Using different piping sizes in forced hot water system

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Old 11-11-06, 07:05 AM
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Question Using different piping sizes in forced hot water system

Hi all,

First time user. Adding a basement bathroom which needs to be heated. Currently there's a forced hot water baseboard system in the surrounding living space. Plumber plans to splice in this room to the current zone loop which has 3/4 copper piping throughout. He plans on doing the splice with 1/2 inch Pex tubing (1/2 was all he had on hand and has done the rough already), so the system would flow from 3/4 down to 1/2 inch for the room and then back to 3/4 through the rest of the loop back to the boiler.
Is it a common or acceptable practice to do this step down-up or is it necessary to keep the 3/4 diameter throughout? Will this cause any issues with bleeding, gurgle, noise, etc? Thanks for any help you can give. There still is a chance to remove and replace the tubing at this point if necessary, but it would entail having to open up a panelled wall or two and the installer may not be too happy about making the change. But I need to know if it's not right. Thanks for any help you can give.

Sincerely,

Lou
 
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Old 11-11-06, 08:07 AM
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do it right

Lou,

There's the right way to do something and there's the cobbler.

What is the pex going to feed? Is it going to be in-floor heat or baseboard in the bathroom? What is up stream, baseboard?

I'm not an expert but I strongly believe in doing things right. If the rest of the living space is baseboard then the new addition should also be baseboard unless there's a good reason for it not to be.

Adding a 1/2" line between to 3/4" lines will reduce the flow and therefore potentially cause a bunch of problems, especially if the loop is long to begin with.

If you HAVE to do something like this then in my opinion it’s better to use a tee on the supply side and a monoflow tee on the return side of the loop going to the bathroom, just like many toe kick heat exchangers do. But again, I would never do this. If it where me and my house I’d pipe it the same way as the rest of the living area.

If this guy is any type of plumber at all he knows how to sweat copper and it sounds like he’s being lazy. Go with your gut and make him do it right (my humble opinion).

Oh, I just re-read the first part of your post that says the rest of the living space is baseboard. Then, as I said above, I'd do the bathroom in baseboard too.
 
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Old 11-11-06, 10:58 AM
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taylorjl, thanks for responding. The bathroom has been done in baseboard as well, yes. I believe the plan is to add the T's you mention and connect it into the existing piping. That's all that's left at this point in the project. I believe the Pex is being used for a few reasons. One is the ability to pipe this loop without major structural issues. The piping is going around/through some tough obstacles. Two, he says it's easier to use, etc. MY concern with the whole thing was the bleeding issue and air noise in the system. I was just afraid we'ld never be able to bleed it properly with these changes in diameter and who knows what else will come up once it's brought on line. This section to the bathroom will probably be about 60 feet total out and back.
 
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Old 11-11-06, 03:53 PM
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Could it be done? Probably. Should it be done? Probably not. I'm with taylorjl on this one.

A 60 ft loop of 1/2" pex, plus elbows (or their pex equivalent) ends up being a pretty long loop, maybe 75-80 ft equivalent length.

If you are piping from an already long zone, this could mess things up. Either or both the flow rate in the existing zone will drop, and/or the 1/2" loop will be hard to get the right flow rate. Inadequate flow equals potential air problems, and reduced output of heat emitters (baseboard).

The 1/2" loop should have a flow rate of ~1.2-2.3 gpm. Here's the heads (resistance to flow) for these flow rates, assuming a 60 ft pex+baseboard run, some elbows, and a ball valve.

gpm head (ft)
1 2.67
1.5 5.43
2 8.99
2.5 13.28

Compare these to what you've got in the main part of this loop and see if you can get there, flow-wise. Remember sticking a diverter tee on the main 3/4" piping adds 29 ft to that loop. If you already have marginal flow in the main loop, this might kill it. Fortunately, most homes are overpumped.

Doing it in 3/4" would help keep the head loss down quite a bit.

gpm head (ft)
3.5 1.89
4 2.39
4.5 2.93
5 3.52
6 4.85

Plumber or you should do the flow calcs for the main loop, regardless.
 
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Old 11-11-06, 09:32 PM
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Thanks for the response. Would this situation affect your opinion at all? Using 2 monoflo tees installed in the supply line of the 3/4 pipe zone with the first tee in the direction to allow flow to the bathroom loop and also to allow the original flow through the 3/4 pipe and the second monoflo in the reverse direction to allow the flow from the bathroom loop back into the original 3/4 pipe zone. In other words, not to route the entire 3/4 pipe zone flow into only the bathroom loop and on, but to allow flow to the bathroom loop and the original 3/4 zone at the same time. We have a toekick heater installed in such a way on the next level and, so far, haven't had any air noise or flow issues (the bleeder on the heater allows for purging if necessary which has only been slight in the past). Thanks again.
 
