need a boiler with an output of around 240,000 to 260,000 BTU/hr. in order to get a house of this size warm from a cold start.
I haven't done a heat loss calculation, but I did calculate the output of my radiators, which showed a total of 1,414 sq feet of radiation. I even added the 200 feet of uninsulated 3" pipe in the basement and rounded that number upwards to 1,700 sq feet of radiation.
Even if my radiators ever get as hot as 140 degrees, which in 25 years they never have, that would mean 153,000 BTU/hr output from my pipes and radiators. At 150 degrees, that would mean 187,000 BTU/hr of output.
Is there any reason to have a boiler that has an output so much greater than what my radiators need to emit on the coldest days? Or should I just go with the Vitocrossal 300 CU3A with a max input of 199 MBH? Assuming a 95% efficiency, it should produce a maximum of 190,000 BTU/hr.
I see my neighbors with smaller houses than mine getting new boilers way bigger than I have. I don't want to make the same mistake.
«1

• Member Posts: 5,737
Boiler size for a hot water system is calculated by doing a Manual J heat loss of the building and sizing according to that. The radiator survey you did would be used in conjunction with the heat loss to predict what water temps you could run.

If you are doing a condensing boiler you can run outdoor reset which will automatically adjust water temps for maximum comfort. The most efficient and comfortable way to run these systems is get as close to them running constantly as possible. The burner will modulate according to the load based on the outdoor temps. If you oversize, you are cutting off the low end which can possibly get you away from this concept. Remember the heat loss give you max load on the coldest day, most of the time you will be well below that.

The contractor you describe are certainly not doing anything right on sizing, they are either using rules of thumb or simply guessing, which isn't correct. I'd keep looking until you find one that want to do their job (Manual J). They might not do it for quote, but it should be mentioned in any quote that they will do it for final boiler sizing.
2014 Weil Mclain EG-40
Boiler pictures updated 2/21/15
• Member Posts: 120
Thank you KC. The owner of the house when the current boiler was installed was a structural engineer at the HOK architectural firm. He said the HVAC expert at HOK came to the house and went to all the rooms on all three floors and did measurements, and that's how the current boiler (196,000 BTU/Hr output) was decided upon, and it has worked perfectly so far. As they say the proof of the pudding is in the eating. If the current rather small boiler has worked so well this long, I don't want to be talked into something bigger that I don't need.
My goal is exactly what you describe: A boiler that cycles as few times as possible.
• Member Posts: 120
Youngplumber, I live in St. Louis. I have actually measured the temperatures of the supply and returning water to my 1970s cast iron boiler with an infrared thermometer with laser distance guidance. When it was 20 degrees outside, with the thermostat set to 70, the water leaving the boiler was around 110-112 degrees and the return was between 96-98 degrees. And the house was perfectly comfortable and even throughout. According to the previous owner, who worked for the architectural firm HOK, this boiler size was chosen in the early 70s after the head of HVAC at HOK went around the house and calculated how big a boiler was needed. That's how they decided on the 196,000 BTU output. I must say that I've never felt as if this boiler couldn't keep the house perfectly warm, even when the temps were blow zero.
• Member Posts: 23,276
I would say, given all that you have said, @Shahrdad , that your existing boiler size is probably just about right. Doing your own heat loss calculation might be an interesting exercise to confirm that; Slant/in has an easy to use calculator here: https://www.slantfin.com/slantfin-heat-loss-calculator/ for that.

Don't, whatever you do, let the contractor talk you into oversizing your boiler. It's common enough, and it's just plain wrong.

@Youngplumber does have a point on the return temperatures -- for modern boilers. That old Weil-McClain of yours, however, simply doesn't care. It may not be up to modern gee whiz efficiency levels, but it's also bulletproof and will likely run for a good time yet, even at condensing return temperatures.

