Matching boiler size to radiator output

GW

I have this simple page in one of my google pics folders 

Shahrdad

Gary,

I watched my gas meter, which has four dials on top, and a fast spinning fifth one on the bottom which says "HALF FOOT" on it. It took 88 seconds for it to make 10 turns, so 8.8 seconds per turn. I'm guessing "HALF FOOT" means half a cubic foot of gas.

If my assumption is correct, it takes the boiler 88 seconds to burn five cubic feet of gas, which is 205 on that chart you posted. The Missouri Dept of Energy gives a value of 1023 BTU/Cub Feet for natural gas in Missouri. 205 x 1023 = 209,715. So let's round it up to 210,000. My boiler is labeled to have an input of 245,000 and output of 196,000, which would be 80% efficiency (when new and under optimal conditions). So at that efficiency, the output would be 168,000 BTU/hr.

Even assuming the gas's heat content is as high as 1050 BTU/Cub feet, that would make input 215,250 and an output of 172.200 BTU/hr at 80% efficiency. And if the efficiency of the boiler has decreased over the years, it could be putting out even less BTUs than that.

These numbers seem to be much more in line with my calculated EDR, and I suspect that my boiler's real efficiency is substantially lower than the 80%.

Does this sound correct to you?

S.

GW

Ok 88 seconds and 5 cu ft, chart says 205,000. 

I predict 80%, so you’re around 164k net heat 

again, do you “really” need more btu for a crazy odd weather even? You could always plug in a heater or so (5000 BTU plug in heaters) 

Shahrdad

No, but I am now thinking that the largest Vitocrossal CU3A should be able to more than match my old boiler's current output. It has a max input of 199 MBH, and with my cool return temps, it should run very efficiently and be able to provide a maximum output of somewhere between 185-190 MBH, perhaps even a bit more. That should more than cover my design day requirements, and give me a bit of extra cushion for the next polar vortex. And being a modulating boiler, it should spend most of the winter at low modulation and run with an efficiency in the high 90s.

Maybe I'll even do an indirect water heater!

Thank you so much for helping me work though this.

S.

GW

Excellent, your system desperately needs/wants a condensing boiler. Make sure you find a viessmann contractor that can set you up with the vitotroll 200/300. I dare say, some Viessmann contractors don’t want to deal with some of this extra stuff. This would make the boiler a constant circulation system, anything else is just silly when we’re talking that much water, that much mass.

edit- Now that I think about it, that’s one clever way to distinguish which Viessmann contractors are really trying to save your energy, and which ones are just slapping boilers in. If the Viessmann guy or gal does not recommend the vitotrol, you may want to keep on looking.

edit 2- I understand that there are Wall lurkers that read my posts. If you have a baseboard system, we are generally not installing Vitotrols. We Did back in the Wb2a and Wb2b days, but the newer Viessmanns are smarter and can handle a basic thermostat call 

Shahrdad

Earlier today, I was looking for the Vitotrol on the Viessman-US website, but couldn't find anything. The UK site seems to have all sort of controls. Does the Viessmann-US also handle these controllers? What is the major difference between the 200 and 300? And how do they help optimize the system?

I also saw a super cool looking square Vitotrol with a round display in the center. . .

From what I have read, my assumption was that I would need a circulator that would run non-stop, since I'm not sure how else outdoor reset would work. Taco seems to have a nice selection of ECM circulator in the Viridian line, where we could set the desired GPM flow, which I think should be around 30 gpm. Since my house is one big zone, I think we should be able to set the GPM and pretty much forget about it.

I'm not familiar with how the indirect tanks and the house heating system share one boiler, but am I correct in thinking that when domestic hot water is needed, the circulator to the radiators stops and the hot water tank circulator kicks in, and the boiler cranks up to heat the tank, and once that job is done, it returns to the temps required by the outdoor reset?

