is a reverse return system useless when emitter piping is over-sized?

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Old 02-16-11, 01:32 AM
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is a reverse return system useless when emitter piping is over-sized?

Yes, you read that correctly?

Let's say you have a 10k btu heat loss and are using over sized piping that carries 20k btu to the radiation, is reverse return actually useless or not as effective?

A reverse return system will give you a more balanced system correct?
What if there is an abundance of hot water going through your piping?
At what point when using larger piping is the system as balanced as when using reverse return? All comments are always appreciated.
 
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Old 02-16-11, 05:48 AM
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Reverse return has nothing to do with flow rates, it just is a piping system that tries to balance all the flows to the feeders off of it (branches, loads, whatever you would like to call them). Flow rate is set by the circulator and you pipe sizing and heat emitter restriction.

Oversizing the emitters lowers water temp, and can allow (in case of CI or steel panel rads) easier pumping. When oversizing copper BB it can actually add more head to the system.

Typically balancing is done with valves in any system that has exposed header.
That and the calculation of branch pipe sizes vs flow rates and head loss.

A proper reverse return system can be very tricky to install, many variables involved
 
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Old 02-16-11, 04:25 PM
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balance all the flows to the feeders
Because the path through each branch is an equal distance.
 
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Old 02-18-11, 07:33 AM
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I'm sorry, maybe I'm not making it clear.

We have 2 identical spaces 20' long x 20' wide heated with 6 cast iron hot water radiators.

These spaces have been calculated to have a heat loss of 35,000 btu.

The cast iron radiators sizes have been matched to the heat loss.

Space A is piped reverse return with a pipe trunk size of 3/4".

Space B is piped direct return with a larger than necessary pipe trunk size of 1".

Please correct me if I'm wrong but are we saying that the last cast iron radiator(s) on the pipe run will not be as hot as the first one(s) when installed on a direct return oversized pipe trunk? If yes, what kind of temperature difference can be expected between the first and last radiators when boiler temperature is at 180 in space A?

Also, would I benefit by connecting the 1" supply to the 1" return at the end of the return to get the water making it a 1" loop? Wouldn't this flow the water with less friction or faster through the main trunk?

Thank you all for your comments and patience.
 
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Old 02-18-11, 06:11 PM
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We have 2 identical spaces 20' long x 20' wide heated with 6 cast iron hot water radiators.
These spaces have been calculated to have a heat loss of 35,000 btu.
Ziz, is that 35K for EACH? or BOTH together? if it's EACH, I KNOW there is something wrong with your calculation, if it's together, it's STILL very high for 800 sq feet.

are we saying that the last cast iron radiator(s) on the pipe run will not be as hot as the first one(s) when installed on a direct return oversized pipe trunk? If yes, what kind of temperature difference can be expected between the first and last radiators when boiler temperature is at 180 in space A?
I don't see where that was said... but it's possible I guess. It's IMpossible to predict that temp difference.

With a DIRECT RETURN setup, as flow leaves the main pipe into each branch, the flow in the main pipe slows down. If it slows down much below 2 FPS, you start to have problems with air getting trapped in the pipes. For this reason, it makes sense to 'step down' the pipe size of the main the further out it runs. This would keep the flow VELOCITY up.

One could conceivably, in a badly designed direct return system, end up with little to no flow in the furthest away radiators. One could install balancing valves on the branches to counter this effect, but the fact remains that at the end of the run, flowing say a gallon a minute through a 1" main is gonna be problematic.

In a REVERSE RETURN system, it's not such a problem as long as care is taken to design properly. The idea is that each run from boiler supply to boiler return is relatively the same length. This means that there is relatively the same pressure difference between each branch supply and return, and the flow will be roughly the same through each branch.

Also, would I benefit by connecting the 1" supply to the 1" return at the end of the return to get the water making it a 1" loop? Wouldn't this flow the water with less friction or faster through the main trunk?
No, absolutely not. If you did this, what reason would the water have to even branch into the radiators? You would remove the majority of the pressure difference between each branch supply and return... remember, water is going to flow from high to low pressure. This would cause MOST of the water to run around the loop and never go through the branches.
 
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