If you decide you need a new boiler, however, you will pretty well have to go for a modulatng and condensing boiler: modulating, to match the heat loss of the house better without on and off cycling, and condensing, to manage the potential for cooler return temperatures. Even with the best, though, I would not expect it to last more than 15 years -- never mind the nearly 50 o your WM. You will also have to go with primary secondary piping revisions and outdoor reset.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 7,829
edited January 2021
You are lucky t have had someone that understood Heat Loss Calculations to properly size the boiler in the the1970s. It would still be a good idea to have that calculation verified today. You may be able to use a smaller than 199,000 BTU input boiler. The fact that your radiators do not require 180° ever is a good thing. and selecting a condensing boiler that modulates will drastically reduce the fuel usage while increasing the comfort (if that is at all possible).

If the boiler you select has an "OPTIONAL" outdoor sensor, cross out the OPTIONAL and be sure the installer includes it in the installation.

Sounds like you have a great system and you are going to make it better.

I can see why @Youngplumber was surprised at the low-temperature operation because we often find a problem with the condensation of flue gasses when cast iron boilers are not kept above 140°F return water temperature. Your selection of a condensing boiler will eliminate that problem because they are designed for that condensation.

Yours Truly,
Mr.Ed
Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
• Member Posts: 120
edited January 2021
The old boiler has so far worked great. It is, however, getting quite long in the tooth, and some of the interior insulation has started to fall off. Also, whenever I clean the boiler in the spring, there is always a lot of rust on the bottom and also inside the burner tubes, so it's just a matter of time before goes wrong with it.
What I'm looking at is the Viessmann Vitocrossal 300 CU3A, which is a high water mass boiler with large passages and nearly no internal resistance to flow, which is designed to minimize cycling. They also say that because it has almost no resistance to flow, it can be piped in without the need for primary and secondary piping, and it does come with an outdoor reset. It looks like it would be a perfect fit for my house. It just has slightly less output than my current boiler (190 MBH at 95% efficiency vs. my current 196,000 BTU/hr). I have read that when systems such as mine that have rather cool return temperatures, these boilers can run at an even higher efficiency than advertised.
One thought that had crossed my mind is was to have the installer install two closely spaced Ts with shut-offs on each side on the return side of the boiler. That way, should this boiler prove inadequate, a small high efficiency boiler (something like a Viessmann Vitodens 100) could be added for additional capacity in the future. It's probably a harebrained idea, but it did cross my amateur's mind.
I did download the SlatFin app on my phone. The house has a several odd shaped round rooms as well as large staircases that go up two and three stories, so I have to figure out how to use the app with these spaces. A snapshot of it is on my profile pic on here.
• Member Posts: 1,215
The most accurate method I've found for calculating heat loss is to use the actual usage of the home and weather data. On that coldest day record your usage over a 24 hour period to see what your average input is per hour. Then deduct the usage for other gas input that you can see in your summer bills. You can also use the fuel bills.

Your boiler may have been piped with a bypass loop to allow the boiler to run hotter, which I have found that is really all that is necessary to reduce condensing in low temp systems. This is usually done by only running part of the water flow from the system through the boiler, so its average temperature rises. When the boiler shuts down, its heat soaks and its average water temperature rises considerably, drying out the castings so there is limited corrosion from running at low return temperatures. If this is a WM CG series, this is especially the case. These boilers only hold a small amount of water, so upon shut down the boiler gets quite hot, drying it out. This is the same methodology used for condensing cast iron boiler designs,.. they dry out the heat exchanger between firing cycles.

I would avoid condensing type boilers. Yes, you get higher gas conversion efficiencies, but at considerable cost. Boiler life is probably about 1/2 and you use much more electricity to run a pump and draft inducer continuously and all the supporting electronics. On a typical properly sized boiler, a single pump only runs about 1/3 or less of the heating season.

