Heat Exchanger - Counterflow VS Parallel
Last Post 25 Nov 2014 08:40 AM by NRT.Rob. 8 Replies.
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clovesUser is Offline
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20 Nov 2014 05:55 PM
Hi everyone,

I was hoping to get some feedback because I am hearing 2 conflicting points of view. I am currently plumbing my radiant heat setup. The basics of the system, AO Smith Vertex (50 gal) providing both domestic hot water and the heating system hot water. System will be closed and a plated heat exchanger (4x14 30 plate) will be used.

Now my radiant tech guy says to plumb it "parallel" but I am reading at another radiant place to "counterflow" is 40% more efficient. Can anyone shed some light here?

Attached is the diagram of what I am putting together. Heat exchanger is plumbed in "parallel".

The article linked below states that "counterflow" is 40% more efficient.

http://www.radiantcompany.com/system...hanger-system/


jonrUser is Offline
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20 Nov 2014 06:19 PM
Use counter-flow. If someone disagrees, ask them to explain why.
newbostonconstUser is Offline
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21 Nov 2014 06:32 AM
Where are you located....not sure what you mean by counterflow vs parallel but....

What I will say is that you are going to loose efficiency through that heat exchanger. And being that the majority (>90%) of your water heater energy is going to go to heating your house. You will be constantly loosing energy through the heat exchanger. When taking a shower the energy to heat water for a shower is around 50 cents.

You might want to use a cheap water heater for your domestic needs and separate the two systems. This will likely simplify the system and save you head aches down the road.
"Never argue with an idiot. They will only bring you down to their level and beat you with experience." George Carlins
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21 Nov 2014 07:15 AM
Like the article you provided states, heat exchangers should always be avoided if possible. Heat exchangers reduce efficiency and increase maintenance. Running the hottest fluid in the opposite direction of the coolest fluid (i.e., a “Counter Flow” arrangement) results in better heat transfer and less inefficiency. Running the fluids in the same direction (i.e., a “Parallel” arrangement) results in reduced heat transfer and greater inefficiency. The reason for this is that heat transfer is maximized when the delta T between the heat exchanger plates is maximized, which occurs with the counterflow arrangement:

Engineers EDGE - Parallel & Counter Flow Designs
Borst Engineering & Construction LLC - Competence, Integrity and Professionalism are integral to all that we do!
NRT.RobUser is Offline
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21 Nov 2014 08:53 AM
Agreed on counterflow.

Re: separating the water heaters, the Vertex is a sealed combustion condensing unit, high efficiency. You would undoubtedly lose more energy with a second cheap water heater than you would with a heat exchanger, just in standby losses. Consider relative surface area and you can figure out why, and that ignores the flue and combustion efficiency differential which further stacks the deck toward the heat exchanger option (or operating cost differential in the case of electric DHW tanks).

If heat loss from a heat exchanger bothers you, insulate it and the single wall round tube water to air heat exchanger carrying heat to and from it . The only other thing that matters is if your supply water temperature requirement is approaching the desired setpoint of the water heater, in which case heat exchanger sizing can be problematic since it relies on a temperature differential to operate...which is often incorrectly interpreted as "efficiency loss", which it isn't in any situation where the tank temp is fixed and the heating temp is consistently lower anyway. But if you need 125 degree water in your heating system, and the tank heater wants to be set at 130, that's a big heat exchanger you need to make it happen, or you need to turn up the water heater, which IS an efficiency loss. Especially with condensing water heaters you want to consider the tank setpoint carefully instead of just jacking it up. Deciding to add emitter to keep water temps down usually makes sense in those cases. If the water temps are high enough, it's time to consider a boiler that at least can stay in condensing range a significant amount of the time with reset control.

there are maintenance considerations to heat exchangers. But then again, there are maintenance considerations to separate water heaters too. Anything installed should be installed with maintenance in mind.
Rockport Mechanical<br>RockportMechanical.com
newbostonconstUser is Offline
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24 Nov 2014 12:45 PM
Posted By NRT.Rob on 21 Nov 2014 08:53 AM
Agreed on counterflow.

