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pipe ID and flow/BTUs (30 Posts)
pipe ID and flow/BTUsI have a 2550 square foot farmhouse. The house came with an outdoor wood furnace that is rated at 225,000 BTU/hour maximum output. 180 degree water pumps from the woodburner through ¾ inch pex tubing into the basement over a 65 foot run. Once the water enters the basement, it flows through another 30 foot run of ¾” pex tubing before entering a SuperStor Ultra indirect fired hot water heater. Water from the woodburner runs through the heat exchanger in the Superstor, thereby heating the water in the Superstor which then heats the house via baseboard heat. As a side note, the water from the Superstor actually flows through a gas fired boiler first before heading upstairs, with the boiler acting as a backup for the house if the wood furnace can’t keep up with demand. However, for our purposes let’s ignore the gas boiler because my goal is for the wood furnace to meet the full demand of the house.
There are a few problems with this set-up, which I inherited from the previous homeowner. First of all, the underground pex tubes are leaching a ridiculous amount of heat into the ground, to the tune of 15 degrees over the course of the run into the house. As a result, I am in the middle of installing insulated pex lines, either Thermopex or Logstor Pex-Flex, on the underground run to the house. Based on the manufacturers rating I am expecting heat loss after install to be 1 or 2 degrees total – a dramatic improvement. However, after spending two days digging, replacing one broken septic pipe, one underground gutter pipe, and tunneling under a sidewalk, I want to make sure I do this absolutely right the first time. Here’s my dilemma:
Thermopex uses ¾” ID pex tubing. Pex-Flex uses a true 1” ID. By all accounts, Pex-Flex is a slightly better product, even though documented Thermopex failures are extremely rare. A buddy of mine is a heating/cooling professional and will be helping me make my final connections in the basement, but he won’t be free until September. Meanwhile, I have an open trench and a wife who wants her gardens back, so I need to make a decision between the two products this week and get the pex buried in the ground. The big question is, is it worth the extra 3 dollars a foot for Pex-Flex, and the extra hour drive to the dealer, to increase the size of my pex from ¾” to 1”? I’m tempted to go with the larger Pex-Flex so I can get more BTUs into the house quicker, but if I do so, then I will need to replace the two 30 foot runs in the basement with either copper pipe or a true 1” non-insulated pex, and I don’t know pricing on these materials. I also don’t know how much more BTU’s the larger ID will give me. Predictably, the Thermopex dealer says the 1/4" difference in ID isn't a big deal and the Pex-Flex dealer uses the larger ID as a selling point. To throw another wild card in the mix, I’m fairly certain the SuperStor setup isn’t ideal: currently, the heat exchanger in the SuperStor can’t keep up with demand, so we may end up going with another type of heat exchanger once my buddy takes a look at everything and tweaks the system. (He mentioned a steel plate heat exchanger and the outdoor wood burner manufacturer included info on “Flatplate” company products in the owner’s manual). Then again, once I’m running 180 degree water into the house with the insulated pex instead of 165, maybe the SuperStor setup will be sufficient.
I’d appreciate any advice on this. I just don’t have the knowledge to run the formulas to see what I really need in terms of volume/flow/btu’s. A few more pieces of information that might help:
· Although we blew in insulation, the house is not very efficient in terms of heat loss (old drafty farm house).
· The wood burner is a done deal – too much money into refabbing it last year so it stays as the primary source of heat regardless.
· Right now I’m using Taco pump model 007-F5, 1/25th HP .71 amp 3250 RPM (I don’t know what the gpm’s are with this pump)
· The woodburner also heats a radiant floor setup in my 4 car garage, so I’m close to the maximum square footage this furnace is rated for (3800 square foot total house and garage, I think my model is rated for 4k or 4500)
· As of right now I’m leaning heavily towards buying the Pex-Flex and having the dealer install fittings for 1” copper onto it (Pex-Flex takes special fittings), I figure that way in the short term we can just use an additional fitting to temporarily step back down to ¾” for the basement run or later on go with 1”. That is, unless I have 20 guys on here telling me I’m crazy because there’s no way I’ll ever need the extra ¼” and ¾” will be sufficient in all applications.
Start with the mathAssuming you are transferring the 225,000 BTU/hr,
A 20F ∆T will require 22.5 GPM, which would be ~20 FPS in 3/4" PEX and ~12.2 FPS in 1" PEX
A 30F ∆T will require 15 GPM, which would be 13.6 FPS in 3/4" PEX and 8.25 FPS in 1" PEX
A 40F ∆T will require 11.25 GPM, which would be 10.2 FPS in 3/4" PEX and ~6.25 FPS in 1" PEX
NONE of these conditions are optimal and only one or two are tolerable. All but the 20F will require mixing on the distribution side, but all would benefit from it, as well as some storage capacity.
