What is the most effective way to handle a MODCON

Harvey Ramer

I recently read an article claiming that a modulating burner starts losing a lot of efficiency at about a 40% turn down threshold. This would be due to a laminar flame and radiant reflection in the heat exchanger. This would in effect cause the stack temp to rise and a loss in efficiency.



Assuming the above is true and correct, this brings a couple questions to my mind.



One of the beauties of a hydronic system is the ability to zone the load. Now the question becomes, what is the smallest zone one should have compared to the total load. Another thing to consider, the smallest zone will only need 15% to 30% of the design, 80% of the time. What effect will this have on the boiler? 



Would it be better to use 2 small modulating boilers versus one sized for design load. Or should we rethink zoning? Perhaps designing a system that monitors the requirements of the building and allows the boiler to turn on only after a minimum load can be met. Then instead of delivering heat only to one zone, it would deliver heat to the whole building. Each zone would have an intelligent control that would allow the proper amount of btu's to be delivered in the allotted time. This would not only prevent short cycling but also allow for a better burn cycle.

Problem with the above method of thinking, would it have a detrimental effect on comfort?

If one designed a system like above, would primary secondary piping be necessary? I doubt it. Primary secondary piping arrangement on a MODCON has one primary job. That is to keep a turbulent flow across the heat exchanger. This results in a better heat transfer and prevents the metal of the heat exchanger from getting damaged from excessive heating and cooling.  If one were to design a system like the one above, the minimum flow rate through the boiler could most likely be met in all system operations. It would also have lower return temps more of the time to help with condensing.



Ya da ya da ya da.. The vicious circle of Hydronics goes round and round.



Some light thinking to occupy a snow day here in the northeast.



Cheers

Harvey

Rich_49

Smallest zone

should be whatever it is required to be .   2 small boilers is always an option but in my opinion not always the only or best option .  Buffer tanks are a great piece of equipment that are underutilized by those of us who know and not known about t=by the other 97.5% of "heating contractors" .  Why not install properly sized buffer tanks and let the mod/con make cast iron temp water and park it in the buffer . No matter what boiler you are using this will insure most efficient cycles .  Low temp systems with an ODR mixing valve can sip and mix this fluid for a good long time before the boiler has to replace the used BTU's . Imagine storing at 180* for a system that only requires 100* and not having the boiler fire until the buffer fluid is 105* . Do you think that will guarantee the efficiencies that made you purchase or endorse a mod/con to begin with ?  Zone away !  A 50 gallon buffer tank for instance would always require the boiler to run at or above minimum modulation and insure nice long run cycles .  Buffer tank sizing is not a difficult equation either ,   Smallest zone   2500 BTUh , 80,000 BTU boiler w/ 4:1 TDR (20K lowest modulation), 20* Delta T system .  Equation  

20,000 minus 2500 = 17,500 .     17,500 x 10 minute cycle = 175,000 .    175,000 divided by 10,000 =  17.5 gallon tank .   Don't forget to add the buffer tank volume for sizing expansion tank .  

   At least that is the way I have handled it for quite awhile .  Did I mention you will always be condensing too ?

Zman

True and correct?

Do you have a link to the article? What type of boiler?

The studies according to ASHREA is contrary., in fact opposite.

This seems like a silly conversation based on "an article I read"

Carl

SWEI

Well-designed mod/con boilers

typically reach their maximum efficiency at their minimum firing rate.  The factory engineers tell us that higher turndowns typically result in both lower efficiencies and higher emissions.



About a year ago, I came to the realization that (assuming the boiler is not actually undersized) its minimum output is actually more important than its maximum output.  The larger the ratio between the minimum output and the design day heat loss, the better.  I'm currently shooting for a 3:1 minimum, though that could change once we have a couple seasons of data to look at.



A mod/con running on a properly setup ODR curve will allow a properly balanced system to run using a minimum amount of zoning.  The overwhelming majority of single zone systems won't even need an indoor thermostat.  Staging two mod/cons will give a higher effective turndown, which is always a good thing, but re-thinking the zoning plan usually costs a LOT less.

Harvey Ramer

Here is the link to the article.

http://versatile-services.weebly.com/uploads/1/9/9/1/19913439/bufferringkpwhitepaper.pdf

The article is 8 years old so parts of it describing the MODCONS may no longer be relevant. I am going to call Triangle Tube and see what they say about the turn down business.



I am not trying to start a silly conversation or an argument. I merely wish to gain an intuitive insight that will allow me to make intelligent decisions when I design my next system.  

SWEI

I remember that article

and I can't disagree with most of what it has to say.  The lesson I hope readers get from it is to re-visit some past jobs to see how those heat loss calcs actually penciled out.  If your results are anything like mine, you will be shocked.



The answer is to size carefully and conservatively, perhaps combining one or more stages of mod/con with a conventional "peaking plant" to assist during cold snaps.

CMadatMe

Biggest Issue with ModCon's

Here in the great USA. Fixed speed boiler pumps. They are the main cause of short cycling most of the time, the reason for elevated stack temps and high boiler return water temps when zoning.