If you really have your heart set on a condensing boiler, repipe the current boiler as a secondary loop off the main system loop and then install a small condensing boiler to handle the typical day heating load...probably about 2/3 the design day load. This will usually allow the small condensing boiler to heat the home for about 90 to 95% of the heating season. Doing this you still have the reliability of the cast iron boiler as back up for the more trouble prone condensing boiler and you will spend less on the boiler and save just about as much fuel as putting in a bigger more expensive boiler.
• Member Posts: 120
Thank you! My old boiler has never had a bypass loop. It just has a Taco 110 circulator (also from the 1970s) on the return side, and the supply side goes straight to the pipes. The old boiler is a WM CGM-8. I am rather surprised that it hasn't rusted through without a bypass, and it's going on 50 years now. But there is definitely rust inside, which falls on top of and under the burners, so I think it's just a matter of time. I just don't want to be left without heat in the middle of the winter if it springs a leak.
I was going to go with two small cast iron Buderus boilers until I saw the Vitocrossal 300 CU3A. It seems the design--with almost 19 gallons of water inside and large passages for water--is a lot more resilient than most high efficiency boilers. That's really what got me thinking about going with this boiler rather than with cast iron. I definitely do see your point about the cost of running a fan and a pump as well.

• Member Posts: 120
Steam Whisperer, the more I think about your idea, the more I'm liking it. The old WM is still chugging along perfectly, and I could use it as a backup boiler for especially cold days or when the other one needs maintenance. A good installer should know how to wire the old boiler to act as a secondary/backup boiler, I'm sure.
• Member Posts: 15,517

Just a few comments. I agree with the other posts above (for the most part).

But there are a few buts.

#1 a mod con boiler only gets 90-95% when it operated in the condensing mode that is return water temps below 130 deg.

You house is a good canidate for this type of boiler and will probably use less fuel than anything else.

Mod cons are finicky and replacement parts are \$\$\$\$ and the verdict is still out on how long they last. If I had to guess I would say 15 years. They cost more and are more expensive to install.

#3 They require a QUALIFIED technician, much more so than a standard boiler.

I am more inclined to install a standard boiler. In your case I would install a 3 way valve if a standard boiler is installed to make sure the boiler return water never drops below 140 degrees.

There is no right or wrong answer to this question.

But to have success with a mod con you must have a good installer, qualified technician and an accurate heat loss.

That is must with a mod con and should be required with any boiler
• Member Posts: 120
@Ed, Thank you! I wasn't even thinking about a mod-con until I saw the Viessmann Vitocrossal 300 CU3A. It's so completely different in design and construction from other mod-con boilers. The return water in my house is always below 130. My radiators don't even need to ever get that warm even on the coldest days.

Interestingly, one of the things I dislike most about mod-cons is seeing the steam coming off the side of the house, as if the dryer were always running, but it seems to be par for the course for these boilers. I do like Steam Whisperer's idea of getting a smaller mod-con and keeping my current cast iron boiler as a backup.
• Member Posts: 15,517
LOL the steam you hate seeing is the fuel you save with a condensing boiler. Weather they save money over the long haul with maintenance, parts cost and longevity is still a topic for discussion here
• Member Posts: 120

LOL the steam you hate seeing is the fuel you save with a condensing boiler. Weather they save money over the long haul with maintenance, parts cost and longevity is still a topic for discussion here

I know! It just isn't an attractive look, but might be worth the savings in fuel costs.
• Member Posts: 241

The most accurate method I've found for calculating heat loss is to use the actual usage of the home and weather data. On that coldest day record your usage over a 24 hour period to see what your average input is per hour. Then deduct the usage for other gas input that you can see in your summer bills. You can also use the fuel bills.

Your boiler may have been piped with a bypass loop to allow the boiler to run hotter, which I have found that is really all that is necessary to reduce condensing in low temp systems. This is usually done by only running part of the water flow from the system through the boiler, so its average temperature rises. When the boiler shuts down, its heat soaks and its average water temperature rises considerably, drying out the castings so there is limited corrosion from running at low return temperatures. If this is a WM CG series, this is especially the case. These boilers only hold a small amount of water, so upon shut down the boiler gets quite hot, drying it out. This is the same methodology used for condensing cast iron boiler designs,.. they dry out the heat exchanger between firing cycles.

