Buffer tank ,Mod Con, how to?

Ronbo

I have a natural gas Weil McClain 155 ultra (128,000 btuh)Mod Con with ODR installed in my home to manufacture spec (primary /secondary) with an indirect as priority 1 but the boiler is 2 times the size needed. My heat loss is 62,500. I have the high fire rate set to 55% (70,400btu) and the low to 20% (25,600) which is lowest setting and might set the high a bit lower to 52%,.Boost is off and it modulates at default 5 on 5 off. Should i raise to 6 on 6 off ? I also have a 008 delt pump for my four zone pumping, is this useful in this application or a problem?

I'm thinking about a buffer tank but not sure how they are installed to cure short cycle issues i am having.Its bumping off the bottom so to speak.I tried slaving two of my four zones together which works at temps greater than 144 targets but would like lower temps.I don't think more base board will do the trick as i have whats needed now to heat at 140/design 0.

I would love some suggestions on buffer size. Is bigger better?How is it piped?

How much lower will it get my temps down?Is a water heater tank a good option/insulated very well?

 I have an 1 1/2 manifold/header with 3/4 branches off the mains, can i just send it from supply via 3/4 branch to the tank and back to the return 3/4 branch with no zone valve so its always pumped through when a zone calls/pump turns on) for heat unless ball valves are closed(this allows me to shut off in summer).

do i need to up size my expansion tank also to match the new volume?

Ty in advance for any help,Ron

Do you live near Newton NJ and want some work/lol?

scott markle_2

mass confusion

Buffers can be good, but keep this in mind. Your boiler works best in low fire, target temp is only part of what determines firing rate, the load is the other part. Since not too many reset curves are load matching, thermostats will cycle. When a system with mass cycles the beginning of each cycle will present a load (ability to suck up btus) to the boiler that is not representative of the load on the house. Consequently the boiler ramps up to a higher firing rate than is actually required. What could be near constant low fire burn becomes cycling and less time in low fire.



Anyway optimizing cycling and firing rate is more complicated than just adding mass.



I'd start by testing your assumptions about your heat loss, it may be lower than you think. (If you are not doing night setback) why don't you see how low you can actually go with the firing rate cap? If winter finally hits you might even get some design conditions, to test.



Zoning is also part of the problem, I favor single "zones" with TRV's doing flow modulation, slow the flow where it's not needed instead of shutting it off totally. For those who want more control but don't want to loose all the benefits of optimized operation, tekmar makes great integrated zone/reset controls that sync. zone operation and give autonomy over reset curve to the thermostats. In this system turning down the thermostats equates to shifting down the reset curve. The new house control series 400,401,402 are awesome, and more affordable than the original 4 wire TN4 stuff.



I'd look at the control side before you start cutting pipe.

scott markle_2

mass confusion

double post

scott markle_2

mass confusion

Dan, while it's not a huge problem, why don't you give your web people some work fixing this double post bug, it happens when you take a long time to finish a post... you get asked to enter your password and security test over again and it results in a double post.

furnacefigher15

Boiler cycling

What is the boiler actually shutting off on?



Loss of demand from zones? If this is the problem, try running a lower reset temperature.



Internal temp controls? If this is the trouble, then I would start looking at volume of water flow, and limiting the firing rate further.



As part of flow issues, what is the zone pump set to do?



other? Shifting priority demand?

Ronbo

ty

well if this is what i hope it can  then I'm in.

can this thing look ahead and see the bottoms pushing up then open up a zone to help keep me stay in low fire? then turn around and turn off the zone that met its need and open another? I'm willing to keep all zones at 1 temp throughout the season if it means constant comfort/savings/condensing/modulation.

I can do all this myself when i see it happening i just turn off  1 zone and turn on h another and so forth.

also is the piping of a buffer as easy as I said in my post? new larger expansion required also? in addition to the control from Tekmar are they both good upgrades? i would like to condensate often as i can......Ty again for help, Ron

Ronbo

ty,ty,ty,ty

"What is the boiler actually shutting off on?



Loss of demand from zones?"

