∆ T, gpm, output relationship
When Desired Delta narrows we would like to slow a circ , when Delta widens past desired point we would like a circ to speed up .
output= 4gpm*500*20 = 40000 btuh supply t= 180f return= 160
if we slow the circ we would have wider delta t? and less heat output for the emitters?
Comments

You will have less output for the emitters as you go downstream. The first one won't see much effect because it will still be getting the hottest water. The last in line will have a more significant performance drop. A good designer will upsize the downstream emitters to compensate for the designed temp drop."If you can't explain it simply, you don't understand it well enough"
Albert Einstein0 
Not really in all cases would you want to slow down flow as delta tightens. Radiant floors for example like a tighter delta compared to panel rads. A tighter delta across a heat emitter, all things being equal (flow rate), would produce a higher average temperature and higher output.chakil said:Hi,
When Desired Delta narrows we would like to slow a circ , when Delta widens past desired point we would like a circ to speed up .
output= 4gpm*500*20 = 40000 btuh supply t= 180f return= 160
if we slow the circ we would have wider delta t? and less heat output for the emitters?
We chose this graph for the cover as it shows how heat output by changing flow rate is very non linear, notice the hockey stick shape to the output as flow increase. At some point the additional pump required to get a small % just isn't worth the squeeze.
A good read on how heat transfers inn hydronics.
https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_23.pdfBob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 
hot_rod said:
Not really in all cases would you want to slow down flow as delta tightens. Radiant floors for example like a tighter delta compared to panel rads. A tighter delta across a heat emitter, all things being equal (flow rate), would produce a higher average temperature and higher output.chakil said:Hi,
When Desired Delta narrows we would like to slow a circ , when Delta widens past desired point we would like a circ to speed up .
output= 4gpm*500*20 = 40000 btuh supply t= 180f return= 160
if we slow the circ we would have wider delta t? and less heat output for the emitters?
We chose this graph for the cover as it shows how heat output by changing flow rate is very non linear, notice the hockey stick shape to the output as flow increase. At some point the additional pump required to get a small % just isn't worth the squeeze.
A good read on how heat transfers inn hydronics.
https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_23.pdf
Hi,
From the pdf :
CONSTRAINED ∆T :
When design load is no longer present in one zone, and it turns off, less heat would be removed from the distribution system. This change would reveal itself as anincrease in return water temperature (assuming that the supply water temperature is fixed). The ∆T of the circuit would decrease. A controller that senses this decrease could respond by reducing the speed of a circulator so that the design load ∆T was reestablished for the zones that remain active. This process would repeat when another zone turned off. This method of control reduces circulator energy use during partial load conditions.
Question:
when water flow slows heat emitter releases less heat so return temperature should be higher then ∆T would decrease
so how can we constrain ∆T by reducing the speed of the circulator?
0 
you've assumed a circuit in seriesZman said:You will have less output for the emitters as you go downstream. The first one won't see much effect because it will still be getting the hottest water. The last in line will have a more significant performance drop. A good designer will upsize the downstream emitters to compensate for the designed temp drop.
0 
Is this a hypothetical conversation or do you have a heating system you are trying to dial in?
I did assume baseboard heat in series, perhaps my mistake.
If the question was hypothetical, Idronics 23 should give you plenty to chew on..."If you can't explain it simply, you don't understand it well enough"
Albert Einstein0 