I would avoid condensing type boilers. Yes, you get higher gas conversion efficiencies, but at considerable cost. Boiler life is probably about 1/2 and you use much more electricity to run a pump and draft inducer continuously and all the supporting electronics. On a typical properly sized boiler, a single pump only runs about 1/3 or less of the heating season.

If you really have your heart set on a condensing boiler, repipe the current boiler as a secondary loop off the main system loop and then install a small condensing boiler to handle the typical day heating load...probably about 2/3 the design day load. This will usually allow the small condensing boiler to heat the home for about 90 to 95% of the heating season. Doing this you still have the reliability of the cast iron boiler as back up for the more trouble prone condensing boiler and you will spend less on the boiler and save just about as much fuel as putting in a bigger more expensive boiler.

This is the best way to do it. Pick a night or morning as close to your design day temperature and log burner on and off time. I did mine the other night- 15 minute burns and then off for 85 minutes. So a 15/100 burn rate, or 15% per hour. So my 200,000/hr boiler * .15 * .80 efficiency is 24,000 BTU per hour @ that outdoor temp delta (Indoor setpoint minus outdoor temp). This includes basement losses.
• Member Posts: 1,215
Actually at that low load, your efficiency is much lower than 80% on a typical cast iron boiler. The attached chart shows atmospheric boiler efficiency as a percentage of peak load efficiency. I have found it highly accurate for determining potential fuel savings of upgrading heating plant efficiency. This is for boilers without stack dampers. At 15% load your efficiency is only .8 x .8 (80% peak load efficiency) = .64. You only get about 64% of the heat out of the boiler compared to the btu input. Stack losses are huge at low inputs and jacket losses can also add up. The penciled additions are the efficiencies of boiler 2 and 3 times oversized.

Back to the original poster, Using 2 small cast iron boilers piped with Primary/ secondary piping and stage fired will likely cut your fuel bills by 20%, assuming you current boiler is about the right capacity.

You could just install a single small cast iron boiler with your current boiler and probably save nearly that same 20%.
• Member Posts: 120
I figured my old boiler is running at much lower efficiencies than 80%. At least it's a much smaller boiler than what is commonly installed in homes of this size. After nearly 50 years, I think it's just a matter of time before it springs a leak.
• Member Posts: 1,215
If it is sized correctly and not fouled with sludge and mineral deposits, it probably still has a continuous firing efficiency near 76 to 78% and a seasonal efficiency around 68% to 70%.
• Member Posts: 120
edited January 2021
It's 18 degrees this morning. My supply pipes are reading 111-112 degrees and the return pipes 98-99 degrees, both measured near the boiler. The boiler runs for about 11-12 minutes, then is off for about 8-9 minutes. The house is a comfy 70 degrees.
• Member Posts: 120
edited February 2021

I would say, given all that you have said, @Shahrdad , that your existing boiler size is probably just about right. Doing your own heat loss calculation might be an interesting exercise to confirm that; Slant/in has an easy to use calculator here: https://www.slantfin.com/slantfin-heat-loss-calculator/ for that.

I downloaded the app, but the house has so many rooms that are not rectangles, that it makes it hard to figure out what numbers to plug into the Slantfin app. There are round rooms, rooms with multiple nooks containing windows, rooms with angled walls, etc. And on the third floor, the upper sections of the walls are angled in and the ceiling is smaller than the floor.

This morning, it's 6 degrees outside. The supply pipes read 123 degrees and the return pipes are reading 111 degrees. The boiler is running for about 13 minutes and then cycling off for about 9 minutes. Again, it's an old WC boiler with input of 245K and output of 196K BTU/hr.

I'm thinking that the Viessmann Vitocrossal 300 CU3A with a max input of 199 MBH might be sufficient to keep the house warm even on cold days like this. Since the return water temps are so far below 130, the boiler should run quite efficiently and condense all the time, providing a max output somewhere in the 190Ks. Since my old boiler is still cycling off at these cold temperatures, I'm guessing the full 196K out put isn't needed to keep the house warm even on super cold days. It seems like the HVAC man from HOK architectural firm must have done a good job of calculating the heat loss and sizing the boiler in the early 70s. And since then, insulation has been added to the attic as well.