The short answer is that if you want to move that many BTUs, you want 1-1/4" or 1-1/2" pipe. What you really need is a proper system design.
Here's some reading homework http://www.caleffi.us/en_US/caleffi/Details/Magazines/pdf/idronics_10_us.pdf
By far, the biggest mistakewhen installing outdoor boiler (furnaces) is under-sizing, sometimes grossly under-sizing the piping runs. Next mistake s grossly over-sizing the pumps because the tube was undersized.
I would shop for a quality insulated piping material properly sized for your application. Putting the wrong tube in the ground is not an easy, or inexpensive mistake to fix.
I know Watts Radiant and several others offer 1-1/4 and larger insulated lines. Or build your own with some 6" PVC pipe as the condiut and spray foam it in the trench.
Add some PVC electrical pipe in the trench also for wiring and sensor runs.
Indeed...You want the bigger pipe. That Watts inch and a quarter looks good. Consider that your pumping loss is inversely related to the square of the size of the pip; that is a one inch will have half the loss of a three quarter, and an inch and a quarter about a third of the loss of the three quarter. So you would need a much smaller pump with the larger pipe; electricity, last I looked, isn't free.
I also completely second hot rod's comment on conduit. Even if you are using insulated pipe -- which I would anyway. 6 inch PVC isn't all that expensive, and you can run your power and signal lines through it as well as the pipes (or, as he suggests, run a separate conduit in the trench for power and signal -- but if you do, make darn sure it's big enough for whatever you might want) and in either case, include your present wires -- and a messenger so that if you want to add another cable later for something, you can do it without trying to push it through. Or digging the whole thing up again.Jamie
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
WoodThe key with wood boilers is storage. They have no real ability to modulate so you have to be able to store the energy until you are ready for it. How much mass does the boiler have? How big is your superstore? If anything you want to add more mass, I would not eliminate the superstore without adding mass. I think it is unlikely the superstore is the problem.
As for you original question, go big!
BTU/Hr=gpm x delta t x 500.
It is hard to tell exactly what your gpm is right now because you are up on the top of the pump curve. I think it is likely 3 gpm +/-.
If you have a delta t of 20 that puts you at 30,000 btu presently. This is a serious SWAG, but gives you an idea.
If you went with an 1 1/4" pex (1" ID) with the correct circ, you would push about 11 GPM.That would give you 120,000. 1 1/2" at 15.6 GPM would be 156,000.
The wood boiler folks in their lust for sales don't share this stuff with you. It is the real math and is unavoidable.
ThanksThanks for the replies. It's clear that I need to go with the Logstor product, minimally the 1" ID, 1 1/4" ID even better. Thanks for the link SWEI, that's very good material. If you can't tell I want to do this job right, and sometimes that means learning on a steep curve. I'm in the middle of reading it now and plan on printing it along with responses to this post and passing it on to my friend before we tweak the system.
Can you clarify by what you meant by mixing on the distribution side? I interpret that as "cooling down" the output by mixing it with return flow. Why would that be necessary? We actually installed a mixing valve in the garage last year so we could run the wood burner higher than 130 degrees without damaging the concrete in the garage. Believe it or not, the previous owner who installed the system was running 130 degree water into the house but had the gas boiler thermostat set at 212 - in other words, the gas boiler was heating the water in the wood burner!
I was already planning on running conduit, not sure what a messenger is but I assume it's a wire that can be used to fish other wires through at a later point (again my inexperience shines through). As for mass, the Superstor is 60 gallons, the woodburner holds 290 gallons.
One concern I have if I keep the Superstor tank in the system but increase btus to the house, is how hot I can heat the water in the Superstor. The past two seasons water temp in the Superstor maxed at 150 degrees (that's as hot as I could get it, and only in between cycles would it reach this high). With more gpm, more btus, and less heat loss to the ground, how high can I heat the Superstor? 180 degrees? Seems to me I remember trying to contact the manufacturer in the past with no luck to get an answer to that question. I do know the warranty states that heating water above 150 qualifies as commercial status as opposed to residential and compromises the warranty.
ControlYou have a reasonable amount of mass. Not great but workable. You can always use add some storage tanks later. You are not going to hurt the superstore with high temps.
How are you going to control it? The backup will heat the wood boiler when the wood fire goes out unless you do something.If you use wood 99% of the time it is not a big deal. I wouldn't elect to put the backup boiler in series as you have it as it just gives up heat and makes control more difficult.
consider solarfor your summertime DHW loads. A few panels and a solar tank with a 4500W top element would probably give you plenty of DHW in the summer.
1" / gpm?As much as I'd like to go with larger pipe, the cost of bumping up to 1 1/4" is prohibitive (almost $20/foot). 1" ID Pex-Flex (actual ID is 32 mm, slightly over 1") is more in my price range, and will still be a significant improvement over the 3/4" ID pex in the system now. Keep in mind I don't have to move all the BTUs at my disposal since the wood furnace also heats the garage on a separate pump / loop. So...
