Required System Flow to prevent short cycling in Mod/Con


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Old 03-25-10, 11:22 AM
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Required System Flow to prevent short cycling in Mod/Con

Just had a Buderus GB142/24 installed. My heatloss is 36k BTU @ design so while its the smallest size they make, it is oversized. I realize it will short cycle during shoulder months but would like to minimize this.

Called tech support and they said the boiler pump will give me 8 GPM @ 7 Ft. of head. This is on low speed on a Grundfos 15-58 three speed cir. A side note is I dont believe this is correct as the pump curve for low speed only gives me 4 gpm @ 7 ft of head(i double checked with the guy and he confirmed 8 gpm).The low firing rate for the boiler is 25k btu.

I am trying to adjust my system pump (grundfor alpha) to the appropriate speed in GPM to minimize short cycling during the shoulder months. I am using the logic found @ http://www.comfort-calc.net/Dont_Ove...on_Boiler.html

Is my assumption correct that if I set the system pump to 2.5 GPM I will successfully pull the max btu from the boiler assuming the one gpm will take 10k btu? Or should I be shooting for around 4 GPM in the system pump (high speed on the alpha) to match as close as possible to the boiler pump flow (based on 7 ft of head and low speed pump curve). Im afraid if I only set the pump to 2.5 gpm, using the logic of the above site some of the flow in the boiler loop will go right back into the boiler causing it to hit the target temp faster.

Again, my goal is to lengthen the firing times at the lowest flow. Thanks for your help.
 
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Old 03-25-10, 02:45 PM
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Just to add some background info, this is a one zone system.
 
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Old 03-25-10, 09:29 PM
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Originally Posted by bovineu View Post
Just had a Buderus GB142/24 installed. My heatloss is 36k BTU @ design so while its the smallest size they make, it is oversized. I realize it will short cycle during shoulder months but would like to minimize this.

Called tech support and they said the boiler pump will give me 8 GPM @ 7 Ft. of head. This is on low speed on a Grundfos 15-58 three speed cir. A side note is I dont believe this is correct as the pump curve for low speed only gives me 4 gpm @ 7 ft of head(i double checked with the guy and he confirmed 8 gpm).The low firing rate for the boiler is 25k btu.

I am trying to adjust my system pump (grundfor alpha) to the appropriate speed in GPM to minimize short cycling during the shoulder months. I am using the logic found @ http://www.comfort-calc.net/Dont_Ove...on_Boiler.html

Is my assumption correct that if I set the system pump to 2.5 GPM I will successfully pull the max btu from the boiler assuming the one gpm will take 10k btu? Or should I be shooting for around 4 GPM in the system pump (high speed on the alpha) to match as close as possible to the boiler pump flow (based on 7 ft of head and low speed pump curve). Im afraid if I only set the pump to 2.5 gpm, using the logic of the above site some of the flow in the boiler loop will go right back into the boiler causing it to hit the target temp faster.

Again, my goal is to lengthen the firing times at the lowest flow. Thanks for your help.
Can't size for best case, need to size for worst case.
You will need to know what the min. flow rate that Budarus recomends for that boiler is.
You will end upi with a low delta t during shoulder months and a high one during cold days.
Leave the Alpha on autoAdapt until it becomes apparent that it needs to be set otherwise.
 
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Old 03-26-10, 07:54 AM
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I have called Buderus on what they feel the minimum system flow rate should be but the tech guy I spoke with was new and is having someone call me back. Its been a couple of days which is why I decided to post here.

The boiler loop flow is set by buderus as the low speed on the grundfos 3 speed which is what I was talking about in the original post.

I did leave it on auto adapt which is what gave me the 2.5 gpm in the system but I found when I put it on high speed, the burner on time appeared to be longer.

I agree that the delta T will be low during the shoulder seasons by nature of less heat given off at the lower temps, and unless I drop the speed during the winter (which I will probably do) the delta T will be relatively low as well.

I just wanted to see how to calculate what speed I could "set it and forget it" that would maximize my run times during the shoulder seasons (@ the lowest speed needed) but also give me an acceptable delta t during the winter months. If I have to adjust speeds during the heating seasons, then it is what it is but I like to be able to calculate things rather than guess.
 
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Old 03-27-10, 12:44 PM
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I believe most mod/cons do not want you to change the vary the flow through the boiler. If the heat loss is 36K you need to move 3.6 gallons at max heat loss. Can you turn the fan max rpm's down to better suit your heat loss? Natural gas you can clock the meter to determine input.
I like the Delta-T pumps as you set for the delta - and that is it. Much easier with the same end result.
 