Next weekend, it's predicted to be about ten degrees colder. I'll check the water temps then too and see how long the boiler runs.

I'll include a photo of the house. It's definitely not a simple rectangular house.

• Member Posts: 4,692
That’s a sweet pad! You seem to already know the answer to your question. If your current boiler BTU works, why would you consider going larger? Or maybe I misunderstood. Clearly, you could go smaller, based on the water temperatures you’re indicating.

I did not read all the messages, is this Gas or Oil? If you get a really cold day you can probably deduce the actual heat loss (skip all the paperwork, manual J stuff). There’s a little bit of guesswork in there anyways. Unless you own a blower door, you got to guess (infiltration). With oil it’s a little bit more of a pain, you would have to know the exact pump pressure, nozzle size and basic efficiency of the boiler to extrapolate BTU. With natural gas, it’s as easy as clocking the meter.

If you do the Viessmann CU3A, you’re going to want to set up constant circulation. To let that big system cool down just seems silly.

The boiler temperature of which the house heats and the the BTU needed to heat the home are not related to each other. With that said, it’s certainly more efficient to have lower temperatures. But that does not change the heat loss of the home.

Gary Wilson
Wilson Services, Inc
Northampton, MA
gary@wilsonph.com
• Member Posts: 4,692
OK, I did misunderstand, the 196 was the output. Not the input

I see that you timed the on/off cycles. Very rough and rounded off math says you’re running about 1/3 of the boilers capacity. Let’s give it a small buffer and say 40%. 196×.4equals 78000 output.

this stuff makes me have a good blood flow, no normal heating contractor will install a Boiler to run 100% on the coldest day of the year. There is absolutely no risk/reward benefit. I’m pretty aggressive with downsizing things.

You could go down to the 35 kW, piece of cake (if your current boilers btu capacity is actually doing what you say it’s doing)

Gary Wilson
Wilson Services, Inc
Northampton, MA
gary@wilsonph.com
• Member Posts: 120
GW said:

That’s a sweet pad! You seem to already know the answer to your question. If your current boiler BTU works, why would you consider going larger? Or maybe I misunderstood. Clearly, you could go smaller, based on the water temperatures you’re indicating.
Thank you Gary! I love the house, and I've tried to be good steward of it. But you can see from the shape of it that doing heat loss calculation wont' be easy for an amateur.

I definitely don't want a 260 MBH output boiler, as several installers had recommended, and as someone installed for my neighbor whose house is half the size of mine. My worry was whether the Vitocrossal would be able to keep up when temperatures dip far below zero, which does a few days a year. I would like the new boiler to be capable of keeping up the way the old one has. The boiler will run on gas.

When we had the polar vortex a few years ago and the temps stayed around -20 for a week, my current boiler with 196K output ran nearly constantly, cycling off for maybe five minutes max an hour. My worry is whether the Vitocrossal could keep up when we encounter another polar vortex. Since the Vitocrossal doesn't give an output number, my worry was whether it could match the performance of my current boiler when the whether is unusually cold. My guess is that with my low return temps, it should run near 95% efficiency, which means around 190 MBH output, which is just a tad below what I have now.

I'm a little confused when you said "no normal heating contractor will install a Boiler to run 100% on the coldest day of the year." I kind of thought that is what we aimed for, i.e. a boiler the maximum output of which would keep up with the coldest days without short-cycling. Wouldn't a smaller boiler such as the 35, 125 be too small on -22 degree days when a 196 MBH output boiler has to run nearly constantly to keep the house warm?

I suppose the most ideal would be to have two smaller boilers one of which would suffice for the average days and both of which together could keep up during the next polar vortex, but that entails the cost of two boilers.

I suspect we will have an ECM pump that will run constantly. I know these gravity systems can do the opposite of what they did under gravity when a circulator is installed, and will require a high enough flow to reach the farthest radiators.

S.

• Member Posts: 4,692
Yes, designing for the coldest day of the year, that’s a very general and vague parameter/statement. Yes, in your example, the 125K is too small, for sure.

