Thursday, June 18, 2009

Properly Sizing PEX Pipe for Remote Boiler Connection When Long Runs will be Necessary

Tarm USA recommends installing solid fuel boilers in the basements or utility areas of homes whenever possible. Heat losses from boilers located within the home are reduced. Furthermore, heat escaping the boiler will diffuse into the home rather than to the outdoors.
Recently, we have seen a trend toward locating boilers in locations remote from the buildings they will be heating. Outdoor water stoves, which demand remote locations due to their tendency to smoke profusely, have popularized the use of PEX tubing buried underground between the heat source and the heat load. While the use of PEX tubing for remote, underground applications has become commonplace, information about properly sizing the tubing to adequately meet heating demands is not readily available. We are concerned that many customers are burying tubing, at a sizeable expense that may be too small to carry an adequate volume of hot water to properly meet their heating needs. Undersized tubing can often be compensated for by very large circulators, but this can lead to piping noise, high electric bills, and premature erosion of plumbing components.
Anecdotal evidence suggests that most of the underground PEX tubing that is now being installed, whether pre-insulated or not, is 1” in diameter. It is not clear whether cost, availability, or marketing is more influential, but 1” underground PEX tubing has become “the pipe to use” despite its limitations. Tarm USA wants its customers to understand these limitations prior to installing the wrong size tubing.
In order to avoid complications due to undersized tubing when installing a Tarm USA wood burning boiler remotely, generally the following tubing sizes should be used between the boiler and the heat source:
CategoryTarm USA Boiler ModelsOne way Pipe LengthUse “X” Diameter Tubing
1Solo Plus 30/Excel 2000/Solo Innova 30/ FrÖling 20/30<50’1”
1Solo Plus 30/Excel 2000/Solo Innova 30/ FrÖling 20/30>50’/<100’1”
1Solo Plus 30/Excel 2000/Solo Innova 30/ FrÖling 20/30>100’/<150’1¼”
1Solo Plus 30/Excel 2000/Solo Innova 30/ FrÖling 20/30>150’/<200’1½”
1Solo Plus 30/Excel 2000/Solo Innova 30/ FrÖling 20/30>200’/<250’1½”
2Solo Plus 40/Excel 2200/FrÖling 40<50’1¼”
2Solo Plus 40/Excel 2200/FrÖling 40>50’/<100’1½”
2Solo Plus 40/Excel 2200/FrÖling 40>100’/<150’1½”
2Solo Plus 40/Excel 2200/FrÖling 40>150’AVOID
3Solo Innova 50/ FrÖling 50<50’1¼”
3Solo Innova 50/ FrÖling 50>50’/<100’1½”
3Solo Innova 50/ FrÖling 50>100’AVOID
4Solo Plus 60<50’1¼”
4Solo Plus 60>50’/<100’1½”
4Solo Plus 60>100’AVOID

