PEX pipe vs copper to use on new heating boiler


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Old 02-16-15, 09:42 PM
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PEX pipe vs copper to use on new heating boiler

I am putting a second heater in a duplex. Should I use PEX pipe with or withour a manifold syste or just put in copper. the heater will be approx. 65 000 BTu wiith old cast iron radiators. What size should the main run be 3/4 or 1"

Thanks
Ed
 
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Old 02-17-15, 05:40 AM
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Hi Ed,

You can use PEX or copper, but either needs to be the correct size.

If using PEX, you MUST use the type made for heating, it MUST have an "Oxygen Barrier". Do not use the stuff for potable water supply.

For 65K BTUH you will need about 6.5 GPM of flow, and you will NOT be able to use 3/4" copper. That will require 1" copper.

PEX has a smaller I.D. than copper, 3/4 PEX is more like 1/2 copper, 1" PEX, closer to 3/4 copper.

You MIGHT be able to get by with 1" PEX though...

I don't understand what this means though...

the heater will be approx. 65 000 BTu wiith old cast iron radiators
Can you explain a bit more? Do you mean that the rest of the building has cast iron and the new unit will be something different?
 
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Old 02-17-15, 09:16 PM
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I am leaving the original heater in for one floor, which floor I am not sure which would be best because that heater is 90 000 btu unit. The new unit will be new runs but connected to the old cast iron radiators. Do I need to use a manifold or can I just branch off with T s? Should I use a larger size pex, 1 1/4 inch to be safe? First floor has five radiators the second has four.
Thanks for the advice.
 
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Old 02-18-15, 07:57 AM
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Sounds like a two apartment building and you want to separate the heating so the tenants can be billed for heat?

Once you remove one floor from the existing system, that system becomes way too large for the load, basically now twice as big as it needs to be.

I believe the new system of 65K BTU is also going to be oversized for the load.

You were previously heating the entire building with 90K, and now you are going to have 90K PLUS 65K installed... or 155K ... to do the same job.

I can understand not wanting to install TWO boiler due to economics, but at least don't oversize the new one.

No, I don't think you will need to go to 1-1/4 " ... 1" will be fine.

What make/model of boiler are you installing?

Since you have cast iron radiators, you will need to be careful about the piping arrangement.
 
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Old 02-18-15, 02:39 PM
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I was thinking a crown boiler . I have installed them before about five years ago and very happy with them. It is a row home coverted into two one bedroom apartments. give me an educated guess on the size you think I should use. I don t want to underestimate the size. the old one is only five years old. The heater is used aobut six months a year in this area, the insulation isn t great, as the house was built in the 1940 s. If I did decide to put two heaters in what would you suggest. Also, I will add a new baseboard heater to the system, about six feet. I have read alot of information ;you have posted and verified it all. I appreciate your help and advice. Any thoughts or changes would be appreciated. Not to change the subject but r u a LEO, as I just retired from an organization from another state. Thnaks.
 
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Old 02-18-15, 02:43 PM
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I have all the tools for copper but thought I should change over to PEX and invest in the tools, your thoughts on that. Thanks
 
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Old 02-18-15, 04:15 PM
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give me an educated guess on the size you think I should use.
I can't. I can only guess because I don't know enough about the building!

How many square feet is the building for starters?

the insulation isn t great, as the house was built in the 1940 s.
A lot of old drafty windows?

I will add a new baseboard heater to the system, about six feet
Mixing fin-tube with cast iron is not always a great idea.

Because the cast iron heats up and holds it's heat and continues to radiate that heat after the boiler has shut down, and fin-tube does NOT hold the heat, imbalances can arise... the rooms with the radiators will likely be warmer than the room with the baseboard.

thought I should change over to PEX and invest in the tools,
If you think you will have a future need for the tools, by all means!

If this is a one time shot, maybe not.
 
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Old 02-18-15, 06:23 PM
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In my system, I have a tiny bit of PEX and a small amount of copper. All the rest is black steel pipe. All are performing satisfactorily, but for my peace of mind, the steel is best. It is way over-designed, pressure-wise, and easy to modify. PEX is cheaper and easy to install. A plumber installing steel pipe will likely set up a threading machine in your driveway.
 