Where I live, the official design temperature is zero. But of course it gets colder than that. Extremely seldom. So, you need to flip a coin, do you want a boiler that will keep your house toasty warm in an extreme outdoor weather anomaly? Either answer is totally fine. You just need to tell the heating guy all that. Before he does the work, not after.

Output rating- Just use the third row down on that chart. It has a buffer built into it, about 15% for piping losses

The notion of designing a boiler to run nonstop in the coldest day of the year is a little bit of a gamble, a gamble stacked against the heating contractor. If that does “literally” happen, You have zero pick up factor. Which may be fine. But, in our society, a vast majority of homeowners insist on fiddling the thermostat settings, it’s like an addiction. Does that make sense? The last thing I need is somebody expecting their house to climb temperature when it’s design conditions. In that case, we just threw out all the science and math out with the bathwater, may as well just throw darts like the heating guys across the street.

The heat last calcs are not an exact science, well..... that’s a whole different topic. Calculations are nothing but theory. Observing run cycles, mathematically backing your way into BTUs per hour, this is real is the sun coming up every day.

If your pipes are giant, you really want to insulate them. Perhaps you already discussed all that.
Gary Wilson
Wilson Services, Inc
Northampton, MA
gary@wilsonph.com
• Member Posts: 120
Gary, I never fiddle with the thermostat. I know these large old gravity systems with hundreds of gallons of water are not quick to respond and work best when just left alone. It's like steering an ocean liner: nothing happens fast.

So that's what the Net AHRI rating means. I had been trying to figure it out.

The pipes are 3 inches in size, and there are two supply and two return ones down there. added together, probably 200 feet of them. The asbestos insulation was removed by one of the previous owners, and I do rather like that part of the basement being somewhat warmer. It's right in the center of the house, and that's where my workshop and treadmill are also. And since it's within the envelope of the building, I don't consider the heat as having gone to waste.

I'm thinking the Vitocrossal 199 with its 5:1 modulation should be able run at low power most of the winter without short-cycling, but also have enough oomph too handle the occasional super cold days. My biggest worry was being left cold when the next arctic vortex heads this way.

S.
• Member Posts: 1,215

Also, time the gas usage at the meter when only the boiler is firing to be sure it is firing at its rated output. I have found most boilers that size have undersized gas lines feeding them and the boiler does not fire at full capacity. In addition it is also pretty common for the gas service to be undersized in the Chicago area and Nicor territory. We've had homes with nearly 1,000,000 btu/hr worth of equipment with a gas service for only 250,000 btu/hr .
• Member Posts: 4,692
Thanks @The Steam Whisperer (Formerly Boilerpro) I must have transposed the math.
Gary Wilson
Wilson Services, Inc
Northampton, MA
gary@wilsonph.com
• Member Posts: 833
Shahrdad your suggestion of two smaller Vitocrossal 300's is a good idea. Most of the year you would be running on one boiler at a time. And you would have two boilers in cascade for the next polar vortex. If one boiler goes "down" you have the other to stand in. Easy to do reg. maintenance on one boiler at a time this way. Could do cleanings in the heating season even! The scarcity and/or delay of Viessmann parts is something to consider (ask me how I know). If you go with Viessmann-- you know that they will be supported for twice the lifetime of the boilers.
• Member Posts: 120
psb75 said:

Shahrdad your suggestion of two smaller Vitocrossal 300's is a good idea. Most of the year you would be running on one boiler at a time. And you would have two boilers in cascade for the next polar vortex. If one boiler goes "down" you have the other to stand in. Easy to do reg. maintenance on one boiler at a time this way. Could do cleanings in the heating season even! The scarcity and/or delay of Viessmann parts is something to consider (ask me how I know). If you go with Viessmann-- you know that they will be supported for twice the lifetime of the boilers.