 I have four baseboard zones all 3/4 copper uninsulated or 3/4 pex supplies/returns depending. my heat loss is based on 0 degree design

zone1 has 43' of baseboard and 100' coppper supply/return zig zagging about the ceiling in basement to feed it. about 18 elbows heat loss is 15,285 in this area i could add/use 2000 more btuh for this area

zone 2 = also in basement can be joined to zone 1 if needed and is 12' of base with 26' of s/r pipe copper pipe 7 elbows. heat loss calculation is 7000 in this area.

zone 3=has 68' of baseboard, 93' of s/r copper and a toe kick 5150 btu.and could use another 2000 btuh to help out. heat loss is 25,038 in this area.

zone 4 = 46' baseboard 130' s/r pex 3/4 could use 3000 to help out in a cold bath with big windows. heat loss is 14459 in this area

Now I can run 2 and 3 together or 1 and 4 together at an ODR of 10 with max high of 170 and low of 120 (target 154 and modualate 20%  till zone temps are good(about 1 and a half hour haul for 2 degree set back).But when I set the high at 160 and low at 110 , the target is 140 and ill short cycle in less then 10 min.

"Internal temp controls? If this is the trouble, then I would start looking at volume of water flow, and limiting the firing rate further."

Could be flow( I have a 008 delta pump set in the mid range for 20 degree delta) I have run tests all the way up and down but the temps just creep higher til short cycle begins. I have the low  fireing rate down to 20% ( lowest setting on priority 2) Have high fire set to 55%



As part of flow issues, what is the zone pump set to do? 55% which should equal about a 20 delta at higher temps.



"other? Shifting priority demand? " Not sure what u mean here

Ty again for help....any ideas?

Jean-David Beyer

running a lower reset temperature.

The trouble with an oversized boiler is that running a lower reset temperature is no help at all for two reasons. First of all, the boiler will not modulate down low enough to achieve that lower temperature except by cycling, and even if you get it, the lower temperature you get, the less heat will be removed from the boiler and into the building, so it will cycle more than ever.



We talked about some things that might help a little in another thread here. I have the same boiler he has, but smaller, and mine is oversized also (though it is the smallest in the product line). Unfortunately, the most effective thing that can be done is to run higher temperatures so that the thermostat shuts off sooner. Trouble with this is the higher temperatures reduce the efficiency of the boiler when it is running, and reduces or prevents condensing. It can also reduce comfort.. It is really important to not oversize the boiler. If it is oversized, everything else is a band-aid.

Ronbo

temp/load/head

The ultra Always starts out at 50% and waits ( at least a min ) before deciding what it needs to modulate up or down to. Now I have a high fire rate of 55% set so it can only increase 5%. If this buffer tank is installed and is insulated anything like my indirect(1/2 degree loss per hr) the temp in it should be close to my target is because of previous cycles.  I would think the mass should be there but not so much load, yes? How this helps I'm not sure of yet,lol.

Does the head it creates become a problem? I would like the buffer installed behind my system and on the floor less than 36" tall (was thinking of a horizontal tank)but I have other options. Would I have to up size the 008 pump or keep the tank higher than the pumps?

Then again is a controller playing/shifting zones a bit the better option as all my homes zones end up a bit warmer/ready for target temps as a result. Like I said I can force it to modulate all day if i had the time to hang around,lol.

But the result is warmer temp in the house across the zones, the indoor feed back can help with some of that I guess, not sure how much the thing can do though or even if that means lower temps to condensate. I assumed from some other reading that the mass of buffer was solution but the controller sounds beneficial also.

scott markle_2

oversizing

It's not all bad, if you look at efficiency charts for mod-cons you will see that the efficiency goes up at lower firing rates, condensing at higher operating temperatures is also an effect of running in low fire. An oversized boiler although bad for many reasons, can potentially be more efficient than a smaller boiler if it's forced to run in low fire.



Try setting your max to 25% (if you can do a DHW demand override), and see if you can still heat your house, it's a great way to see what your actual heat loss is (minus passive gains).



The zone sync. feature alone on the tekmar controls are very helpful with cycling. It's not the end of the world for your boiler to turn on and off, you just don't want a bunch of small zones calling randomly and causing your boiler to have lots of short (less than 2-3 min.) burns.



I think buffers are more useful for fixed firing rate boilers, the problem of mod cons "seeing" mass as load complicates the use of buffers.

Paul48

delta-t

The way I see it.....Running a delta-t circulator will actually make the short cycling problem worse.Keep in mind the system(delta-t) circulator is basically scavenging what it needs from the boiler primary loop(circulating at 12.1 gpm). When the delta-t sees cold return temps, it will slow the pump, allowing more water that has not been affected by radiation to immediately return to the boiler.

furnacefigher15

boiler cycling

What is the boiler shutting off on? Is it the zones getting warm and causing the demand for heat to go away, or something else?



