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Author Topic: Another BTU question  (Read 3041 times)

woodman

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Another BTU question
« on: January 05, 2018, 12:57:46 PM »

I seem to have a lot of hypothetical questions this year. I know target delta t for most hx's is 20, but why? Does a delta t of 20 amount to a given amount of btu's regardless of water temperature?  For example, with everything else being the same (blower cfm, system gpm) are there the same amount of btu's derived from 180-160 as there is from 140-120? If so, and you disregard return temperature and had a delta t of 60 degrees would that be the equivalent BTU's to 3 times the gpm of a delta t of 20? The reason I ask is I am trying to get more BTU's through my 1" Thermopex lines, without using a larger circulator. It seems you hit a wall at 7.5-8 GPM with 1" pex.

I have been watching my water temps with this cold snap, (low teens for highs, lows 0-10 below) and am able to HEAT my house with 110 water, and MAINTAIN house temps down to 90-100 degree water. With 160 water and a delta t of 20 the temp at my registers is 130. What would the temp be at the registers be if I slowed the water down and pulled water temp from 160-100 at the hx?     
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mlappin

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Re: Another BTU question
« Reply #1 on: January 05, 2018, 03:04:16 PM »

Unless you have very good mixing at your boiler, returning water under 140 can cause some serious issues.

I think we’ve all seen when first starting a boiler for the season it sweats a LOT until water temp is over 140, same effect if your returning cold water and that hits a spot on the firebox before it has a chance to mix.
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woodman

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Re: Another BTU question
« Reply #2 on: January 05, 2018, 03:30:33 PM »

Unless you have very good mixing at your boiler, returning water under 140 can cause some serious issues.

I think we’ve all seen when first starting a boiler for the season it sweats a LOT until water temp is over 140, same effect if your returning cold water and that hits a spot on the firebox before it has a chance to mix.

I understand. The water will be going back to storage. Once again this is to promote stratification and low flow. It's hard for me to explain everything I have going on. Mostly looking for a hypothetical answer. I'm trying to get my head wrapped around a concept.
« Last Edit: January 05, 2018, 03:33:55 PM by woodman »
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E Yoder

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Re: Another BTU question
« Reply #3 on: January 05, 2018, 07:14:11 PM »

I would say a big delta isn't an issue IF your emitter is something like a hot air coil where all the heat exchange happens at one place. Providing it's sized correctly of course, counterflowing really helps with low flow rates too.. Something like copper baseboard can't run a large delta because it heats unevenly as it makes the loop around the house.
A large delta t is a great way to reduce flow requirements, but there are obviously limitations. Your air temp will eventually drop enough it won't heat the house. Upsizing the coil can help with that.
The sweating on the boiler because of low return temps very much depends on mixing inside the boiler and if the cool water impacts the tank wall or disperses in an open area in the tank. Like you said in your case doesn't matter as you have no flue gasses.
I've read Jon Seigenthaler and Dan Holahan promoting a more nuanced approach to the whole delta t thing and push back against a standard 20℉, just depends on the system.
« Last Edit: January 05, 2018, 07:31:39 PM by E Yoder »
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E Yoder

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Re: Another BTU question
« Reply #4 on: January 05, 2018, 07:29:49 PM »

Pumping out of storage I'd look into something like a B&G Vario, can be turned way down for stratification, and you could easily bump it up during colder weather for hotter air temps.
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mlappin

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Re: Another BTU question
« Reply #5 on: January 05, 2018, 07:31:20 PM »

The way I look at it, if the system works with lets say a 30 degree delta T, thats fine and dandy if your running a  tight differential so your return temps are never much lower than 150 degrees F.

However, if your running a 30 Δ because you can’t get any more flow, now you are locked into what you have now, issues will arise in the future if you want to add on to the house, add a hot tub, heated sidewalk, add heat to a solarium, etc etc etc.