Thanks! Parts availability and service are some of the things I've thought about. The Vitocrossal seems to be beautifully engineered, and with high water mass and large passages, I am suspecting its a good fit for these older systems, and it's less liable to clog up, as other boilers with tubular heat exchangers can do. Another attraction was that Viessmann says the system doesn't require primary and secondary pumping, but I'm sure they mean that with just one boiler. What's your experience been with this brand?
• Member Posts: 120
Gary, just an update: these last few days have been as low as -3 degrees and as high as 6 degrees. When the temperatures is closer to zero, my boiler runs continuously. When it got to six degrees, it did cycle off and on. The Supply temp is 132 and the return is 120. Since it's a converted gravity system, I'm assuming my Taco 110 is pumping around 32 GPM.
Now I'm wondering of the largest residential Vitocrossal will be able to keep up on weeks such as this. The Net IBR on my current boiler is 170, and the Net AHRI rating on the Vitocrossal is 161. I am assuming they refer to the same thing.
• Member Posts: 5
edited February 2021
I'd primary/secondary pipe two smaller cast boilers, that when running together, will have a similar output as your current unit. Set one up as a booster with a open on the rise aquastat controlling it's operation. You will also need a relay to have the same thermostat control both boilers.

I did this over 20 years ago on a large gravity conversion and it still works well. You also have to run a small circulator on the primary piping to mimic the gravity flow. I use a B&G NRF-22 as the system pump.
• Member Posts: 73

Shahrdad your suggestion of two smaller Vitocrossal 300's is a good idea. Most of the year you would be running on one boiler at a time. And you would have two boilers in cascade for the next polar vortex. If one boiler goes "down" you have the other to stand in. Easy to do reg. maintenance on one boiler at a time this way. Could do cleanings in the heating season even! The scarcity and/or delay of Viessmann parts is something to consider (ask me how I know). If you go with Viessmann-- you know that they will be supported for twice the lifetime of the boilers.

If you are running condensing boilers in parallel cascade operation does not provide the best efficiency if the return temperature is cold enough to condense. From Shahrdad's measured return temperatures are low enough for condensing.

Have a look at https://www.hpac.com/heating/article/20925496/maximizing-smallboiler-efficiency that talks about running boilers in tandem rather than cascade. Each boiler is the running on a smaller load so efficiencies go up. You get the increased reliability from having two boilers.

For this to work in your setup the boiler flow temperature probably needs to be controlled using the outdoor temperature.

Regards John
• Member Posts: 120
Thank you John and Gary.

Another option for me would be to have two Buderus GC144/5 cast iron boilers each with an output of 112 MBH and efficiency 85%. The two together cost a fair amount less than the single Vitocrossal and I am guessing that since they're are regular cast iron boilers, they would require less costly maintenance as well. I should also be able to vent both through my existing 7" lined flu. I think this option would give me few more BTUs than I have now for the next polar vortex comes along with even colder temperatures, and I also like the idea of having some redundancy should one boiler go on the fritz.

Buderus calls this model a "Compact boiler with advanced low temperature technology." I'm not sure whether this means that it's more tolerant of thermal shock or whether it can run at lower temperatures. They also say the Logamatic 2107 can be used for outdoor reset, and I'm guessing Tekmar or another brand could provide the same function.

I really love the idea of the Vitocrossal, as it seems so beautifully designed and built, but I'm starting to think it might be just a shade too small for these super-cold weeks. After talking to a few installers around St. Louis, not many are that familiar with this Viessmann, which could be an issue as well.
• Member Posts: 4,692
Yes pros and cons to everything. If it’s a deep freeze, do you really need to have the same comfort level that you enjoy the other 99.5% of the winter? Minus 20 isn’t normal at all

if you’re happy with the capacity of the current boiler, just match the output. Clocking the meter would be a little more accurate, unless you happen to know the gas  pressure (3 1/2” is normal, you could rely on the name plate BTU)
Gary Wilson
Wilson Services, Inc
Northampton, MA
gary@wilsonph.com
• Member Posts: 627
Sounds to me like your boiler is perfectly sized. At -3F you were probably getting really close to the actual design point.

Whatever the output of that boiler is.....match it. It seems perfect.