I don't know what ty means.

Jean-David Beyer

If it is like mine,

it shuts off on the high limit of the differential around the reset curve. The default differential is +|- 5F. In my small zone, I changed it to +7F | -8F to lengthen the cycles. I also moved the reset curve up a little to make the load consume more heat. This lowers the efficiency and condensing somewhat, but it is probably better than rapid cycling.



When the boiler is too large, the U-control reduces the firing rate as much as it can (to 20% of maximum), and that is so much more than the load that the temperature increases until it hits the high limit. Then the circulators continue to run and gradually lower the boiler water temperature. Eventually it gets down to the low limit whereupon the boiler fires again, etc. That is all it can do.



Now the U-control cannot really manage if the boiler is significantly oversized. One problem is that it starts at about 50% firing rate and since the water temperature is below the set point, it gradually increases the firing rate until it gets close to the upper limit. But the temperature goes up much faster than the control can follow it, so it hits the top. Similarly on the way down. Lowering the maximum firing rate allows the temperature to go up more slowly and gives the control more time to lower the firing rate. I have diddled mine so that works pretty well except in warm weather, when it is hopeless. But no matter what I do, it cycles on and off because it cannot modulate down to the rate actually needed by the building.



Even if I had a boiler half the size, it would not work in warm weather, though it would be a lot better.

Ronbo

test

OK, so the pump was part of the problem yes, but not all. I took the return sensor from the pump off to force it into full speed all the time. it let my burns get longer and the bottom temp did not push up so quickly. The top temp stays pretty steady until the bottom one goes up a degree there bye making the boilers 20% low fire "too much",(its easier to heat warmer water). So if I had more baseboard/better baseboard/larger mass or more to heat it may not happen. Hey Jean , good news, you can lower the high fire rate, it still starts at 50 to 55 then right away jumps down to set temp, i tried 45% today and it worked. I'm glad i have a 008 instead of a 007, it would happen even quicker as the 007 pumps a bit faster. So i would love to know more about a buffer and how its piped. TY means thank you......but really I thank you all.

PS, the controller looks like a great option if it can mange all the parameters

furnacefigher15

Boiler cycling

Okay, looks like you've made some progress.



Zone 2 with only 12 feet of baseboard is too small of a zone, and it would be best to physically re pipe that zone in series with one of the other zones, preferably one on the same floor of the house, and on the same side also, so the need for heat would be at the same points of the day.





For example: the boiler cycles every 5 minutes on for 5 and off for 5, so that's a 10 minute cycle. or 5 cycles per hour



So you need to add a 5 minute buffer to eliminate cycling on limit.



http://www.cemline.com/products/brochures/seb.pdf

Paul48

firing rate

According to the manual, you can adjust the max firing rate down to 21%, and minimum to 20%

Jean-David Beyer

Installation manual version...

"According to the manual, you can adjust the max firing rate down to 21%, and minimum to 20%"



I have the Ultra 3 manual, and it says the minimum setting for the maximum firing rate is "ignition rate".

The Ultra 3 UE manual (for the current boilers), says 21%, as you say. So it will depend on just what boiler and controller the o.p. has.

Paul48

I can't find that

I looked at the ultra 3 manual, and see the same thing. Can you post a link to your manual.

Jean-David Beyer

post a link to your manual.

See page 89 of this manual. This is for Ultra 3 UE boilers. Since mine is the earliest Ultra 3, it is not listed in the manual. Actually, I may have the U-control for a UE model in there because the original U-control got flooded (long story) and the W-M rep gave me a new controller. There were quite a few different versions of the Ultra 3. Mine was installed May 2009.



http://www.weil-mclain.com/en/multimedia-library/pdf/weil-mclain-pdf/products/boilers/gas-boilers/ultra-series-3-ue/ug3_boiler_manual.pdf



If you go here



http://www.weil-mclain.com/en/our-products/boilers/gas-boilers/gas-boiler-ultra-series-3-ue.aspx



and select Discontinued Boilers, you can see some of the earlier ones and their installation manuals. Called Boiler Manuals.

Tim Potter

How to plumb & control

If you have a mod/con & wish to add a Buffer Tank, how would you plumb it and how would you setup the controls on the boiler to maximize the benefits of being able to run the mod/con for longer periods on low fire & low return water temps?



Thank You:



Tim

Paul48

Options

In this case, we are investigating possible alternatives to adding a buffer tank.

furnacefigher15

How to plumb

There are a few different ways to plumb in a buffer tank.



