New boiler has smaller pipe than old one. Effect of PEX

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Old 12-26-09, 09:58 PM
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New boiler has smaller pipe than old one. Effect of PEX

I am replacing a 30 yr old cast iron boiler with a Viessmann WB1A condensing boiler. I will also replace the circular, expansion tank, etc , but will keep the 3 existing Taco zone valves and as much of the original piping as I can. (One circulator is for all 3 zones).

The existing system uses 1" copper pipe from the boiler to the 3 zone valves. Each zone uses 3/4" pipe. The return piping is similar. The 3 zones of 3/4" copper pipe combine to a 1" pipe before going to the circulator and boiler.

The new boiler has 3/4" connectors for both the the supply and return. I have 2 questions that deal with the size of the new pipes to be installed between the boiler and zone valves, and the return to the circulator.

The first question is would there be an advantage to use 1" copper pipe (like the existing system) from the boiler to the zone valves even though the boiler only has 3/4" connectors on it? I want to be sure I get enough flow.
The performance curve of the Wilo Star 16 FX circulator that has been recommended to me is at http://www.wilo-na.com/cps/rde/xbcr/...SCISD-0609.pdf At a head of 14 Ft it can pump 4 gpm if I read the curve correctly.

The second question has to do with PEX tubing. If I use 3/4" PEX tubing, the connectors appear to reduce the flow to 1/2" because they fit inside of the 3/4" tube (as opposed to the Ls and Ts of copper pipe which are on the outside). Should I be concerned about the 1/2" restriction and the increase on the head, and if so, how do I solve it?

Since this is my first post, let me know if more details are needed. Thanks in advance for your help.

Ron
 
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Old 12-27-09, 05:30 AM
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You have to size the pipe to handle the flow required. If you need more than 4-5.5 gallons of flow than you need the 1" pipe. The tapping size of the boiler does not have anything to do with the pipe size.
Multiple_boiler Installation
This link does not directly apply to boiler piping but does explain why pipe sizing and tapping size does not always agree.
 
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Old 12-27-09, 07:47 AM
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Originally Posted by Ron51 View Post
If I use 3/4" PEX tubing, the connectors appear to reduce the flow to 1/2" because they fit inside of the 3/4" tube (as opposed to the Ls and Ts of copper pipe which are on the outside). Should I be concerned about the 1/2" restriction and the increase on the head, and if so, how do I solve it?
Please don't think that a 1/2" ID fitting will block flow as if the whole system were 1/2".

But, since you're concerned about this, why are you reducing the pipe size from 1" to 3/4"? If you're not prepared to calculate the system pressure drop or compute the required flow, I would just install the same size pipe as before.

You refer to a point on the pump curve at 14' head. What is the significance of that point? You'd need to calculate the total system pressure drop for the required flow, and compare that to the pump curve.
 
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Old 12-27-09, 08:43 AM
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Which 'sub-species' of the WB1 ? Looks like there are two, 80K and 100K BTU.

I've got a question related to this... why would a boiler manufacturer supply a boiler with 3/4" tappings if it were say an 80K BTU unit? According to common piping practice, 3/4" would be good for say 40K BTU at a 'proper' flow. (note: with a 20F Delta T).

Is the idea that these boilers run a higher DT? In this case perhaps recommended DT is 40 ? (I haven't studied the Veismann I&O yet...)
 
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Old 12-27-09, 04:18 PM
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Originally Posted by NJ Trooper View Post
... why would a boiler manufacturer supply a boiler with 3/4" tappings if it were say an 80K BTU unit? According to common piping practice, 3/4" would be good for say 40K BTU at a 'proper' flow.
Here is my speculation: the boiler manufacturer will provide pressure-drop data through the boiler. It's up to the installer to deal with it (or compensate for it) through pumping and/or system pipe sizing. The boiler manufacturer saves a few dollars.

I am familiar with a 1950s-era boiler, about 120K Btu/hr (net), with 3" tappings (back when quality, not price, was king). But, maybe it was set up to be compatible with gravity flow? The original installer used reducers to 1-1/2" system piping (pumped).

I'm thinking the old-timers would rise up out of their graves if they knew what was going on.
 
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Old 12-27-09, 04:21 PM
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Trooper Burnham does the same thing on the ALP210's and up. You also see this a lot more on indirect water heaters and hydro-air coils.
The Burnham Alliance indirect has 3/4" tapping's but the flow is 6 gpm. Therefore it should have 1" pipe. I teach in all my seminars now that the age of looking at a tapping and thinking that is the pipe size is gone.
The ALP210 requires a 15.5 GPM flow which requires a 1-1/4" pipe. As the flow is introduced to the 1" tapping on the boiler the velocity increases through the boiler to keep a 25 delta T. When it exits to a 1-1/4" pipe that can handle the flow it actually slows down a bit. Water exiting a smaller area into a larger area slows flow (like an air separator) and a larger area entering a smaller area increase in flow.
If an 1-1/4" tapping were installed on the boiler the flow in the boiler would be slower and the delta-t would increase or need a larger pump to keep the delta-T higher.
 
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Old 12-27-09, 04:30 PM
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Ron: my advice is the same as Rbeck's. Ignore the 3/4" boiler tappings, and install the original size pipe, 1" (assuming you were satisfied with the original perfomance). Use 3/4-1" adapters.

Have you compared the Wilo-Star's pump curve to that of your original pump? The Wilo-Star seems to be a Taiwan manufacturer: http://www.allproducts.com/environme...116112852.html

Just wondering - how did you select that particular pump? The data says it's 220V. Make sure yours is 120V. What was wrong with your original pump?
 