As for a replacement boiler/mod-con its a matter of opinion. Get a good quality non-condensing boiler and it may last 20-30+ years but at a lower efficiency and a bit more fuel. Or get a mod-con, save some fuel but you'll likely have to replace the mod-con when the standard boiler is only halfway through its life cycle.

I do wonder though, as it's not an apples to apples comparison as your old boiler can handle the low return temps. Newer non-condensing ones are going to want to have return temps much higher, and your radiators will be hotter.
• Member Posts: 120
edited February 2021

Sounds to me like your boiler is perfectly sized. At -3F you were probably getting really close to the actual design point.

Whatever the output of that boiler is.....match it. It seems perfect.

As for a replacement boiler/mod-con its a matter of opinion. Get a good quality non-condensing boiler and it may last 20-30+ years but at a lower efficiency and a bit more fuel. Or get a mod-con, save some fuel but you'll likely have to replace the mod-con when the standard boiler is only halfway through its life cycle.

I do wonder though, as it's not an apples to apples comparison as your old boiler can handle the low return temps. Newer non-condensing ones are going to want to have return temps much higher, and your radiators will be hotter.

That's kind of what I'm trying to figure out. If I hook up two cast iron boilers in this configuration, with each boiler having its own bypass (orange and green) or a common shared bypass (blue), I should be able to keep the boiler return temperature high enough and also regulate the secondary circuit's water temp perfectly with an outdoor reset, especially if the main circulator runs constantly.

I have been very interested in the Vitocrossal CU3A, since several commentators on The Wall have called it hands down the finest boiler made today. However, it's about 10,000 BTU/Hr below what I have now, of course assuming that my current old boiler is still putting out 196,000 BTH/Hr. If the old boiler's efficiency has degraded even 5% over the last 50 years, the two might be about equal.

What do you think of the Buderus GC144 series? Two of the ones with 112 MBH output would more than equal my current boiler, and give me a bit of extra cushion as well. I've heard good things about the Buderus quality, though I have a bit of a crush on the Vitocrossal.

S.
• Member Posts: 4,692
It’s allowed to have a crush on the CU3A. It’s the modern day C3PO. Either way you’re good. The big viessmann is built to handle 130 degree water, the GC144s would appreciate hotter water
Gary Wilson
Wilson Services, Inc
Northampton, MA
gary@wilsonph.com
• Member Posts: 120
Gary, with total EDR of 1,700 sq ft (1,414 for the radiators and 200 and change for the exposed pipes), shouldn't it require only 153,00 BTU/hr to get the supply temp up to 140 degrees? And yet, my supply temp never got over 132 despite the boiler running constantly. Does that mean that the boiler is putting out substantially less than 196 MBH, or are all my assumptions about EDR wrong?
• Member Posts: 4,692
Well it's very cool that you're trying to match the numbers, but 'real world' and calculations don't always agree. If you boiler is maxed and you're getting 132, that tells me that the extra large rads are shedding 'that much' BTU at 'that low' of a water temp. IF you want hotter water (more BTU), you will have to burn more BTU's to achieve that. OR, get someone to reduce the heat loss of your home, that's probably a better dollar spent. You have natural gas right? Just time how long it takes for the 2 cu ft dial to make a full turn (when only the boiler is working) and post it here- if you can't find a simple calculator/sheet on the web. Then you can deduce the BTU if we do a little guessing. OR get a heating guy in there and see how hot the stack is. Even modern combustion analyzers have a little pixie-dust built into their calculations though
Gary Wilson
Wilson Services, Inc
Northampton, MA
gary@wilsonph.com
• Member Posts: 120
Yes, I have natural gas. I'll record the gas-meter turning with only the boiler running when I get home.

I'm realizing what you mean when you speak of calculation vs. real world. Going by my calculations, it should have only required 119,000 BTU/hr for 130 degree water, and with 196,000 BTU/hr, my old boiler should have been able to get the water much hotter without any problems. But in the real world, that's not what I saw on the design day.

Thank you again. I'm learning a lot.

S.