The PexFlex distributor recommends a Taco 0014 pump for my application, with 32 mm ID pex. Not sure if it matters but each way there is about 125 feet of line from the furnace to the heat exchanger. How would I calculate my gpm with this pump? And once I establish what my ∆T is with the new setup, how do I calculate the BTUs being transferred to the house? Thanks again.
Universal Hydronic Formulagpm = btu/hr / (delta-t x 500) The 500 is constant unless you have a glycol mix. Once you calculate your flow rate you then need to find the pressure drop chart for that PEX and figure out head of foot. From there find a pump curve chart on Taco's web site and find your pump.."The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."This post was edited by an admin on August 14, 2013 6:52 PM.
TranslationIf you can double the ∆T, you can carry twice as many BTUs in the same pipe.
Doing so requires a more in depth analysis of the system, but that may well be a worthwhile investment right now.
Taco 0014is a pretty steep curve pump . Does not sound to me that once you increase the tubing size that you will have such high head . But in the instance that you need to move the entire 225,000 the 0014 will not do it at more than 8" of head with a Delta T of 20 , a higher Delta could work . Where I am going with this is that for the same almost exact money you could purchase and install a 0013 VDT and keep your system at whatever Delta you choose and the ability to pump more volume at higher head pressures .You didn't get what you didn't pay for and it will never be what you thought it would
I agree..."Doing so requires a more in depth analysis of the system, but that may well be a worthwhile investment right now."
I agree 100% - if I only knew who to call in my area I'd gladly pay to have an expert come out and tell me exactly where I stand and what I need. What level of expertise is required for that kind of analysis? Should any decent home heating professional in the yellow pages have that kind of knowledge? Last guy who came out to service my gas boiler didn't want anything to do with figuring out the wood stove. How would I go about searching for a hydronics expert in my area?
Also, I will never need to move all 225000 btus on that one pump. A separate pump moves water to a radiant floor system in the garage from the same furnace and keeps up nicely - garage is nice and cozy. I'm guessing from what I've read that the house itself requires maybe half that.
That's WhyYou want a buffer tank..A place to park the btu's being made by the wood boiler. Where are you located?
By the way, 1" Pex with 100 percent water at 180 degrees moving 10gpm, 150 feet, has a head loss of 17.2ft at a velocity of 5.5 ft per second.. Still want to use 1" pex?
1-1/4" Pex with 100 percent water at 180 degrees moving 15gpm, 150 feet has a head loss of 13' at the same 5.5ft per second velocity.
1-1/2" Pex with 100 percent water at 180 degrees moving 20gpm, 150 feet has a head loss of 10' at the same velocity as above.."The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."This post was edited by an admin on August 14, 2013 10:18 PM.
Rochester NY...and doesn't the Superstor tank count as a "buffer"? Water from the wood burner heats water in the Superstor, thermostat kicks on, circulator pulls water from the Superstor to heat the baseboards.
NoIts acting like a heat exchanger. What size is it? Here's the question of the day...Have you done a heat loss of the house? I haven't read in any of your posts as to what the heat loss is..I'm a betting man,, 3800 sqft of heating space let's see,,, I'd say around 70,000 or so.."The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."This post was edited by an admin on August 14, 2013 10:03 PM.
...60 gallon. The wood burner holds 290. What about running the wood furnace to a plate exchanger then into the Supersor, which holds the water until called for - would that be considered a "buffer"? What is the advantage?
Buffer tankChris , I believe that since the 2 pumps never see each other that the Superstor could technically be classified as a buffer tank . It does decouple primary from secondary in a sense . 4 holes does not a hydraulic separator make . An HX will do .You didn't get what you didn't pay for and it will never be what you thought it would
True But Not Big EnoughHe can't move the required flow across it's HX. Pressure drop across that coil is 8'. Add the numbers I posted above to that and he needs one big pump.. His biggest problem is his pipe size choice. If he wants to stick with 1" he needs a tank much bigger then 60 gallons.
By the way...you guys are scaring me. I called the PexFlex distributor today and ordered 90 feet of 32mm/1" ID dual pex for pick up 3 hours from here on Friday. It's probably already cut and ready for me. The wife had already expressed her concern at the $1200 price tag and the 1 1/4" costs 6 bucks a foot more. Without someone on the ground here to advise otherwise, I estimated that the 1" ID could get enough BTUs to the house to be worth the investment. Past winters, the wood furnace could hold the house at temp as long as outside temps stayed 27 or 28 degrees or higher with calm wind. Really cold days, lots of wind, the gas boiler did half the work. I figured insulated pex without the heat loss plus bumping from 3/4 to 1" ID, based on Zman's numbers, even if they weren't right on, I would at least be doubling the btus I'm currently getting.