1- Install into the return side of the boiler loop. Causes 100% of the flow to the boiler to go through the tank first.



2- Use the buffer to create a p/s piping interface.

Supply and return of the boiler on one side (supply from boiler on top, return to boiler on bottom) And on the opposite side (supply to system on top, and return from system on the bottom.)



I generally prefer method 2 for heating application and method 1 for chiller application, but if the tank is big enough, either way will work.



In either method, no additional controls are required, but you can add a tank sensor to prevent the boiler from running if the temp in the tank is warm enough, or as the sole boiler demand control.



The Buffer tank will not cause a lower return temp





Also important, in applications where indirect dhwh is connected, pipe in a way that the dhwh loop is not effected by the buffer, otherwise the buffer may get to hot for its intended purpose.

Ronbo

Have fun with this one

OK today i wanted to run a test to help me determine a few things and would love to know what you all see. By the way my manual says 21% was my min low high fire rate(but does say model specific).

Test is  to determine if I'm getting all the heat/flow /BTU to and out of my baseboard.

settings, its 50 degrees outside today,120 min 180 max and set fire rate to 65% high, 20% my settings are to promote a  140 target. which slant fin gives me ratings for. 10 ODR. All stats would ask for 2 degree set back.all four zones.My oo8 taco is set for full output. Test takes 46 min to complete when all 4 zones met the 2 degree climb. I ran a 10 degree delta he whole way and was modulating at 34% within 5 min and through out. it would mod down to 20% or up to 40 now and then to keep pace. flu temp was 129, blower was 2000,

now slant fin says 300 btuh per ft but they add 15% so lets call it 255 per ft

i have 170' of baseboard x255 = 43350

plus a 5150 btu at 140 degrees  toe kick which works well on high its a 6/8 total = 48500

219' of copper connecting these together some on cold walls some in ceilings none of it is insulated i think this losses 50 btu per ft = 10950

196 of pex i guessed 10 per ft  x 10 = 1960 

so 61,410 btuh of radiation at 140 degrees.

my tests resulted in the boiler modulating at 32% which i think means either 155,000x.32% btu ultra (49600) or 128000 actual output ( 40960) which do I use? its called a 155,000 but the output says 128,000?

So 61410 less the 40,960 is big to me but the 49,000 could mean lots o dust bunnies and 50 % of the basebaord is painted.

Could this be a flow problem or sounds normal?

Now what did you mean that a buffer will not lower temps? would I not be able to run cooler temps? is there another way when set up that i could? mixing of some kind?

furnacefigher15

Buffer

In order remove heat from the water, you need load. The buffer tank will not add load. All the buffer tank does is add a time delay to the flow of water / btu's , and therefore extend run time.



Now depending how it's piped, it will act as a mixing point, blending different water temps together, but p/s piping does that on its own. The main thing the buffer will do is add a time delay to the flow of the boiler loop. The pump for the boiler will move 14 gallons in one minute. If you add a 70 gallon buffer tank to the boiler loop, it will take 5 minutes for water leaving the boiler to return back to it, hence time delay. The purpose of the time delay would be to allow for the boiler to run longer, and in that longer run time, hopefully the return from the system will drop on its own giving the boiler something to heat.



The rated EDR is based on clean baseboard, and a specific room temp and boiler water temp.

Paul48

primary

I'll put this out there, for what it's worth, I do not like manufacturers drawings for boiler primary. Under all conditions, too much boiler temperature water is recicrculated immediately back to the boiler. I would prefer the boiler loop expanded and the system within, using closely spaced tee's as primary to insure the return temps reflecting the system. After all, the system is desiged based on return water temperatures being a certain temperature.I wouldn't consider something inefficient to try and create efficient boiler operation. I would rather run a second indirect off the boiler  loop and preheat my DHW rather than waste it.

Jean-David Beyer

Would that really do much good?

My indirect runs two or three times a day (40 gallon nominal) for about 10 minutes each time. That is about all the hot water I use.



So if I stuck another indirect in series with the primary loop to preheat the water going into the indirect, just how much heat could I put in there in 24 hours? It would be less than what now goes into the indirect because the indirect I now have gets 175F water, and the primary loop, when heating the house never exceeds 134F, and on design day it is closer to 112F.