PS. I wish you served this area. We sure need good hydronic specialists. I was looking at my neighbor's newer system, with a Lochinvar Knight water tube boiler capable of producing up to 378,000 BTUs for a house smaller than mine. And the boiler started and quickly ramped up to 100% and then cycled off, and the circulators turned off as well. And this was not even a very cold day. I don't think I'm using that company.

GW

Not sure what their site shows, it may be lacking on the control side. The 300 allows you to make changes up at the living space, the 200 is limited but probably good enough for most people/users.

https://www.viessmann-us.com/content/dam/vi-brands/CA/pdfs/doc/vitotrol/vitotrol_300a_oi.pdf/_jcr_content/renditions/original.media_file.inline.file/vitotrol_300a_oi.pdf
I found this on the Pro Login

30 GPM, not sure you need that much. If your net btu is 200 or so, then the rough math is 20 gpm (1 gpm per 10,000 btu). That's presuming a 20 degree drop (supply minus return temp). Did you even measure the supply and the return temps?? What's your deltaT?

Indirect---yes, as you suggest/imagine. The boiler knows it's making DHW, and kills the space heating pump. In summer, the boiler does not send heat to the home. The Viessmann brain knows what's what.

Your neighbor, that's a super bummer, there should be a crime against humanity when people oversize like that. I was kinda dumb 30 years ago and WAY oversized some stuff (but I was cheap!)

Shahrdad

Thanks for the link to the Vitotrol. It looks like it also acts as a room thermostat to keep an eye on the temps in the living spaces, is that correct? Knowing me, I'll definitely go with the 300!

As for circulators, I came up with the number using the old B&G chart that was on the the article by Frank "Steamhead" Wilsey discussing circulator sizing for old gravity systems. I thought that since my radiator EDR is around 1,400, the circulator should be pumping between 30 and 35 GPM. I am estimating that my old Taco 110 has been pumping around 30 GPM as well.



As for the Delta T, it seems to run between 10 and 12 degrees F. When it was 0 degrees outside, the supply temp was 132 degrees and the return 120 degrees F. I've read that gravity systems that have had a circulator added tend to have a 10-15 delta-T range, so mine didn't seem too out of the ordinary. On warmer days, the Delta-T is closer to 10 degrees.

What are your preferred circulators? The thing I liked about that Taco Viridian is that the GPM can be adjusted between 0 and 52 GPM, but I'm sure all manufacturers have something very similar. Since my almost 50 year old Taco 110 is still running without breaking or leaking, I think the quality must still be good. Grundfos seems quite popular as well, though the boiler pump on my neighbor's Knight boiler stopped working at the lowest speed at which it was supposed to run, and is now running on the medium speed.

Also, I think I will need a good dirt and magnetic separator. Caleffi's seems very good, and does both functions, and Taco also has one. BoilerMag out of England has a wonderful looking powerful magnetic separator with a max flow of 29 GPM. They claim 90% ferrous particle removal at first pass. I think I definitely will need to protect the ECM pumps from ferrous particles.
https://www.boilermag.com/na/products/commercial/

GW

Wow that low of a boiler temp and that low of a delta, that’s wild. The rads must be Yuge. 

Certainly others know more than me in a system this size. 99% of our jobs are running well under 10 gpm in a zone. However, not sure if that much flow is needed—-the original system was no pump at all. Perhaps a concern is the heat won’t make it to the extremities of the piping system, yet Mother Nature has a way of moving heat. 

Our small systems- we use the Grundfos Aplha. I haven’t done a job that alphas don’t work. Smaller house stuff is what I do mostly. Commercial stuff and giant systems- just don’t want commercial and we don’t have giant homes in my wheel house 

I’d be curious what your 110 is pulling for watts. You might even see a drop on your electric bill when you get a new boiler system

Boilermag seems legit 

Shahrdad

I'm sure the old Taco using way more electricity than the new ECM pumps do today, but it just keeps going and going and going.

