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Author Topic: Determining heat load and sizing of pex??  (Read 2465 times)

Jd79

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Determining heat load and sizing of pex??
« on: February 11, 2014, 11:28:30 AM »

I'm still in the planning phases of my build (mostly due to weather not cooperating with my off days from work!) but I am having trouble figuring out how to calculate my heat load.  I've seen rough estimates of 40 btu per sq foot for my area of the country (western KY), but I have no idea what I need for the domestic hot water.  I know the BTU ratings of the heat exchangers are the max output of that particular exchanger per hour, but how do I know what I need exactly?  My house is 1800 sq feet, fairly well insulated (built in 2004) and I plan to run the plate heat exchanger for domestic water heater in series first, then to the heat exchanger in the ductwork, then return to boiler.  The water heater is 82 gallons.  I was thinking about using 1” pex for the whole loop (roughly 130 feet each way with a TACO 0015 3 speed), until I saw that 1” pex is only good for about 7.5 GPM, which would likely be about 75,000 BTU per hour, if what I read was accurate.  I’m not sure that would be enough.  Should I opt for 1” pex-al-pex or Logstor to get higher GPM or am I over-estimating what I will actually be seeing for heat load?  Some help or direction with accurately calculating my heat load would be really appreciated.  Thanks in advance.
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LittleJohn

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Re: Determining heat load and sizing of pex??
« Reply #1 on: February 11, 2014, 11:42:07 AM »

If you do plan on running DHW and fan coil in series, definately use a Thermostat mix valve on potable side of hot water heater.  Cause I think that temps requried for fan coils are in the range of 160-180; where most DHW likes a top end temp of 120 to prevent scalding.

If you run in parallel, you could zone each applicance and potentially reduce the head pressure or GPM required to operate both in series. 
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Sprinter

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Re: Determining heat load and sizing of pex??
« Reply #2 on: February 21, 2014, 08:09:47 AM »

Use 1.25"ID minimum. This will allow you to use much smaller circulators and use less electricity.  1.25" pexalpex is equal to 1.5" regular pex.

Taco website has plenty of video and design sizing info. Head loss climbs exponentially as flow exceeds 4 feet per second, which equals expensive pumps, lots more electricity, impeller wear, less heat control, lots more issues. If not now , down the road. If sized correctly, you can run half a dozen 007's or 15-58's versus one large pump like a nrf36 for example. 80kbtu or larger is good for water heater for quick recovery. Look at indirect water heater specs for examples
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