When my main (radiant) zone is running, it takes about the same flow as the boiler loop. When both zones are running, it takes more flow in the secondary than in the boiler loop, so none returns directly. Only when the small zone is running alone does most of the water flow backwards through the closely-spaced Ts. Why is this a problem? The boiler already modulated down as far as it can in this condition, so the return water temperature is about as low as it is going to get whether it goes it goes through through that zone or not.

Ronbo

how much?

So what is the boilers max fire rate 100% ..... is it 128,000? or 155,000?

if it is 128,000 as I think , then I have a water flow problem or my baseboards are clearly not radiating as intended by a drop of 33% on top of the 15% i already removed from fin rating chart yes?

I think I'm going to take some temps from the individual zone 3/4 returns and see how close they are to readings on the boilers. if they are close to the boilers/manifolds readings then its my baseboard thats not up to snuff(matching up to test ).....if they(temps) are off by 33% or so then the flow would be a problem(faster needed). Am I right in this thinking?  Now Im sure some heat mixes from the manifold and creeps away from the tees to readings along the manifold (location of boiler sensors)and could be skewed a bit. But my problem of bump up from bottom temps is real, So I need to remove doubts about flow from all this thinking before moving forward.

My thinking is if my return temps are actually lower then stated by sensors by 20% or more it tells me that I need to get more of those temps(water) into boiler loop. I could do that by volumne(buffer) or by speed yes?

CMadatMe

Wider

The first thing I would look at doing is to get rid of the pri/sec. It is most likely only allowing a 20-25 delta across the boiler. You need to widen that delta enabaling you longer run times. I'd pop on a low loss header and size my boiler pump for a 40.



What delta are you running the VDT 's on? If you are running a 25 or 30 you also not pulling many gpm off the secondary side on the small zones which is just sending hot water back to the boiler.

Ronbo

My feable mind,lol

I currently have the 008 set to max flow / not at variable speed because it was fighting the whole process, it would slow or (speed up rarely) So because it spent most of its time running slow and the boiler loop was all that was being accounted for. Things got better right away in tests once I got more flow.

Does this design  do the trick and still keep the warranties from Weil Mclain?

http://www.cemline.com/products/brochures/seb.pdf

I keep thinking , why cant I just pipe from my supply header with 3/4 like all the other zones into a buffer tank then out to the return like all the other zones with 3/4.This as my feeble mind sees it just creates a large zone (mass) that's open all the time(no zone valve) waiting for the pump to kick on.

Then when a zone calls ,the buffer and the zone calling both flow when the 008 comes on. The buffer adds mass (or does it piped with only 3/4?)that cannot be heated so fast and keeps me in p/s to Weil Mclains liking.

I would love a delta of 20  though. thats why i got the pump in the first place. But the lower water temp for modulating takes priority I think, yes?

PS. I have another issue to think about/ lol I did switch the direction of flow through a zone to better accommodate hierarchy of need on zone 3 when I re did things and zone 3 has  a toe kick heater. The heater is run with mono flow tees x 2 off the main with 1/2" copper ......I think that still works correctly yes?

furnacefigher15

40 degree delta

Running a 40 degree delta would be fine, but that has to be accomplished by load, not the boilers pump. The best way to accomplish that is by what is called load stacking.



Load stacking would be more like what Paul48 is eluding to. Form a "extended primary" and have all the secondaries tie back to it with close tees (requiring a pump for each zone/secondary). What this would do is cause the first secondary to be the hottest, then the next would be a little cooler, and the next a little cooler still, and so on.



But this strategy would require a different control set up, and a lot of re piping.



In order for this to work, the U control would have to have only one in put demand to run, and no reset for the b/c of the dhwh



The way it would work would be as follows: boiler would have to run hotter on a call for heat like 180 supply. That 180 supply would first be sent to the dhwh and and it would remove some heat leaving say 170 supply after mixing into the header, then the next zone would draw on it and remove more heat leaving say 165 after mixing into the header, and so on. It also would require the use of pump controllers to provide pump start/stop and boiler demand.



I don't believe the boiler in this case would benefit, b/c all zones require the same temps. A better application would be radiator or baseboard loop, then in floor heat, cause the in floor can utilize the lower temp water during all outside conditions.



A low loss header is no more than a miniature buffer tank.



Sizing the boiler pump for a 40 degree delta would cause the flow through the boiler to drop by 50% at least. This will cause an increase in delta, but by causing the supply to get hotter, because there would be more dwell time in the boiler.



Certainly before I would recommend adding a buffer tank, I would try tweaking the system to get better performance with what is there.