On one of the threads here regarding pumping an old gravity system (https://forum.heatinghelp.com/discussion/153602/ecm-and-converted-gravity-system/p1), Harvey Ramer wrote, "Now if we put in a pump that moves enough water, it will pump through all the rads on the first pass. Reason is, we are now moving enough water so that even the path of least resistance offers enough flow restriction to ensure we are getting flow through every other circuit." Harvey's explanation seemed to make the most sense to me as to why it was recommended to go with 50% higher flow for a certain EDR when the system was originally a gravity system.

An interesting thing I witnessed a few times was when relay switch inside the boiler that activates the circulator has slipped out of its socket: the boiler was firing, but the Taco wasn't turning. So the system was operating on gravity again. I would have expected the hot water to shoot straight up to the third floor, like a hot air balloon, but instead, the big radiator in the front hall was hot, some other radiators on the first floor were warm, and the ones upstairs were cool. Of course, the water was going up a 1 1/2 inch pipe rather than up two 3" pipes so it couldn't have flowed with the same facility it did originally.

One reason I liked this Viridian pump was that it would allow me to play around with the flow and find the sweet spot for my house. I didn't think I would need a Delta-T pump for this system.

As for radiator size, honestly I don't think they are that YUGE compared to some of my neighboring homes. But apparently, they're big enough to keep the house at 70 degrees while running with 130 degree water. As you say, I think a relatively low temp system such as this is begging for a condensing boiler.

GW

Try it again and give it time, the whole system will heat up. Mother Nature will do her job. Sure- if you’re doing a basic on-off basic thermostat and you want even heating sooner than later, then yes. Constant Circulation—still not sure you need that much gpm 

Shahrdad

GW said:

Try it again and give it time, the whole system will heat up. Mother Nature will do her job. Sure- if you’re doing a basic on-off basic thermostat and you want even heating sooner than later, then yes. Constant Circulation—still not sure you need that much gpm 

Excellent point. I think an ECM pump will allow for finding the optimal flow at the lowest power consumption. I am sure I'll need a boiler check valve too, like the Taco Flo-Check or the B&G Straight Angle Flow control to prevent gravity circulation when heat is not needed. Do you prefer one brand over the other?

GW

yes you'll need a check, the water heater will come on and you don't want heat creep (I just made that up, I think i like it, not sure though) I like the taco 200 series (221, 222, etc). On the cheap jobs/small jobs we just use the internal flow check or zone valves. Of the bigger high tech jobs we do the taco flow checks. I'm sure the B and G is just as good. I bet a dollar and a doughnut a small Alpha or a Viridian would heat your home. They won't kick 30 gpm though

Shahrdad

My next door neighbor has his system set up without check valves. When the weather is mild, he has to go in the basement and turn off the boilers altogether or his radiators get warm by themselves. I messaged Harvey to get his thoughts on how much flow was needed, and he also repeated what you had said about constant circulation requiring lower GPM flow. Thank you again.

Shahrdad

How about “Gravity Heat Creep?” It’s pretty descriptive.

GW

It was a little bit of a tongue-in-cheek gesture. Gravity heat or ghost flow is the normal expression

Shahrdad

Gary, This is the piping diagram Viessmann gives for the Vitocrossal in their Application Guide. I assume the check valve on the main house circuit goes on the outlet side of the circulating pump. Do you also put a check valve on the circuit to the indirect water tank?

Do you do also install your diaphragm compression tanks the way they show in this graphic?

One installer wasn't sold on diaphragm tanks, because they have to be replaced after a few years, though looking at the price of a 20 gallon Amtrol compression tank, it's a lot less than a 40 gallon B&G steel tank. I could replace the diaphragm tank a few times for the price of the steel tank, though I find the physics of air control pretty fascinating.