I try if possible to stick to a 20Δ using a Grundfos 15-58 when designing an install, if they can get enough flow with a 15-58 and stick to the desired 20Δ then the sky is the limit on adding to the system in the future simply by swapping the 15-58 out for a 26-99.
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E Yoder

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Re: Another BTU question
« Reply #6 on: January 05, 2018, 07:33:40 PM »

You're right. 90% of our jobs use a NRF-22, which gives some wiggle room.
« Last Edit: January 05, 2018, 07:35:35 PM by E Yoder »
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E Yoder

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Re: Another BTU question
« Reply #7 on: January 06, 2018, 02:15:20 PM »

I seem to have a lot of hypothetical questions this year. I know target delta t for most hx's is 20, but why? Does a delta t of 20 amount to a given amount of btu's regardless of water temperature?  For example, with everything else being the same (blower cfm, system gpm) are there the same amount of btu's derived from 180-160 as there is from 140-120? If so, and you disregard return temperature and had a delta t of 60 degrees would that be the equivalent BTU's to 3 times the gpm of a delta t of 20? The reason I ask is I am trying to get more BTU's through my 1" Thermopex lines, without using a larger circulator. It seems you hit a wall at 7.5-8 GPM with 1" pex.

I have been watching my water temps with this cold snap, (low teens for highs, lows 0-10 below) and am able to HEAT my house with 110 water, and MAINTAIN house temps down to 90-100 degree water. With 160 water and a delta t of 20 the temp at my registers is 130. What would the temp be at the registers be if I slowed the water down and pulled water temp from 160-100 at the hx?     
But to answer your questions-
BTU rate is GPM x Dt x 500. So the bigger the delta the more heat you are extracting. Never mind the water temp. However as you mentioned you can't heat the house below a certain temp water because the heat exchanger doesn't transfer heat nearly as well as the water temp drops. But if you can get it to heat the house pumping a really low rate off a storage tank should pose no issues that I can think of. You'll have to play with the flow rate and see what works for you, then take whatever delta you get. The larger the hot air coil the bigger delta you should be able to tolerate. I've known guys to double stack coils to get hotter air temps, counterflowing through both. You want the air to exit at the hottest end. Would need to watch airflow for AC if this system has it.
I don't mean to imply that no issues could crop up in other scenarios like mlappin mentioned tho.
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Sprinter

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Re: Another BTU question
« Reply #8 on: January 06, 2018, 04:39:39 PM »

a lot of good answers have been pointed out already.  The 20 degree DT is just a very basic design metric more than anything, and most metrics are based off of closed loop boiler designs. All designs are calculated to heat the structure evenly most efficiently. The temp design is derived from type of equipment and type of heat exchangers.
Boiler manufacturer design specs are what you should go by for DT and lowest operating return temp. Smaller DTs, regardless of temp, will have more even consistent burns. Versus big long burn times to recover more btus when using larger DTs.
In the end, is the house comfortable? Is the hot water there when you need it in the shower?
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woodman

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Re: Another BTU question
« Reply #9 on: January 06, 2018, 05:56:05 PM »

Yes, lots of good answers here and I appreciate all of them. If I can figure out how to get pictures up I will try to post some of what I have been working on. A Taco 0011 delta T circulator will probably be what I end up going with, but I have a new unused 15-58 3 speed that I am going to try for now just to see how low I can go. My lineset is 120 feet one way plus another 30 or so feet of indoor pex and 3 heat exchangers so I end up with around 20+ feet of head at 6.5 gpm with my current  B and G pl-30 on the back of my boiler.   
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mlappin

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Re: Another BTU question
« Reply #10 on: January 06, 2018, 06:01:00 PM »

Are the HX in series now? Primary/secondary loops here might work quite well in this case.
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woodman

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Re: Another BTU question
« Reply #11 on: January 06, 2018, 06:08:05 PM »

Are the HX in series now? Primary/secondary loops here might work quite well in this case.


Yes currently in a series. I am planning on Primary/secondary loops at a later date. I am slowly making changes to my system that I would have originally done 10 years ago if I would have known what I was doing then.
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