@chakil going to try and answer your last question.
As you shed load by zone valve shutting, you need less total Btus.
So where you once needed 15k BTUs from three different zones. Two zone valves shut, now you only need 5k BTUs for one emitter. Do the hydronic formula and you come up with less GPM needed. That's assuming the three zones have the same Delta T.
Change the delta T in any of the zones and the gpm needed for that zone will fluctuate.
I think hot rod was saying that delta T is not linear with gpm not that they aren't related.
Does any of that help?1 
conventional Boiler doesnt modulateYoungplumber said:@chakil going to try and answer your last question.
As you shed load by zone valve shutting, you need less total Btus.
So where you once needed 15k BTUs from three different zones. Two zone valves shut, now you only need 5k BTUs for one emitter. Do the hydronic formula and you come up with less GPM needed. That's assuming the three zones have the same Delta T.
Change the delta T in any of the zones and the gpm needed for that zone will fluctuate.
I think hot rod was saying that delta T is not linear with gpm not that they aren't related.
Does any of that help?
at first it produces 15k for the design load and ∆T 20 then load become only 5k
the boiler will still produce 15k the supply temperature would increase and ∆T decrease and shortcycling with same gpm?
if we lower gpm, ∆T would increase ? but
if we lower gpm less heat would be relased by the emitter so return water temperature would be higher so ∆T decrease ?
0 
The answer to your question about the relationship between GPM and delta T is pretty simple. If the supply temp stays the same and you reduce your gpm, your delta T will widen and the emitters will put out less heat. I can't think of a system where this is not the case.
Whether it is a good idea to do so depends on a whole lot of factors: mass of system, output of boiler (modulating or not),laminar vs turbulent, series or parallel to name a few.
It sounds like you are are asking for opinions about a particular system (because you correct people who make assumptions) but don't want to give specifics about the system.
As with anything heating, the answer is "it depends".....
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein0 
Don't forget the load. The delta on a fixed speed fixed output boiler will drop as the heat load drops. The heat emitters are in charge of the boiler operating condition, always. We put limits and temperature controls on boiler to prevent them from working at unsafe or undesireable conditions.
With a fixed speed circulator and on off burner control, as the load lessens the boiler will short cycle. All a Delta T circulator does is "pulse" the heat into the loop to lessen the on/ off cycling when you have a fixed output boiler, for example. In some cases the gpm drops to a point where the energy being pulsed will not cover the load as the graph shows. So you would need to put a low speed limit as most delta T circs show where the bottom of the curve is truncated.
If the boiler allows, it is much better to lower the SWT, modulate the burner to match the ever changing loads. The loads change as zones kick off, or you are at the point where the thermostat shuts off, load is covered.
As the hydronic formula shows wider delta operation can move or carry more BTUs. The math and basic physics also show that higher temperature emitters, of any type transfer more heat energy with higher AWT. This attached examples shows a constant SWT and 3 different flow rates.
So in my simple way of thinking, design around a delta T, pick one out of thin air, 23.7 for example. You may at some point actually see the system operating at that exact delta. It may be a 30∆ on a cold start, it may drop to a 5∆ just before it satisifies, really no harm in that movement.
If a fixed output boiler short cycles with a low load condition, it's not the boilers fault, the load is the issue. So lower the SWT with ODR if possible. That will help but not completely eliminate short cycles. A buffer tank is another option. The older large mass cast boilers and large volume steel pipe systems were "buffered" by default. Todays cast boilers hold a few gallons and weight 1/2 of the older iron, so that cycling can be more of an issue.
IF we could exactly match the SWT to that ever changing load, it is possible the boiler fires and never shuts down, the output is always exactly matched to the moving loads. With todays deep modulation and ODR, ramp delay and other functions you can get real close to that condition. If the weather cooperates and a load within that modulating windows is present.
A fixed speed, fixed output boiler and fixed speed circulator it will not be easy to get to and stay at a steady state condition, zoning, especially micro zoning makes it worse.Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 
@chakil The answer I gave was pretty complete. I didn't bring up the boiler because we're not talking about the boiler. I broke it down pretty simply for you. Not really sure what more of a question you could have.
Do you know the hydronic formula?
As Z man says you need a specific example to punch into the formula. Then you can play with those numbers to answer your question. Or design what you want to design.0 
Hi thanks for the replyYoungplumber said:@chakil going to try and answer your last question.
As you shed load by zone valve shutting, you need less total Btus.
So where you once needed 15k BTUs from three different zones. Two zone valves shut, now you only need 5k BTUs for one emitter. Do the hydronic formula and you come up with less GPM needed. That's assuming the three zones have the same Delta T.
Change the delta T in any of the zones and the gpm needed for that zone will fluctuate.
I think hot rod was saying that delta T is not linear with gpm not that they aren't related.
Does any of that help?
in your example, by reducing GPM we retablished the delta t for the 5k zone
a conventional boiler would still short cycle isnt it? what would be the impact of reducing GPM on the boiler, in your example?
0 
Well I hadn't considered the impact on the boiler since you were asking about the relationship between delta t and gpm.
It could, but it really depends on the boiler and piping arrangement, is there a buffer tank, heat loss etc.
Is all a conglomeration of factors that make a sytem run at peak efficiency.0 
Hypotheticaly
considering simple system with conventional boiler pump and emitter
boiler set 180 f with 20k btu h 4gpm in design load delta t 20
and two zones each 10k
lets say only 1 zone on certainly the boiler would short cycle
by reducing gpm we would keep delta tee for the zone
what would be the impact of reducing gpm on the boiler, would it keep short cycling ? more short cycling or less?
0 
So in your theoretical setup. Each 10k zone heats in a different amount of time? That doesn't really compute, unless the emitters are sized incorrectly. But if for some reason it did. I'm following you, although the logic is flawed (because each zone needing 10k btu would heat in the same amount of time.) Your boiler would already be cycling for a given time seeing as it already pumped 10k btu into one of these zones. So it shouldn't be short cycling unless one zone and only one zone calls. But heat loss is the rate at which something loses heat.
Your fundamental question is, can you alter the delta t between zones to make your boiler run more effeciant, with a conventional boiler? It depends
I feel like we're two monkeys trying to read Shakespeare here.2 

