Short cycling, flow rate, delta t, relationship

chakil

Hi,

From youtube video, Taco After Dark, Part 4 "Pipe Sizing, Air Control & Pumping Away", minute 50.
It is said that choosing a pump with higher flow rate than needed (or estimated) would lead to boiler short cycling
I don't understand how an increase in flow rate would lead to the boiler short cycling?

Thanks

hot_rod

Depends on how the boiler is sized to the load. If you have a 50,000 BTU/hr load and the boiler output is exactly that, increasing flow rate would change the delta t that the system is operating at.

If the system is zoned, a 50,000 load spread across 5 zones and the boiler is a fixed output then you will see cycling based on what zones are calling, etc.

The best solution for a zoned system would be a modulating output boiler, zone valves and a delta P circulator that adjusts flow rate based on zones opening and losing. Additionally a outdoor reset control would help modulate the boiler temperature to the ever changing load.

chakil

hot_rod said:

Depends on how the boiler is sized to the load. If you have a 50,000 BTU/hr load and the boiler output is exactly that, increasing flow rate would change the delta t that the system is operating at.

Even if the delta t changed and decreased why would the boiler short cycle ? the boiler would produce the same amount of heat per hour wether flow rate has increased and delt t decreased or vice versa, in accordance with the universal hydronic formula.

HomerJSmith

The boiler short cycles (turns on and off) because the temp of the return water is close to the set max temp of the boiler programing. A close delta t.

The boiler BTU output should be met by the heat emitters BTU outflow to the space. Because the heat emitters remove heat energy as the water move thru the sys, the water entering the heat emitters will be hotter than the water leaving the heat emitters.

The BTU's available to the heat emitters depend on two things. The water temp and the flow thru the heating circuit. If you increase the flow at a given water temp and at a set temp difference between the heat emitters and the space, the return water will be warmer at the boiler. A closer delta t. The closer the delta t, the less the time that the boiler will turn on and off. So, at a given temp, flow has everything to do with delta t.

I hope I got that right.

Rich_49

chakil said:

hot_rod said:

Depends on how the boiler is sized to the load. If you have a 50,000 BTU/hr load and the boiler output is exactly that, increasing flow rate would change the delta t that the system is operating at.

Even if the delta t changed and decreased why would the boiler short cycle ? the boiler would produce the same amount of heat per hour wether flow rate has increased and delt t decreased or vice versa, in accordance with the universal hydronic formula.

Short answer is the boiler would then begin to climb toward it's high limit and turn off when that is reached . It is called thermal equilibrium , the boiler is making the exact number of BTUh the emitters are delivering . When that delta narrows your boiler is making more BTUh than the emitters are delivering

chakil

HomerJSmith said:

The boiler short cycles (turns on and off) because the temp of the return water is close to the set max temp of the boiler programing. A close delta t.

The boiler BTU output should be met by the heat emitters BTU outflow to the space. Because the heat emitters remove heat energy as the water move thru the sys, the water entering the heat emitters will be hotter than the water leaving the heat emitters.

The BTU's available to the heat emitters depend on two things. The water temp and the flow thru the heating circuit. If you increase the flow at a given water temp and at a set temp difference between the heat emitters and the space, the return water will be warmer at the boiler. A closer delta t. The closer the delta t, the less the time that the boiler will turn on and off. So, at a given temp, flow has everything to do with delta t.

I hope I got that right.

Let's say we have a 60,000 BTU/hr load and the boiler output is exactly the same with 6gpm and delt t 20f (160f-180f)
we replace the pump with new one with 12gpm and delta t would be 10f (170f-180f)
Why the boiler should short cycle?
My understanding is it would still need to be on the whole hour to supply 60000 btu

EBEBRATT-Ed

@chakil

your right in your last example. if the load is more than the boiler will produce it won't short cycle but you may have other issues like condensing in a CI boiler.

One thing I earned from @hot_rod is the "radiation drives the system" ....or something like that.

The radiation and the boiler will constantly battle to balance heat out put versus heat input and the radiation will always win

Rich_49

chakil said:

HomerJSmith said:

The boiler short cycles (turns on and off) because the temp of the return water is close to the set max temp of the boiler programing. A close delta t.

The boiler BTU output should be met by the heat emitters BTU outflow to the space. Because the heat emitters remove heat energy as the water move thru the sys, the water entering the heat emitters will be hotter than the water leaving the heat emitters.

The BTU's available to the heat emitters depend on two things. The water temp and the flow thru the heating circuit. If you increase the flow at a given water temp and at a set temp difference between the heat emitters and the space, the return water will be warmer at the boiler. A closer delta t. The closer the delta t, the less the time that the boiler will turn on and off. So, at a given temp, flow has everything to do with delta t.

I hope I got that right.

