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Author Topic: Primary loop PEX size  (Read 4879 times)

Flyboy206

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Primary loop PEX size
« on: December 16, 2017, 11:04:16 AM »

Been lurking and reading on here for a year or so. Finally getting ready for the install.
Have a few questions, and this board is a wealth of knowledge.

I am setting up in slab floor heat, 3 zones. Two for the house and one in the attached shop. Have a mixing valve, relay, and each zone has a dedicated pump.

Total BTU required when all zones call for heat is calculated at 94,732 BTU. Will be using a G200 for heat.

My question is sizing the primary loop and max flow rate. Total run is about 214 ft.  My numbers tell me I need 9 to 10GPM in the primary to keep a 20 degree temp diff on the return when all the zones are calling for heat. Once the floor is up to temp, will the 1" Thermopex work to keep the temps stable?

Thanks,
Terry
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Flyboy206

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Re: Primary loop PEX size
« Reply #1 on: December 17, 2017, 09:47:10 AM »

wreckit,

Appreciate the reply. I had already posted it this, but am on moderator probation. LOL

I would like to separate the systems with a heat exchanger so the floor is a closed system. I have a 1740 sq. ft. house and a 2480 sq. fit connected shop I am trying to heat. Eventually I will add DHW, but for now, just want to get the floor heat up and running

Looks like I should use 1 1/4 for the long run. Taco chart shows a max flow rate of 11.2 GPM for 1 1/4.

Suggestions on what size heat exchanger to use that won't restrict the flow rate? If all zones are calling for heat, flow rate will be around 15 GPM on the radiant side with 120* water.

Have 2 zones in the house, 1/2 inch, 3 loops each. 1 zone in the shop, 7/8 inch, 7 loops. All loops are 300ft.

What am I missing or need to do different?
« Last Edit: December 17, 2017, 12:12:16 PM by Flyboy206 »
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wreckit87

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Re: Primary loop PEX size
« Reply #2 on: December 17, 2017, 10:22:53 AM »

I would look into 1-1/4" to carry that load. Rehau makes a nice foamed 1-1/4" similar to Thermopex with a true 1" ID, and Logstor 32mm is also 1" true ID (I've found Rehau to be more cost effective). Thermopex also makes 1-1/4" but for some reason they are a true 1-1/4" ID which is great, but possibly overkill for your application depending on system setup. I personally use the Rehau on my installs, unless the customer provides something else or I need the extra 1/4" of Thermopex. 1" Thermopex (or any other 1" PEX) maxes out at about 7 GPM with a reasonably sized pump. A guy can squeeze 8 or 9 out of it with a monstrous pump like a PL-55, but the cost of that pump and electricity to run it ends up costing more than just buying bigger line.

As for the radiant, would you mind explaining your setup a little bit? 3 zones with 95k BTU heat load says you've got a pretty substantial floor plan. You mention a single mixing valve, most mixers max out at 1" CTS which is going to be pushing the limits of your system. Will this be all tied into the boiler's primary loop, or will you be separating it with a plate exchanger and pressurizing the radiant side? Anything else being served (i.e. domestic water or forced air coil) from the boiler loop or only radiant? The square footage of the floor and number and length of the loops in each zone would be very helpful information in this case
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Flyboy206

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Re: Primary loop PEX size
« Reply #3 on: December 17, 2017, 05:08:22 PM »

My thought was to install the mixing valve on the supply line from the plate exchanger, prior to the pumps. Whenever a zone calls for heat it will get tempered water. The primary from the boiler will be continuous circulation through the plate exchanger back to the boiler.
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E Yoder

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Re: Primary loop PEX size
« Reply #4 on: December 18, 2017, 05:13:45 AM »

Yeah,pulling it all through one mixing valve will create a fair amount of restriction. I'd probably do one for each pump, or at least one for the house, one for the shop.
Is the 7/8" you mentioned 3/4" Pex (7/8" ID)?
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Flyboy206

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Re: Primary loop PEX size
« Reply #5 on: December 18, 2017, 06:58:49 AM »

Should be 3/4 ID, I'll have to check for sure. I can install one mixing valve for each zone if that will help with the flow. Any recommendation on what size heat exchanger?
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wreckit87