GW

yes and yes, if we use the taco flow check we just use a 5 or 6" black nip after the pump. Cheapie job we use the internal flow check, they are very reliable but with old school cast iron system i would want the taco iron flow check. Yes the water heater needs one too as to not overheat the tank

We don't use air elimination on CU3A, we let the high vent on the boiler do the work. Which is depicted in the image you pasted. It comes with the boiler. We go an extra step and install a small 3/8" ips ball valve as well so when the hi vent pukes someday down the road the process is cake to change it. (3/8" black coupling needed if this is done)

Location of the tank----anywhere! Sometimes we tag it off the bottom like what your pic shows, other times it's off the top (supply) of the boiler. I think you're gonna want a big floor mount though, if you go with a compression. Again, lots of isolation, once you fill that beast of a system up, you don't want the heat guy to drain it for some silly reason down the road (a repair of some sort)

If the heat guy insists on a steel expansion, just go with it, don't battle him on too many things.

Shahrdad

Thanks Gary! I love how those Taco or B&G check valves look so sturdy and old-school. I already have a 1 1/2" Taco Vortech air separator, which we can use. I think they stopped making it, but it's a real beast. I also have an Airtrol fitting that I never installed.

I think the air-separator will have to be placed right off the boiler, before the supply flow splits to the house and to the DHW tank. Since the boiler has to reach much higher temps for DHW, that's when even more air will come out of solution, which can be expelled if a diaphragm tank is used, or be routed to the steel tank, if that ends up being used. Hot Rod even suggested that if we use a steel tank to connect a small copper tube from the air vent that's on the drawing to the top of the tank. If we do that, I guess we could put the air-separator just on the house side of the flow.

On a separate thread, I read that a Viessmann rep suggested to another potential customer that the 79 gallon tank with the twin coils be used for DHW, and to have the coils connected in series, to bring the return temps from the DHW tank to sub-condensing levels. Seems like an interesting idea, but I wonder how much it reduces the actual volume or the tank, or if it's even worth it. It might take a decade to recoup the price difference.

I'm contacting different contractors to see who best has their s*** together and seems to know what they're doing. I've seen too many iffy installations around here.

S.

GW

Yes if the Viessmann guy is sleeping you’re certainly gonna wake him up, you’re really grabbing on to this stuff. It’s a little tricky though; if your personality it smooth, you’re good to go to interject your ideas with the heating guys. 

I would skip the air elimination- once you fill the system there’s no way air will come back to the boiler, it simply ends up in the rads. Purging the  tank is cake (assuming you have normal plumbing pressure). If some air “does” come back it will simply float to the top of the boiler and out the vent. But it you really want to keep the air elimination that’s fine. 

Two coil tank- I am stumped- I disagree. If you keep heating the top of the tank while the bottom heats the cold water you will overheat the water. If you’re gonna use two coils you need two controls (dare I say “zones”). Now if you have a mixing valve then that’s different. If we are talking heating up a stone cold tank then sure- but how often will you be smoking the entire tank? Seldom to never?

This is crazy talk but if your max water temp is 140 or so for the heating, you don’t “really” need a flow check on the water heater (if you have a mixing valve). I’m starting to split some hairs here though. 

Shahrdad

One of the attractions of the CU3A for me is the lack of the need for primary and secondary pumping. But I keep thinking about the large water volume inside the boiler, the low temps my radiators generally require, and the high temps needed by the DHW tank.

Most of the winter, my radiators are just lukewarm, so a condensing boiler like this would run at very high efficiency. But then when you add the DHW tank, then the pump to the house has to stop, the boiler has heat up 19 gallons from lukewarm to hot in order to heat the DHW tank, and then switch back over to heating the house. How efficient is a large water volume boiler when serving a hot DHW and a house that doesn't need very hot radiators when hooked up without primary/secondary pumping?

I wonder if anyone on here has had any experience with a system like this, using this particular boiler piped like the diagram above? It wouldn't be a big deal of my radiators ran hot, but even on sub-zero days, they don't need to go above 130 to keep the house at 70. Perhaps the largest DHW tank with its lower surface to volume ratio would retain the heat better and require less heating cycles.

S.