Let's say we have a 60,000 BTU/hr load and the boiler output is exactly the same with 6gpm and delt t 20f (160f-180f)
we replace the pump with new one with 12gpm and delta t would be 10f (170f-180f)
Why the boiler should short cycle?
My understanding is it would still need to be on the whole hour to supply 60000 btu

That would depend on if the aquastat differential was changed

Zman

I am having Deja Vu all over again:( ...
Didn't we settle this nonsense a couple of months ago?
https://forum.heatinghelp.com/discussion/comment/1673128#Comment_1673128

hot_rod

maybe this journal will answer some of your questions.
https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_23.pdf

Rich_49

Zman said:

I am having Deja Vu all over again:( ...
Didn't we settle this nonsense a couple of months ago?
https://forum.heatinghelp.com/discussion/comment/1673128#Comment_1673128

Apparently not . here is another similar question , this is a help site , maybe we can help this person understand it . Sorry we bothered you Carl

HomerJSmith

Zman, that Yogi Berra saying is really funny. I read that post that you put up again, word for word. I forgot how contentious it was.

The question was, "I don't understand how an increase in flow rate would lead to the boiler short cycling?" Haven't we answered this.

As hot_rod said, a delta t is a point fixed in time. The delta t changes with changing conditions during boiler operation. This can be minimized with sensors and a variable speed pump and mod/con boiler, I think.

Zman

Here we go pretending you can create or destroy energy again .
I'll stay out of it this time...

HomerJSmith

Zman,

Albert settled this with his famous equation, E=MCsquared. But, is energy destroyed when energy is changed into matter? hmmm

Please, don't stay out of it, I read your responses with rapt attention.

hot_rod

HomerJSmith said:

Zman, that Yogi Berra saying is really funny. I read that post that you put up again, word for word. I forgot how contentious it was.

The question was, "I don't understand how an increase in flow rate would lead to the boiler short cycling?" Haven't we answered this.

As hot_rod said, a delta t is a point fixed in time. The delta t changes with changing conditions during boiler operation. This can be minimized with sensors and a variable speed pump and mod/con boiler, I think.

But why would you want to minimize or constrain the changing delta T of the system? Recently we had a poster here stating he was seeing a 60 delta on a cold start up of a new system, the space was warming quickly. So why would you impose a fixed delta, say 20 on the running full bore system? Basically slowing the warm up times?

As he experienced and the hydronic formula predicts the wide delta was an indication of a high amount of heat transfer into the space. Perhaps in hours or a day the delta closed to maybe 15 as the load becomes less. And at some point the delta would be 0 when the load satisfies, again proving the hydronic formula in fact works just fine

Adjusting the heat transfer rate to the space is accomplished quite eloquently by changing the supply water temperature, the "mod" part of mod con boilers. It is very predictable and nearly a perfect linear relationship. It's the concept of temperature reset control proven out for many decades now by the early implementation of tekmar technology and their understanding of heat transfer.
tekmar brought this mainstream although Honeywell had cap tube reset controls many many years ago, in the 1950 maybe?. tekmar brought microprocessors to the table, put it in a pleasing, blue plastic box for us.. The boiler manufacturers were wise to embrace the reset technology and offer it a mostly standard equipment on mod cons. The con part of mod con allowed for no low limit on return temperature.

Purists will incorporate an indoor feedback function to the mix, via a RTU, again a concept pioneered by tekmar in the 1990s. Really no need to try and reinvent the wheel, properly dialed in and installed ODR and indoor feedback gets you as close to a perfect control, perfect comfort, highest efficiency, and constant circulating system as possible.

A boiler that could modulate down to 1000 BTU/hr would give us systems that may start on a first heat load call, modulate SWT and BTU supplied and never shut down until the building or space no longer needed heat input.
I feel it is critical that those of us in the heating industry "understand how heat energy travels, and how best to manage it efficiently", it should be in our job description, actually

As a trainer you get used to saying this over and over.

https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_23.pdf

Rich_49

Zman said:

Here we go pretending you can create or destroy energy again .
I'll stay out of it this time...

What'd I miss ? Who said we could create or destroy energy ?

HomerJSmith

Oh, boy! I quoted hot_rod when I said that. As hot_rod said, a delta t is a point fixed in time. What I understood from that statement was that when people discuss Delta T that they think of it as always being fixed and always unyielding to differing conditions, which of course it's not. Why would one want to constrain the changing delta T of the system. That's probably an impossibility.

Rich_49

To have authority over what the system is doing and how it's performing for the longest part of the season . That is actually what people pay us for

con·strain
/kənˈstrān/

verb
verb: constrain; 3rd person present: constrains; past tense: constrained; past participle: constrained; gerund or present participle: constraining

compel or force (someone) to follow a particular course of action.
"children are constrained to work in the way the book dictates"
h
Similar:
compel


severely restrict the scope, extent, or activity of.
"agricultural development is considerably constrained by climate"
h
Similar:
restrict

limit
curb
check
restrain
regulate
contain
hold back
keep down


Maintain
Definition of maintain

transitive verb
1 : to keep in an existing state (as of repair, efficiency, or validity) : preserve from failure or decline maintain machinery
2 : to sustain against opposition or danger : uphold and defend maintain a position
3 : to continue or persevere in : carry on, keep up couldn't maintain his composure
4a : to support or provide for has a family to maintain
b : sustain enough food to maintain life
5 : to affirm in or as if in argument : assert

HomerJSmith

Rich_49, thanks for the English lesson.