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Re: Primary loop PEX size
« Reply #6 on: December 18, 2017, 09:07:53 AM »

Keep in mind that you'll only need about 11 GPM to satisfy that floor, and a certain percentage of that water will be recycled through the mixing valve(s) to get it down to desired temp. 120 is typically a little high. If you have 140 coming off the plate at 7 GPM will be more than enough to serve 110 to the equivalent of 15 loops of 1/2" as your return temps will likely be in the 80 degree neighborhood, so pretty much any 1" plate will flow 7 GPM. As for the heat load however, if it were my job I'd use a 60 plate, 5"x12" HX with 1-1/4" ports and build the main loop from 1-1/4" copper with three 1" branches for the pumps. I recently did one similar to this in a big shop, also 3 zones but over 9,000 sq ft. I'll attach a picture once I get on my phone, but I used 3 individual mixing valves to help dial in the temps and help flow. I can't say I've ever seen 7/8" PEX, but 1" is 7/8" ID and 3/4" is 7/8" OD, what is it that you have exactly?
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wreckit87

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Re: Primary loop PEX size
« Reply #7 on: December 18, 2017, 09:11:48 AM »

Third mixing valve is hidden behind the expansion tank, but this should give an idea
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Flyboy206

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Re: Primary loop PEX size
« Reply #8 on: December 18, 2017, 09:32:51 AM »

It's 7/8 OD pex (orange) 3/4 ID, 7 loops, 1 zone in the shop. House is 5/8 OD, 1/2 ID, 6 loops, 2 zones.

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wreckit87

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Re: Primary loop PEX size
« Reply #9 on: December 18, 2017, 09:39:59 AM »

Any reason you chose those sizes? Seems odd for a residential application. Either way that doesn't change the BTU output, the aforementioned flow rates and plate size will still be accurate to transfer that amount of BTU regardless of pex size
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Flyboy206

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Re: Primary loop PEX size
« Reply #10 on: December 18, 2017, 10:27:18 AM »

Had one of the supply houses do the specs for the floor before I had done much research. So that is what I have in the slab to work with.
Initial design was for an open system. I decided I wanted to keep the boiler water out of my floor and use a heat exchanger, may add glycol in the future.
After all the advice, plan now is 1 1/4 pex from boiler to HE and return. 1 1/4 loop from HE, 1" branches to each zone, individual mixing valve and pump for each zone, expansion tank and air eliminator (in the main loop from HE?). Thermostat for each zone with a 3 zone controller.

Anything else I need to consider? This will be my first go around with an OWB. G200 coming Friday.

Appreciate all the help.
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wreckit87

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Re: Primary loop PEX size
« Reply #11 on: December 18, 2017, 11:00:37 AM »

Looks good to me. 1-1/4" air separator and a 4 gallon expansion tank you should be set to go! If you're not adding glycol right away, I might suggest a corrosion inhibitor in your water to keep the oxidation from building in the metallic components like pump impellers. A Y-strainer is never a bad idea either, nor is a purge valve to bleed air from all floor loops. This can usually be done without a purge valve, but they are a handy thing to have. A couple boiler drains and a ball valve will do the same
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Flyboy206

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Re: Primary loop PEX size
« Reply #12 on: December 18, 2017, 11:44:45 AM »

What about a check valve in the loop from the HE?
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wreckit87

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Re: Primary loop PEX size
« Reply #13 on: December 18, 2017, 12:01:23 PM »

I always use the check valves that come with the pumps, but if backfeeding is a concern you'll need a check in each zone. A single check in the main loop wouldn't gain you any ground the way I look at it. Oh and don't forget to counterflow your HX; boiler water one direction and floor water the other direction
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E Yoder

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Re: Primary loop PEX size
« Reply #14 on: December 18, 2017, 02:21:06 PM »

It's 7/8 OD pex (orange) 3/4 ID, 7 loops, 1 zone in the shop. House is 5/8 OD, 1/2 ID, 6 loops, 2 zones.


Could it possibly be 5/8" ID, 7/8" OD, with would be standard 3/4" Pex? Not a big deal, just sounded odd.
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