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Old 11-12-06, 01:09 PM
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Sixty Feet

I think sixty feet is going to be more than a couple of mono-flo tees will want. I am by no means an expert on mono-flo tees but every installation I've seen has had the tees close to radiation. I have seen them in a basement & serve a second floor but even that was only about 10-12 feet from the radiation & verticle for all practical purposes.
 
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Old 11-12-06, 01:25 PM
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A monoflow aka diverter tee, or a pair of them, divert some flow, not all (you can't fit 3/4" of water into a 1/2" pipe...).

If you're going to do this, you or your plumber should do the head calculations and make sure your loop and pump can support this. 3/4" provides less resistance than 1/2".

You should also have an idea of how much baseboard is required in the new space, and ensure that 1/2" at whatever gpm will provide the necessary heat.

Here's some info on diverter/monoflow/venturi tees

http://www.taco-hvac.com/uploads/FileLibrary/100-3.6.pdf
 
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Old 11-19-06, 05:45 AM
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Hi Guys,

Help again! Have not been able to reach plumber working on this project so will be forced to perform calculations myself. Can anyone link me to or explain the formula(s) for calculating whether the monoflo addition with the 1/2 in tubing will work in this situation? Thanks again, Lou.
 
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Old 11-19-06, 06:11 AM
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Try this:

http://www.taco-hvac.com/uploads/FileLibrary/SelectingCirculators.pdf

Shoot me a PM with your email and I can send you an excel spreadsheet that I use based on the Taco sheet. Probably still have the numbers from earlier in the thread.
 
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Old 11-19-06, 10:14 AM
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Is PEX tubing even appropriate for the temperature that the baseboard loop runs at ? I'm under the impression that it might not be ???? I'm pretty ignorant about some things though...
 
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Old 11-19-06, 04:39 PM
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I believe Uponor/Wirsbo Pex-Al-Pex is rated 200F at 100 psi.
 
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Old 11-20-06, 09:30 AM
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Originally Posted by NJ Trooper
Is PEX tubing even appropriate for the temperature that the baseboard loop runs at ? I'm under the impression that it might not be ???? I'm pretty ignorant about some things though...
The pex tubing the plumber installed has this on the tube. This is only some of the info: 1/2 inch CTS-Watts Radiant pex with DIN 4726 oxygen permeability SDR-9 100 psi @ 180 F 0.070 min wall.
 
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Old 11-20-06, 11:25 AM
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If you have any iron piping in your system, then you want oxygen barrier pex. Typically, PEX-AL-PEX (aka PAP) is used for this application. The AL is for aluminum, which forms a layer of the tubing and provides a 100% oxygen barrier. My _very_ quick reading of DIN 4726 says that your pex qualifies as oxygen barrier under certain conditions, but it's not pex-al-pex. The "qualifies" part means that under certain conditions the pex rates as oxygen barrier, and in others it does not. One example I read applied to very long loops. So there is permeability, but you need a lot of tubing before the cumulative permability begins to affect your system. My _very_ quick reading suggests that a short length like you describe probably falls within the DIN 4726 oxygen barrier spec due to the short length involved, and for your application it would be considered to have sufficient oxygen barrier properties. So based on a _very_ quick read, you're probably fine.

But would I do it in my house? No. I'd do pex-al-pex. I'd also look into the various flow rate and BTU output issues that come with what the project entails, which we've discussed above. They seem to me to be potentially more significant. Good luck.
 
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Old 11-20-06, 01:53 PM
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Xiphias,

Thanks for the info on the permeability issue. The system is iron out of the boiler then to copper feeding each of the 2 zones. The total piping involved in this loop turns out to be a pretty accurate 84 ft. Sent you a PM as requested.

Thanks, Lou.
 
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Old 12-01-06, 09:39 AM
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Well the problem has been solved. We used avoidance. Since we were not able to get the original installing plumber to return, we decided to bring someone else in (who does not how to work with pex so it will be removed). He will install 3/4 M to match the rest of the system and then just tie directly into the loop (no monoflo's needed either). This should solve all the problems. Sometimes it's better to take a step backwards so you can take two forward. Thanks for everyone's help throughout.

Sincerely,

Lou
 
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