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Old 02-18-15, 06:54 PM
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the building is 1590 sq feet
the windows are newer, not drafty
i will be adding a cast iron baseboard
looking forward to working with pex
the crown boilers are 38,000 then jumps to 70,000 btus. or can you suggest another make/brand heater
thanks,
 
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Old 02-18-15, 06:57 PM
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I have used black iron in the past but tired of hand threading the old school way. However, I wasn t sure of mixing up the pipe type. It is an idea, but looking forward to using the PEX. (never used it)
 
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Old 02-18-15, 07:36 PM
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Mixing any of the three pipe materials is not a problem, it's done all the time, including by me - but you can find contrary opinions.

There may be a slight issue with PEX - it is nonconductive, electrically. So, joined metallic pipe may not be grounded - which can be a safety issue per the National Electric Code. You can jumper around the PEX if necessary.
 
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Old 02-18-15, 08:13 PM
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the building is 1590 sq feet
That's the total for BOTH apartments?

A GUESS at the heat loss I would put at 55K BTUH for that square footage, and I'm probably somewhat high on that number. This GUESS is based on 35 BTUH / SQUARE FOOT

How much in just the apartment with the new boiler?

If it's half and half, figure 30K BTUH for each apartment.

These are GUESSES, but should not be too far off. There's no substitute for a real estimate being done.

i will be adding a cast iron baseboard
Good, should be no problem.
 

Last edited by NJT; 02-19-15 at 04:49 PM.
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Old 02-18-15, 08:35 PM
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1020 sq ft. 1st floor including basement.
570 sq ft. 2nd flr.

do you think two 38000 btu boilers or a 65 and 38btu? Just looking for a little guidance. I appreciate the follow up questions
 
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Old 02-19-15, 04:46 PM
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570 sq ft. 2nd flr.
You could almost heat that with candles!

Even if the heat loss in that space is an unGodly high 50 BTUH / square foot, you would not need more than 30K BTUH.

Ditto with the downstairs... no more than 50K BTUH.

It's more likely that the heat loss is even less than that... maybe 40 BTUH/SF ... or even 35 ... or who knows? maybe even less...

So more realistically, let's GUESS at 35 BTUH/SF of heat loss.

Upstairs --- 20K BTUH

Downstairs --- 36K BTUH

Keep in mind that these are GUESSES based only on what I have found to be BALLPARK heat loss numbers.

It should also be possible for you to measure the radiators and determine their heat output by googling up the tables that give this info. One should NEVER size a boiler using this method, but you can learn something from it... for example:

Let's say that you know in a particular building that the heat OUTPUT of all the radiators combined is 45K BTUH with 180F water running through them at a proper flow rate.

This does not mean that the building's heat loss is 45K, it's just what's installed in radiation...

Let's say you have a 'history' with the building and know for a fact that at no point during the worst cold winter has the building lacked for heat.

This tells you that the building's heat loss is 45K BTUH OR LESS.

It also tells you that you would never need MORE than a 45K BTUH boiler.

Again though, this is NOT the way to size a boiler, because the building's heat loss could be less than that.

Now, let's say that when it's zero degrees outside and a stiff north wind, the building struggles to stay warm, or worse, cools down slowly, and the boiler is running NON-STOP.

This means that there is not enough RADIATION installed in the building. No matter HOW BIG the boiler is, if there is not enough radiation to get the required heat OUT of the water and into the building, the heat would be lacking. You could hook those radiators up to a NUKE PLANT and the building would not warm up.

MANY buildings have MORE radiation than needed. This is a GOOD THING. It means that the building can be heated with COOLER WATER which would save fuel. The cooler the water needed, the less fuel burned.

There are LIMITS though... you can't run TOO COOL water to a conventional non-condensing boiler for any long period of time. The water returning to the boiler MUST warm up to AT LEAST 135-140F to prevent damaging the boiler. This means about 20-30F HOTTER water out the SUPPLY side of the boiler.
 
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Old 02-19-15, 04:48 PM
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All the info in the previous message aside;

There is NO SUBSTITUTE for a proper Manual J method heat loss estimate.

It's the ONLY WAY to properly size a boiler.


I mentioned careful piping when connecting a boiler to a system with cast iron radiators. This speaks to my statement about running TOO COOL water back to the boiler.

Cast iron systems have a LOT of water in them. Much more than fin-tube baseboard systems.

The radiators heat up more slowly, thus the water coming back to the boiler stays COOLER for extended periods of time.

The piping must be designed so as to protect the boiler from this cool water returning.

This is what BYPASS PIPING is all about.
 
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Old 02-19-15, 09:33 PM
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Thank for all the information. I appreciate spelling it all out. I have great amount of information now to work with. A few things I was not aware of .
Thanks.
 
 

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