Ronbo

reply

"Certainly before I would recommend adding a buffer tank, I would try tweaking the system to get better performance with what is there"

Yupper, thats what I hope to do 1st get stable. Get my lowest temps with good cycles then see where to go from there. Ill post back with results on testing today sometime.

Hey Furnace, what say you on my piping idea for buffer? Its the easiest fix /piping right now as im set up for adding a zone now. Am I off my rocker?

furnacefigher15

Because

If it is piped like a zone, it will only every see one temp, hot supply, and will have a hard time ever cooling down.



In your case I think programming, and maybe minor piping tweaks to balance the zones is all you need.



You may never get the Ideal b/c the boiler is too big, but I can tell you that you can make it workable w/o major reworking.



Adding a buffer is a last resort. Buffer tanks are not cheap, most are built for commercial use, and therefore require ASME rating, which adds big money to the cost.



When you first posted about your system, I was under the impression that it was all new, baseboard and all, and sized to run low temps. I now know that is not the case.



With dirty baseboard, you will not get the heat emission you planned on, especially at low temps.



The first thing you should do is break out the vacuum and clean the baseboard, and be sure to keep curtains away.



Next, I would try bumping up the low end of the reset warmer, to force more btus to emit into the space.

Paul48

ODR

Raising the min OD reset will force the max temp to be the target. Won't that just make his short cycling problem worse?Wouldn't lowering the max ODR to try to force the controller to make the min temp the target be a better solution? A little foggy this morning.

CMadatMe

Boiler Delta

there is a reason they give different flow rates and boiler head. Boiler is oversized, by increasing the delta on the boiler side and lowering the flow rate you can then exhaust the primary loops btus into the secondary side running at a 20 or 25. The use of a LLH will allow for a blend with the warmer return water back into the supply keeping the cooler water heading back to the boiler. It is also the least intrusive piping change as well as more cost effective. Boiler pump change to a UPS15-58 or Taco 00R.

furnacefigher15

Paul 48

Maybe you misinterpret my meaning.



By increasing minimum reset temp, I'm referring to the water temp, not outside temp.



Increasing the minimum reset temp will cause the boilers target temp to go up, but it will also cause more btu's to emit from the baseboard to the air.



The higher the water temp is compared to the space temp, the more btu's will transfer to the space, causing a higher spread from supply to return. But, too high a temp will prevent condensing. Originally I believe the minimum target was 80, and the boiler short cycled on internal temp limit, because the water temp was too close to the space temp and the btu's were not transfering to the space.

furnacefigher15

The buffer won't work

piped that way, like I said before, it would only see hot water. And once hot, it would stay hot.

furnacefigher15

If you could

change the way the boilers algorithm worked, I would agree with you, but unless you use a 3rd party control, and reset the boiler to use a remote firing rate, you cant.



As far as I'm aware of there is no setting in the boilers control to tell it what delta you want to maintain.



So, my point remains, reduce the size of the pump, and reduce the flow through the boiler.



Reduce the flow, and you will increase the delta, but by way of increasing supply temp. The boilers control does not attempt to maintain a specific delta t, but only uses the supply and return sensors to see whether its making progress or not, and how fast.



By the way, a third party (tekmar for example) firing rate control, may be a good option. This is something that would be minimally invasive.

Paul48

Foggy

I thought his problem was that he could not transfer enough btu's to the space without the boiler hitting its limit. I didn't realize he was not getting enough heat to the living areas.

furnacefigher15

Its not

that he does not get heat to the space, but that the boilers heat output to the water does not match what the space is absorbing, so the boiler shuts off on limit.



Meaning there is more boiler capacity then available load.

Paul48

flow

Flow through the boiler is fixed by the boiler circulator.The boiler maintains the Delta-t in a round-a-bout way, based on the information you give it vs its own sensors.It adjusts its own firing rate accordingly. Raising supply temps will always raise return temps regardless of radiation.Thereby shortening the time it takes for the boiler to reach its high limit.Especially if the problem already exists.

Jean-David Beyer

Raising supply temps will always raise return temps regardless of radiation.

That is true. But the situation is really quite complex. If I raise the supply temperature, the emitters will emit more heat. If the boiler can modulate down nearly far enough, raising the supply temperature may just sink enough heat into the house zone so that the boiler need not drop out of modulation and hit the high limit -- not the one at about 210F, but the one just above the reset curve. In the case of my boiler this is not enough. My minimum temperature is now 110F (I wanted it to be about 80F) and it still cycles, but not as fast as it used to.