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Old 12-27-09, 04:36 PM
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the old-timers would rise up out of their graves if they knew what was going on.
At the very least, they'd be scratchin' at their skulls!

I'm thinking that the smaller tappings would also cause a slight pressure increase inside the boiler... and this might prevent air from coming out of the water INSIDE the boiler.

Also, as rbeck explains, the water would SHOOT into the boiler, rather than 'dribble' in, and probably cause more turbulence inside, which might result in better heat transfer.

The MPO series has an 'injector' on the return, and presumably this is for the same reason... and one of the reasons it will tolerate slightly cooler return water... (along with the fact that it doesn't return the water to the bottom of the boiler).

OK, I guess I should look for the I&O for that Veismann unit now.
 
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Old 12-27-09, 05:00 PM
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Ron, are you planning on using the 'low loss header' that Veismann so strongly recommends in their I&O manual?

Also, I see that they actually DO recommend a Delta T of 30 to 40 across the boiler...
 
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Old 12-27-09, 11:28 PM
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My answers to some questions. Thanks!!

Wow! Thanks for the many quick responses. I really appreciate this info. I wish I would have asked these questions last week when I was researching the Viessmann Boiler. Things are happening faster than I anticipated. A friend of mine who is doing the actual install has been in the HVAC install/repair field for 15 yrs. He now has his own business. Although he has plenty of experience installing conventional boilers, he wanted some experience installing a newer technology unit. This is his first install (we both agreed to this and he gave me a great price). A lot of the initial install is now complete, but I see some things need to be corrected. He now plans on getting some formal training from Viessmann. (In a perfect world, this should happen first, but...) We both knew that the install of a condensing unit is very different from an atmospheric one

I'll do my best to provide answers

Why are you reducing the pipe size from 1" to 3/4"? Before reading the post from rbeck, I was thinking that the 3/4" connection on the boiler would limit the flow thru the entire system, so why spend the extra $$ on the larger size of pipe? After reading these responses, I will ignore the 3/4" tapping and re-do it with 1".

What is the significance of that point (14' head) on the pump curve? This is the max head the circ can handle and still provide the minimum flow. (Viessmann Tech Data Manual says typical min is 3.6 GPM.) Maybe I should calculate the head on the various zones, but I am not sure what the head will be if >1 zone is open. It is like having 2 resistors is parallel. (I know a lot about volts, amps, ohms, impendances, VARS, etc. but sometimes the analogies to pressure, flow, and head don't hold up).

Which 'sub-species' of the WB1 ? It is the smaller one. Viessmann Vitodens 100-W Model WB1A 8-24. CSA input 29-80 kBTU/hr
My existing 30 yr old boiler is rated at 128,000 BTU input (a tad oversized). But it does include a coil for domestic hot water. Using the Manual J method, I estimated 55,000 BTU/hr is sufficient (using the ODT of 5 deg lower than the one for my area). The new installation will have a separate conventional hot water heater, so the new boiler is sized only for space heating.

Why would a boiler manufacturer supply a boiler with 3/4" tappings? One post speculated to reduce their cost. In addition, it could be to sell more of their low loss headers.

Is the idea that these boilers run a higher DT? Say 40 deg F? The Viessmann Tech Data Manaul says typical temperature rises are from 20 to 40 deg F.

The Viessmann Tech Data Manaul says typical boiler flow rates are 3.6 GPM for a 40 deg rise to 7.2 GPM for 20 deg rise

The Wilo circulator is made in Canada. The Wilo is said to be a high efficency model. It uses 120 VAC. The old circ was leaking a lot (about one cup per 2 weeks.) I got tired of emptying the coffee can under it each week, so I wanted to replace it.

Just wondering - how did you select that particular pump? My friend who is installing the system recommended it. All recommendations come from Viessmann or his supply house.

Are you planning on using the 'low loss header' ?. I wasn't planning on using it. However, the installer was told one was necessary, so he put one in. However, it 'does not work'. Or, to be more precise, it 'does not work' with only one circulator (which is installed in the primary loop). After searching the Internet and looking at the pics of typical installations, I believe that you need one circ on the primary loop, and one circ in each zone (secondary loop). Too bad the diagram in the Tech Data Manual doesn't mention that. Although to be fair, some of the example system layouts have the extra circulators..

Thanks to all who help explain things to me.

One Question on PEX. For replacement boilers like mine, should the new piping be copper or is PEX OK?
 
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Old 12-28-09, 04:49 AM
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I'm thinking that the smaller tapping's would also cause a slight pressure increase inside the boiler... and this might prevent air from coming out of the water INSIDE the boiler.

Also, as rbeck explains, the water would SHOOT into the boiler, rather than 'dribble' in, and probably cause more turbulence inside, which might result in better heat transfer.
Yes as flow increases the resistance to flow increases. That is why many of these mod/cons are calling for larger pumps. Increasing turbulence doe increase thermal transfer as in the ES2 cast iron boiler. Most boilers of yesteryear are lineal flow where mod/cons and the ES2 are turbulent flow. The other thing with mod/cons the water flow is opposite of the flue gas flow.
I would stick with 1" copper on that install.
 
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Old 12-28-09, 07:09 AM
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Originally Posted by Ron51 View Post
One Question on PEX. For replacement boilers like mine, should the new piping be copper or is PEX OK?
PEX is OK, but use the version with an oxygen barrier. Otherwise, oxygen can diffuse through the PEX into the water. Of course, copper doesn't have this problem.
 
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