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Old 03-28-10, 08:38 AM
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If you really have issues with short cycling, consider adding a buffer tank to the system. The radiator (or whatever heat emitters) loop will draw from the tank, which will add mass to the load, and fire the boiler for longer and more efficient cycles.

Another way to accomplish something similar is to use a hydraulic separator between the boiler and system. Look into these options rather than try to nail down a perfect flow rate. It may never happen.
 
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Old 03-28-10, 11:52 PM
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If (and only if) you have a one zone system, why do you even need P/S piping, or the Alpha? The end result, regardless of how you pipe the system, is governed by thermodynamics: The boiler puts out heat, and the floor absorbs that heat from the water. If the system isn't in balance, and the boiler puts out too much heat, there's nothing that you can do to change that, regardless of how you have the system piped. Anything other a single loop will only make matters worse - there's nothing that you can do to make it better without a buffer tank. Think of it this way: That heat is going into your water, and it's much better to spread that water out through the entire system rather than keeping that hot water in the boiler loop.

If you're not into changing your piping yet, I'd match the Alpha's gpm to be at least the gpm of the 15-58, and you'll accomplish the same thing. (Do not let the Alpha do what it wants to do - tell it to operate at one of the fixed curves, or at one of the fixed pressures. There's nothing the Alpha can do for you here.) Then, the radiator (the floor) will absorb all of the heat that it can, and when the water gets too warm, the boiler will cut off. At least you'll know that the floor took all that it could take.

But, if your entire system holds, say, 12 gallons (100 pounds) of water, it'll at least take 3,000 BTU before the temperature goes up 30 degrees. 3,000 BTU on that boiler is at least 7 minutes, and if one assumes that the floor takes up another 3,000 BTU while the water is getting hot, you'll at least get 6,000 BTU's, or 15 minutes, of run time out of the boiler. Again, the most important thing is to get the water out of the boiler loop. That's why you see longer burn times when you run the Alpha on a faster speed.

But again, anything that you do to un-match the speeds of your two loops will only make matters worse. The only way to make things better is to add water mass somewhere, but frankly, I can't believe that you need it. Short cycling is generally a problem with systems with small, micro-zones that don't chew up heat fast enough, and that's not what you have.

Jeff
 
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Old 03-29-10, 02:28 PM
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Thanks for your responses. Jbaron, I think you answered my question as far as logic. I will try to get the system flow to be as close to the flow in the boiler loop (15-58) which I think will be the high setting on the alpha. that will hopefully extend the burn times to as long as possible giving the boiler the most mass to work with.

To answer some questions, I did try the taco delta t cir. first. I wanted to play with the delta t to keep the boiler condensing as much as possible. The problems were that it screwed with the modulating logic of the boiler as it kept adjusting the flow in the system. Also when the boiler called for temps for 100, the flow went to the lowest speed possible and that caused the boiler to hit its target too quickly. Also as the flow was so slow, at times the return water temp was actually higher than the supply causing the pump to keep going into an error mode. it worked fine with higher boiler temps but it gave me too many problems. I chose the alpha as I will be adding a zone soon and also for efficiency purposes as I will pretty much have constant circulation.

I piped my system as P/S as buderus requires it. they actually supply the boiler loop piping.

i dont plan on adding a buffer tank. i am aware of its benefits for longer burn times but dont feel I would ever recoup the cost.
 
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Old 03-29-10, 06:02 PM
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Am I wrong or is the operation basically the same on both the Taco and the Alpha. the logic is definitely different but they both operate by changing pump speeds.
Any problem experienced with one should be experienced with the other. The boiler flow should not be changed for best results.
 
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Old 04-01-10, 10:28 AM
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The boiler loop flow is constant and is run by the grundfos 3 speed on low speed.

yes the logic is different with the two pumps and they run differently at least from what I observed.

The alpha will change speeds when either a zone opens or there is a call for heat. What I found was that with an ORD the heat call does not increase temp and as a result, the pressure, high enough to change the speed of the alpha (on auto adapt). I imagine that if on a call for heat the temp went from say 120 to 180 then that change in pressure would increase the speed as its running on a delta p. But as Im only going from say 95 to 115, the pressure of the system is not changing much and the expansion tank is probably helping to keep the pressure constant as well. As such, I didnt experience the problems I did with the taco.

On auto adapt, the pump seems to have a higher "floor" or bottom speed which for me was 2.5 GPM approx. The floor of the taco is far lower as its trying to give me (in my case) a 20 degree delta t and with the low temps that I am working with it will never quite get there.

Another observation I have of the of the auto adapt function is when I had to move the outdoor temp sensor. My boiler ran for an hour at full temp but I did not notice a change in the alpha speed.
 
 

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