It's possible that the PexFlex hasn't been cut off the reel yet and if I call first thing in the morning MAYBE I can still spring for the 1 1/4" ID...if absolutely necessary (?). You guys impress the heck out of me with your knowledge and I really appreciate the input, just pulling out my hair not having the professional background to make the right decision.
So, not understanding the head loss numbers, MY question of the day is: is the extra $540 and an angry wife worth the 1 1/4"?This post was edited by an admin on August 14, 2013 10:23 PM.
Question ishow angry she may be if we should experience an extended cold snap ? Does she like the COOLLLDDD ?You didn't get what you didn't pay for and it will never be what you thought it would
Actually My Head Loss Calc's Are WrongI didn't see that extra 60' of 3/4" pex in the basement. 65 to the house then 30 to the Indirect is 95' and there has to be a return so that's another 95' for a total run of 190'.. I added in the 8' of head for the indirect pressure drop..
1" Pex 10gpm @ 30' of head
1-1/4" Pex 15gpm @ 25' of head
1-1/2" Pex 20gpm @ 21' of head
I don't think his existing pipe is leaching btu/hr, it's that his existing pipe size and pump are so small he can't move water..This post was edited by an admin on August 14, 2013 10:39 PM.
Sorry...but as long as you're punching numbers, the final measurement for the new line is 90 foot (65 foot was a bad estimate with a tape before I dug the trench), plus add that to the 30 feet inside, for a total of 120 feet each way.
The Superstor doesn't carry any of the BTUs for the garage, just the house.
You're right, we have a lot of heat loss in the house and garage. My buddy estimated I needed 130,000 btu's/hour on our coldest days to heat the house alone.
An extra 1/4" worth it? Sounds like it might. For example, that's extra electricity spent running a bigger pump - electricity isn't free either.
1-1/4" Pex Is The Way To GoI'll just use 10gpm based off the specs on the indirect. I'm assuming its an SSU60 (Stainless Steel). The head has the indirect pressure drop in it.
1" Pex = 10gpm @ 35' of head
1-1/4" Pex = 10gpm @ 19' of head
1-1/2' Pex = 10gpm @ 13' of head
In the end your going to still need that boiler as back up.
10 GPM@ 19 Ft Hd is easily achieved with the 0013 VDT also . The 1 1/4 is the ticket . Would not worry too much about the electricity . The taco 00 line is gonna run right around 80 Watts no matter the size , difference comes in the impeller type and HP . Shame you are at where you are with this already , Uponor makes an 1 1/4 Ecoflex thermal twin with S&R in one jacket that I can get for about 15.00 per foot I believe , but then there is a 150.00 custom coil charge also so the money would be the same outlay .
Kinda sux that physics don't care about budgets doesn't it ?You didn't get what you didn't pay for and it will never be what you thought it wouldThis post was edited by an admin on August 14, 2013 11:27 PM.
UpdateThanks for all the comments posted. I will be going with the 1 1/4". I actually got hold of a friend of mine who is, quite literally, a rocket scientist. Turns out that fluid mechanics - and hence hydronics - is right up his alley. He ran the numbers on my system and confirmed that 1 1/4" is a better choice. His numbers weren't quite the same, but then again he had the benefit of gathering some additional information from me. Bottom line, very few circulator choices would get me even remotely close to the gpm I need with the 1" ID, but 1 1/4" opened up several options.
Next, I need to find 1 1/4" ID pex for the shorter run in the house. I don't need oxygen barrier nor insulation on the pex (I will wrap it myself), so I'm just looking for plain old 1 1/4" pex that is rated for 200 degrees. Will this be hard to find? Would anyone like to recommend manufacturers?This post was edited by an admin on August 19, 2013 10:19 PM.
Curious As ToWhy no O2 barrier? Is the wood boiler and all the components being used non ferrous? Here's the spec sheet for Uponor.
Pex AUphoner 1 1/2" would be a good choice. Aquapex is a bit cheaper than hepex if you don't need the barrier.Either way make sure it is Pex A.
Good to here the folks here aren't being put to shame by a rocket scientist.
Open systemNo O2 barrier because it's not a closed system - I can stare right down the top vent of the stove into the water. So it's not a true "boiler". Since Oxygen can already enter the system there, I'm assuming O2 barrier on the pex wouldn't make a whole lot of difference?
why not leave the 3/4 in service with the 1"presumably the 3/4" is some kind of reasonably insulated install already. even thought it does not have the sole capacity you might as well keep using it.
so don't abandon it. parallell manifold the two. that gives you a cross section almost precisely equivalent to 1 and 1/4.
I don't know the physics of the flow interaction with the pipe wall and the theoretical btu's available might be less than 1 and 1/4 inch, but it would be more than 1.
the only installation issue I can see is whether you know where the existing line is so you can dig a trench next to it without hitting it.