Get variable speed boiler pumps controlled by the boilers logic to match its flow with the modulation rate of the boiler and they go away. The attached chart is provided by one mfg as the modulation rate in comparison to efficiency in its boiler. I don't know anyone else that publishes this.

Zman

Buffer

Harvey,

Thanks for the article. It makes me think even more about buffer tanks. The jockey boiler idea is very intriguing as well. How nice would it be to have a 55 k boiler for everyday loads and something bigger for the design day? It also makes me wonder how to control the mis matched duo.

I will have to start paying more attention to stack temps at low fire.

I am curious what triangle tube has to say. I also wonder if other designs fair better,

Carl

CMadatMe

Carl

All the buffer is really doing is allowing the full boiler flow rate a place to go no matter the modulation rate. The pump is fixed speed. If you had a variable speed pump being controlled by the boilers control logic to match the modulation rate based on the systems demand that issue goes away and there would not be a need for the buffer tank.

Zman

Pump

Chris,

I understand what you are saying about the boiler controlled pump. I agree.

The article also points out that the same issue is present on the fire side of the boiler and that efficiencies are reduced at firing rates lower than 40%.

I will be testing the articles statements as soon as I get home. If the flue temp rises at low fire, we have an issue.

Carl

Harvey Ramer

Chis

I tend to agree that the boiler should control the pump, however, looking at that graph you posted. It only goes down to 30% What happens to the boiler after you get below that threshold? The article I posted suggests that a modern, zoned hydronic system will spend a great deal of time below 30% of design load. In that scenario you would still need a buffer tank to keep from short cycling the boiler.



HTP makes a high mass ModCon which would address many of these issues but I know very little about the company or the quality of their product.

hot_rod

high capacity heaters

I have been running a HTP solar Phoenix for 6 years now, in my shop and office. It is an 80 gallon, 130,000 input. It works great with the 3 small zones I have. Lower turn down compared to most modcons, but that large volume works wonders. I run to a 135F setpoint.



i also generate DHW with an external plate HX, so no stored DHW. in the summer months the solar coil provided enough energy for DHW production.



At home a 120 Lochinvar Cadet coupled to a 50 gallon ThermoCon buffer does about the same as the Phoenix, but I run it on ODR.



If you are micro zon ing the buffer really smooths out the cycling.

Rich_49

HTP

is a strong company that has been around for awhile now , long enough not to be considered new or suspect . They only do high efficiency and nothing else .  The Phoenix that Rod talks about is THE BEST Water heater in its class if there was another in its class . The boiler you speak of is probably the Pioneer with 55 gallons of mass in 100 , 130 , 160 , 199 BTU sizes .  The Versa Hydro is a water heater that uses the heated DHW being stored and runs it through an HX for space heating , again using this technology you can expect performance similar to having a unit with 10:1 TDR . I suggest you check out their website and explore the stuff they have engineered to make our lives easier and more profitable . The company is owned by a single man , Dave Davis and he is very involved in every aspect of the company , service is very good also .

Mark Eatherton

While I have to respect the author of the report...

because ANYONE who has been graced by the presence of Gil Carlson is truly a blessed person, I can't whole heartedly agree with the premise of his article, and in fact have found that with the equipment I have dealt with that the contrary happens. Smaller modulated flame equals a lower flue temperature. If in fact everyone was seeing flue stack temperatures of 350 degrees F, we'd be talking about melted flue pipes and such.



With that said, and not knowing for sure which product and its ability to modulate both air and fuel accordingly he is referring to, it IS possible with a 2 stage burner, and a single speed fan that the conditions he speaks of can be seen. Have seen it on non modulating gas fired mid efficiency equipment, but not on the Gianonni or Fire tube modcons I've had the opportunity to observe over the last 10 years.



Even in my own home, where I did perform an actual detailed heat loss calculation, when it gets to design conditions, which we have had the opportunity to see in the last few weeks, my UNDERSIZED mod con boiler still only runs at around 50% fire rate and the home is snug as a bug in a rug. What this tells me, is that even though we do our due diligence, we are still grossly oversizing our heat sources. I think the answer may lay in "Bench marking". By bench marking given buildings in given cilmatological conditions, we can establish a decent baseline of energy consumption per square foot per hour for the purpose of sizing the heat sources.



Either that, or a heat loss calculation that takes the real world, real time mass effect into consideration…



I'll sit over here and hold my breath and wait for that to happen…



ME

Harvey Ramer

Thanks for the input guys.

As always, I hold your opinions on these matters in the highest esteem.



I truly believe our quest to improve hydronics learn the proper implementation of hydronic equipment will slowly but surely change the way the public and average contractors view these systems. There is a lot to be said for a product such as the HTP Pioneer. Not only does it solve some problems for us "in certain applications" but it also looks appealing to homeowners. No complicated piping system with valves and circulators and wiring going everywhere. Just a simple two pipe connection. It looks simple to them and that's what they like.



I agree with you on the heatloss Mark. While the software we have available to do load calcs is much better than nothing, it still leaves a lot to be desired.





Harvey