These suggested tubing sizes are based on a 20 degree temperature drop using a 50/50 water/glycol blend. Flow rates are assumed to be 10 gpm for the category 1 boilers, 14 gpm for the category 2 boilers, 18 gpm for the category 3 boilers, and 20 gpm for the category 4 boilers (See the formula at the end of this document to see how these flow rates are derived). In all cases these recommendations enable the use of readily available and reasonably sized circulators such as the Grundfos 15-58 Superbrute (3 speed) or the Taco 0014 by keeping head losses below 20 feet through the tubing itself. Of course there are other sources for head loss such as flat plate heat exchangers, fittings, and/or fan coils. These other sources for head loss are not inconsequential and should always be considered when calculating head loss.
By increasing temperature drop to 40 degrees, flow rates can be cut in half, which reduces head loss by a power of 1.75. So if flow is cut from 20 gpm to 10 gpm head loss is found by multiplying the head loss at 20gpm X .5 1.75. Some applications will be acceptable with a 40 degree drop, others such as fan coils may not. When a larger temperature drop is not possible, it is possible to increase flow rates through undersized tubing to improve heat transfer, however, as flow rates increase past 20 gallons/ minute head loss begins to increase dramatically. Keep in mind that if a person is planning to connect a remote wood boiler to a storage tank in another location, high head losses may be encountered. As head losses increase, flows will decrease. When the temperature differential between the boiler and tank is high the tank will still absorb all of the heat the boiler can make. However, as boiler and tank temperatures get with 15 degrees of one another for instance, it may be impossible to add any more heat to the tank. Consider the following example:
A Solo Plus 60 is installed in a shed 100’ from a home and a 20 degree temperature drop is required. A 50/50 glycol/water mix will be used. A 20 gpm flow is required. One inch PEX is the tubing. Head loss is 47.94’ through the tubing alone. Two Grundfos UP 43-70 F circulators will almost provide the necessary performance, 44’ of head @ 20 gpm (by placing identical circulators in series we can double the head produced at each flow rate). There is simply not a good “single circulator” solution for this amount of head loss as circulators that can handle these head loss conditions provide enormously high flow rates. For demonstration purposes only, these circulators draw 6.8 amps together (800Watts). Assuming that electricity costs $0.10 per Kw/hr. and that the circulators will operate 3000 hours/year, total electrical costs not adding for inflation will be $4,800.00 over a 20 yr period.
If we took the same example, but now used 1 ½” tubing, we could use two Grundfos UP 26-64F circulators. These circulators draw a total of 3.4 amps (370 Watts). Electrical costs over 20 years with this arrangement would be $2,220.00. The larger pipe in this example saves only $2,580.00 over 20 years.
Below you will find similar electricity cost scenarios, but in table format. The following examples involve a 50/50 water/glycol mix and 20 degree temperature drop:
FlowPipe LengthPipe DiameterHead LossCirculator20Year Power $
14100’1”36.4UP 26-99F x 2$2,940.00
14100’1¼”10.36UP 26-64F$1,110.00
14100’1½”4.68UPS 15-58FC$522.00
10100’1”14.5UP 26-64F$1,110.00
10100’1¼”4.34UPS 15-42F$390.00
10100’1½”1.96UPS 15-42F$390.00

Flow (Gal/Min)TemperatureWater MixPipe Dia.Pipe LengthHead Loss

Above data courtesy of Wirsbo, Inc. Pressure loss for hePEX and AQUAPEX were converted to head loss by multiplying by 2.37.
H= (144ΔP)/D
H= head added or lost from the liquid
ΔP= pressure change
D= density of the liquid at its current temperature (approximately 60.75lbs/ft3 @180deg.)
In piping, as flow rates increase, head loss increases by a factor of 1.75. This means that if we double flow rates, head loss increases by 3.36 times (H X 2 1.75). If we triple flow rates, head loss increases by 6.84 times (HX 3 1.75). Using the Solo Plus 60 example from above: We have a Solo Plus 60 installed in a shed 100’ from a home. A 20 degree temperature drop is necessary. A 50/50 water/glycol mix will be used to prevent freezing. One inch PEX will be used between the buildings. Because we have a supply and return we are faced with 200’ of tubing overall. Head loss for 1” pipe at 10gpm is 14.64. If we double the flow rate to 20 gpm head loss becomes 49.19. IT IS NOT A LINEAR RELATIONSHIP!
Flow requirements can be calculated with a simple formula:
f= flow
q= rate of heat output in BTUs
k= a constant based on the concentration of antifreeze
ΔT= temperature drop of the loop in degrees F.
k factors to be used in the above equation:
100% water = 500
70% water, 30% propylene glycol = 477
60% water, 40% propylene glycol = 465
50% water, 50% propylene glycol = 449
These k factors courtesy of Ipex “Manual of Modern Hydronics 3rd Edition”


pex tubing said...

Read with interest ... did not know that in Germany PEX tubing enjoyed the same popularity as in US

Sacramento Heating And Air said...

I didn't know that the tubing was as popular in Germany as it was here either. Fascinating!