i dont understand how can we constrain delta t
let say system with conventional boiler with 20k, 2gpm, delta 10f, load is 10k, certainly the boiler would short cycle
so can we reduce gpm so we increase delta t and then no more short cycling? is that possible?
0 
OK I totally edited and deleted this from my original.
The hydronic formula gives you gpm based on your Delta t and how many btu's needed.
So you need 20k btu with a delta t of ten.
10*500=5000
20000÷5000=4 gpm.
Change the delta t in this equation. Use all different ones you'll get different gpm needed. Does that help?
I belive I asked you before if you knew this formula so you could play with it.1 
how could it be possible, whatever you do with gpm the boiler still give 20k and load take only 10k
where would go the other 10k of the boiler?0 
I do understand what your saying. Your asking in the fall can you change gpm and get longer cycles. You should be able to.0

But your boiler should heat as fast as it cools on the coldest day of the year. Which means your only gonna be able to do so much. You will at some point need shorter cycles. Gpm is a factor... Of many which determine run time.0


Keep your load the same say 20k btu but 25 delta t.
20000÷12500=1.6 gpm
0 
The other 10K causes the high limit to stop the burner. Oops if the burner stops then the other 10K isn't there.
You can get 40,000 BTU per hour from an 80,000 rated boiler by shutting it off for 30 minutes every hour. or you could use a 40,000 BTU burner for 60 minutes and get the same amount of heat... In a perfect world.
We don't like to do that because there is some added energy lost on startup and there is also more energy loss on shut down and then there is standby loss too. To pump or not to pump... That is the Question!
Edward Young
Retired HVAC Contractor from So. Jersey Shore.
Cleaned & services first oil heating system at age 16
Specialized in Oil Heat and Hydronics where the competition did Gas Warm Air
If you make an expensive repair and the same problem happens, What will you check next?3 
Another way of looking at it besides @EdTheHeaterMan 's comment is that the extra 10K of heat goes into raising the overall temperature of the water leaving the boiler. Now at some point the boiler is going to hit its high limit and shut down. Then the circulating water will gradually cool down. At some point later, the boiler hits the low limit and fires up again.chakil said:how could it be possible, whatever you do with gpm the boiler still give 20k and load take only 10k
where would go the other 10k of the boiler?Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), WeilMclain 580, 2.75 gph Carlin, Vapourstat 0.5  6.0 ounces per square inch2 
That is a nicer way of sayin' that @Jamie Hall But you always speak better Shakespeare than @Youngplumber and @EdTheHeaterManEdward Young
Retired HVAC Contractor from So. Jersey Shore.
Cleaned & services first oil heating system at age 16
Specialized in Oil Heat and Hydronics where the competition did Gas Warm Air
If you make an expensive repair and the same problem happens, What will you check next?0 
Lol. I'm not even sure where I am right now.0

