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Author Topic: GPM?  (Read 3292 times)

donniet79

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GPM?
« on: December 12, 2017, 04:40:38 AM »

I'm sure this question has probably been asked, but I couldn't find anything on here. I was wondering if GPM has any bearing on wood usage or boiler efficiency? I guess what I mean is if I am running low flow(not what I should be running) will it cause my return temp back to the boiler to be lower and cause the it to drop the boiler temp faster than if I run at a higher GPM? The reason I ask is because after actually sitting down and figuring how much ft of head I have after 2 years of having this boiler, I figured out that I need a pump capable of 15.284 ft head to get my full 7gpm of flow through 1" pex. The pump that I have is not even capable of over coming that. On high speed my pump is only running around 5gpm according to the performance chart, so I'm not sure how what effect that has on my system. Maybe I'm figuring wrong. I'll tell you what I have. I have the small badger pump. My lines from boiler to the house and back round trip is 230' and my boiler is about 6ft lower than where the lines go into my house. They first go through an 20 plate hx then to a 16x19 hx in my forced air plenum. I run my boiler at 180F with 10 degree differential. My boiler takes quite a while to recover once it kicks on so I'm just trying to figure out if I need to change something in my system. I hope I worded all of this correctly. I appreciate any feedback from you guys. BTW, thank you to CountryBoyJohn for giving me advice to help me get my boiler burning like it should be.
« Last Edit: December 12, 2017, 04:51:37 AM by donniet79 »
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mlappin

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Re: GPM?
« Reply #1 on: December 12, 2017, 06:47:04 AM »

Usually difference in height unless extreme doesn’t change head, you have yo push water up to the house, but the return water is flowing down so they negate one another somewhat. You can only push so much thru 1” pex which is actually more like 7/8” ID, however a larger pump will help, I usually stick with grundfos, sounds like you may have the equivalent of a 15-58, I’d try a 26-99 after checking your temperature differential, I imagine it’s quite large. There is nothing special about a 20 degree differential but its what most people design for. Take black electrical tape and wrap the pipe at the back of the stove on the inlet and outlet then take a IR gun and place it directly on the tape and see what you have.
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wreckit87

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Re: GPM?
« Reply #2 on: December 12, 2017, 09:06:53 AM »

This was discussed on another page a couple days ago, with an individual claiming he burned a fraction of the wood by doubling up a pump that was already overkill in singular form. The fact is, that your building(s) require X amount of BTU. Your stove needs to burn X amount of wood to make that BTU. The flow rate to get it there will have no bearing on the amount of BTU used because the heat load remains the same regardless of flow. Now, if you have low return temps, your burn cycles may be longer, but considering your exchange rate is slower, it balances out. I have a hard time believing 5GPM is accomplished through 230ft of 1" with that little pump, but I'm not doing the math either. Seems to me that pump should push around 3.5-4 on that loop. A larger pump like a 26-99 or even the large Badger or Liquidus if you want a cheap one, should bring you back up to the 7 range and keep your return temps up. As was mentioned above, the 20 degree design temp is kind of a wive's tale. As long as return temps stay above 140, there is no problem. However, there is some merit in that it takes less wood to keep 200 gallons of water at 160 vs 180. So if you can keep your supply temps low to save on a little wood, a tight delta T is necessary to keep return temps above condensation. If you need to run 200 degree water, you can have a 55 degree delta and the boiler won't care. It's all about keeping return temps up
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E Yoder

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Re: GPM?
« Reply #3 on: December 12, 2017, 02:03:15 PM »

I agree ^^^, if the building is being heated then the BTU load is the same regardless of flow. Upping flow might allow the heating system in the building to run less, but it'll shut off quicker. Think of the flow and return water temp as a seesaw, one goes up the other goes down as long as there is enough flow that the heat transfers over in the building.
Checking return water temps would tell you a lot.
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hondaracer2oo4

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Re: GPM?
« Reply #4 on: December 12, 2017, 04:38:38 PM »

Darin don't be spreading the truth like that! When I told that guy what you just said he told me he has been heating with an owb since I was in diapers and promptly blocked me! :o
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donniet79

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Re: GPM?
« Reply #5 on: December 13, 2017, 04:17:56 AM »

Thank you guys for clarification on that, I appreciate it. I will get some supply and return temps and go from there. It may be the weekend before I check them, but I'll post them on here when I do. Thanks again guys
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hondaracer2oo4

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Re: GPM?
« Reply #6 on: December 13, 2017, 07:07:01 AM »

What means do you have to measure the supply and return temps?
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wreckit87

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Re: GPM?
« Reply #7 on: December 13, 2017, 09:08:55 AM »

If you are going to be using an infrared thermometer gun, be sure to wrap a single wrap of black electrical tape around whatever pipe you are measuring (ideally steel or copper) and hold the gun physically against the tape when measuring. PEX tends to give a lower reading than a metallic pipe would, but if that's your only option, tape it as well
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donniet79

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Re: GPM?
« Reply #8 on: December 13, 2017, 09:27:48 AM »

I have both infrared and k-type thermocouples. I don’t really care for the IR myself, I prefer the thermocouples so that’s what I’ll use.
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donniet79

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Re: GPM?
« Reply #9 on: December 13, 2017, 06:51:59 PM »

Did a quick check on the supply and return right at my stove with my house calling for heat and had 176 on supply and 158 on return. I wasn’t running the hot water though and my furnace is variable speed so it only runs at 675 cfm. I’ll do another check this weekend with furnace call and running hot water.
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E Yoder

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Re: GPM?
« Reply #10 on: December 13, 2017, 08:06:46 PM »

The MF5e doesn't require high flow rates and you don't have unusually cool return temps. I think you're very normal and working fine.
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donniet79

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Re: GPM?
« Reply #11 on: December 14, 2017, 10:31:29 AM »

Great! Thanks for all of your input gentlemen, I appreciate it.
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Re: GPM?
« Reply #12 on: December 14, 2017, 01:34:22 PM »

 :thumbup:

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