so the boiler would run 30min/h straight or it gonna short cycle ?EdTheHeaterMan said:The other 10K causes the high limit to stop the burner. Oops if the burner stops then the other 10K isn't there.
You can get 40,000 BTU per hour from an 80,000 rated boiler by shutting it off for 30 minutes every hour. or you could use a 40,000 BTU burner for 60 minutes and get the same amount of heat... In a perfect world.
We don't like to do that because there is some added energy lost on startup and there is also more energy loss on shut down and then there is standby loss too. To pump or not to pump... That is the Question!
0 
@chakil Like @EdTheHeaterMan said, if the system isn't shedding the load, aka heating a room, it sends that heat back to the place with the fire, aka the boiler, making the return temp climb. If the temp climbs too high the internal safety for the boiler shuts it down until it cools off. Slowing the flow down on a fixed output boiler will speed up the time it takes to thermally shut it off because its flame puts the same amount of BTU into the chamber all the time. The fluid flow is what takes it away and keeps the chamber below its high limit shutdown. Slow down the flow and...You can have it good, fast or cheap. Pick two0


but from the formula q=500.gpm.delta tCanucker said:@chakil Like @EdTheHeaterMan said, if the system isn't shedding the load, aka heating a room, it sends that heat back to the place with the fire, aka the boiler, making the return temp climb. If the temp climbs too high the internal safety for the boiler shuts it down until it cools off. Slowing the flow down on a fixed output boiler will speed up the time it takes to thermally shut it off because its flame puts the same amount of BTU into the chamber all the time. The fluid flow is what takes it away and keeps the chamber below its high limit shutdown. Slow down the flow and...
by slowing gpm we would have a bigger delta t which should prevent short cycling?0 
No, it won't. The short cycling comes from the boiler putting out more heat than the emitters can get rid of. Changing the flow rate won't change that.Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), WeilMclain 580, 2.75 gph Carlin, Vapourstat 0.5  6.0 ounces per square inch0 
Yes, yes, yes! hot_rod, "The heat emitters are in charge of the boiler operating condition, always." In fact, heat emitters is what it is all about as far as a comfortable environment is concerned. The boiler is there just to satisfy the heat emitters.
One does a whole house heat loss calculation and determines what size boiler to install without taking the heat emitters into consideration may end up as someone stated, "You have a Ferrari up against a tree with your foot on the gas, the wheels are spinning, but you're not going anywhere." The boiler is putting out the heat energy, but it hasn't anywhere to go. Sizing boilers is a juggling act. There are so many variables that one has to consider and heat emitters are just one.
I posted earlier that the BTU transfer from a heat emitter to a space was dependent on flow and SWT. That was simplistic although true. The forgotten element was physics that is the space environment. Heat energy moves to cold. What if the SWT to the heat emitter is 100 deg and the room temp is 100 deg, what would you say the heat energy transfer would be?
Maybe a big goose egg? Maybe a ∆T of 1deg? Environmental factors play a larger part in your planning.
What's so sacred about a ∆T of 20 degs? Wear & tear on the boiler electronics with a tighter ∆T? Boilers have circuit boards that have relays that have points that arc when they close and pit. I can see that. But, HXs that heat up from 70 deg to 180 deg is more stressful, I would think. Better to run the boiler with a tighter ∆T, less stressful. You would still need to put out the goods to the house environment.
Years ago, I was talking to a Glowcore engineer and he said the best operation was for the boiler to run 24/7 and never shutting down. Long, long, long run times while delivering the goods.
It's so complicated, which is why hydronics is so exciting. There are all these puzzles to be worked out and there is a pleasurable satisfaction when everything works out the way you planned. So, what's so sacred about a 20 deg ∆T?
1 
Here is some perspective on the points that Homer and Younge and Jamie are trying to make
Now the piping to and from the sections of this much radiation will be different in order to maintain the same ∆ T on the 4GPM loop and the 1GPM loop(s) But it is possible with creative piping design.
This discussion reminds me of a story I heard when I was 14 years old.
There were 3 men who wanted to stay at a hotel but they were on a budget and decided to share the room. The Assistant was on duty and was not very goods with fractions and could not figure out how to divide the $25.00 cost of the room equally among the 3 men. He decided to charge them $30.00... $10.00 each. and all was good in the world.
When the manager returned, he was unhappy with these customers being overcharged and told the assistant to return the $5.00 that he overcharged them. Still uncomfortable with the mathematics the assistant decided to return only $3.00 to the 3 men, $1.00 each, and keep the other $2.00 as a gratuity. The men were happy for the corrected overcharge (the refund) and all was good with the world.
So, as I see it each man paid $10.00 and was refunded $1.00 so they actually paid $9.00 each for the room... Right?
3 men times $9.00 is $27.00... Right?
The assistant has a $2.00 tip... Right?
$27.00 plus $2.00 is $29.00... Right?
So where is the other dollar?
This is an example of using incorrect information that sounds perfectly logical to get an impossible result. Think about it. Where is the extra dollar? is the wrong question based on the information provided. This info improperly presented will never get you a correct answer. Can you see the flaw in the logic of the presentation?
GPM will not get you the answer you are looking for because the end result is not a primary function of GPM but a function of the BTU output of the boiler and the capacity of the emitters to release the heat available. GPM is only a small part of the equation. Like that extra dollar that isn't really gone!
Respectfully submitted.
Mr.Ed
Edward Young
Retired HVAC Contractor from So. Jersey Shore.
Cleaned & services first oil heating system at age 16
Specialized in Oil Heat and Hydronics where the competition did Gas Warm Air
If you make an expensive repair and the same problem happens, What will you check next?2 

That's how banks make money.
I've actually heard that one before. The guy who told it to me, kept telling me the 9 dollar part every time. I tried to correct him. But he would say no they paid 9$.
They paid 27 dollars for a 25 dollar room the guy got 2.
I saw where he was coming from though.
Deja vu.
2 
Adding $2.00 is what is wrong. it sounds right but it is not the correct parameter. Each man actually paid $9.00... You either Add $3 t get $30 or subtract $2 to get $25. But you don't add $2.00 to the $27.00.
That is how Uncle Sam gets you to pay for entitlements. LOL tell you you only are paying $3. but actually charging $5.00.
How does that relate to Gallon per Hour and BTU per minute... Did I say that right?
This sounds like the perfect time to bring up Tigerloops and Circulators are not Pumps.Edward Young
Retired HVAC Contractor from So. Jersey Shore.
Cleaned & services first oil heating system at age 16
Specialized in Oil Heat and Hydronics where the competition did Gas Warm Air
If you make an expensive repair and the same problem happens, What will you check next?2 

@HomerJSmith, to quote our illustrious president... "Comeon Man"
if the company that makes it calls it a pump... Then it IS a pump
I guess "That depends on what the definition of IS is!" To quote a former illustrious president.
And where is @STEVEusaPA when you need to talk about Tigerloops anyway?
WOW this has gone way off topic... That is what you get when you have a hypothetical quwey. If the OP just wanted a real problem solved, I guess we could hypothetically help him.
Edward Young
Retired HVAC Contractor from So. Jersey Shore.
Cleaned & services first oil heating system at age 16
Specialized in Oil Heat and Hydronics where the competition did Gas Warm Air
If you make an expensive repair and the same problem happens, What will you check next?1 
The problem is that the heating load is ever changing, so the boiler would like to modulate along with the changing load. To a point a modulating boiler can follow the load. Here is an example for upstate NY, notice how small of a % you are at design condition. So with multiple zones, especially micro zones even a modulating boiler will cycle. Ideally lowering the SWT will help eliminate short, less than 10 minute on cycle. If the cycling drives you, and your boiler, crazy a buffer tank will extend on/ off cycling.Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1
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