Re: separating the water heaters, the Vertex is a sealed combustion condensing unit, high efficiency. You would undoubtedly lose more energy with a second cheap water heater than you would with a heat exchanger, just in standby losses. Consider relative surface area and you can figure out why, and that ignores the flue and combustion efficiency differential which further stacks the deck toward the heat exchanger option (or operating cost differential in the case of electric DHW tanks).

If heat loss from a heat exchanger bothers you, insulate it and the single wall round tube water to air heat exchanger carrying heat to and from it . The only other thing that matters is if your supply water temperature requirement is approaching the desired setpoint of the water heater, in which case heat exchanger sizing can be problematic since it relies on a temperature differential to operate...which is often incorrectly interpreted as "efficiency loss", which it isn't in any situation where the tank temp is fixed and the heating temp is consistently lower anyway. But if you need 125 degree water in your heating system, and the tank heater wants to be set at 130, that's a big heat exchanger you need to make it happen, or you need to turn up the water heater, which IS an efficiency loss. Especially with condensing water heaters you want to consider the tank setpoint carefully instead of just jacking it up. Deciding to add emitter to keep water temps down usually makes sense in those cases. If the water temps are high enough, it's time to consider a boiler that at least can stay in condensing range a significant amount of the time with reset control.

there are maintenance considerations to heat exchangers. But then again, there are maintenance considerations to separate water heaters too. Anything installed should be installed with maintenance in mind.


The efficiency looses in a heat exchanger are from pumping looses and having to heat the water to a higher temperature to get good heat exchange. Also the heat exchanger for use with potable water has to be double walled which doubles your cost and doubles your inefficiency.

Also you are constantly heating your house up to 100% of the time....so that inefficiency is all the time and not just when you are heating water to take a shower... I really really think you are going the wrong way with having a heat exchanger.

The cost of a potable heat exchanger and pump and larger water heater because of the inefficiency's and also the plumbing associated with it will pay for a really nice water heater or on-demand heater. And you are going to be paying for the inefficiency's year after year....
"Never argue with an idiot. They will only bring you down to their level and beat you with experience." George Carlins
kromUser is Offline
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24 Nov 2014 06:53 PM
I guess you can count the pumping power required as an inefficiency, but other than small losses to the surrounding air, you aren't losing or wasting heat, and there are 0 standby losses, any heat that isn't transferred through the exchanger returns to the heater.
NRT.RobUser is Offline
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25 Nov 2014 08:38 AM
Yes, you are using another pump. However, that pump can very easily be set up for variable speed injection which makes that parasitic draw absolutely trivial, and which improves the entire heating system performance! I would never install a heat exchanger system without VSI... the cost of the controller would be worth it in comfort improvements alone.

Also, you do not "have" to heat the water temperature to a "higher" level with a heat exchanger. You're using a water heater. It has to be set at a minimum of 120 and a recommended level of 140 for most tank heaters anyway for bacterial reasons. As long as your radiant supply water requirement is 5-10 degrees below those setpoints on design day, the presence or absence of a heat exchanger means nothing to the temperature of your source, the tank. IN THOSE CASES, which are usually pretty easy to hit with radiant, there is absolutely no "loss" associated with the heat exchanger, thermally.

If you're right up to the line of your desired tank temp, THEN it matters. a little or maybe a lot depending on what your water temps are. If you need higher than 135 or so (higher than condensing threshold on space heat return water) in your heating system then I recommend going to a boiler in any case, unless you've got some mod/con water heater you can vary the setpoint on with an external controller that is using at least outdoor reset (I've seen exactly one of those, but maybe there are others)... any fixed temperature heat source that must stay fixed above 135 for space heat is losing the efficiency battle from the get go, even if it's a mod/con water heater. Go boiler with outdoor reset in those cases.

Compare a small amount of pump energy to the jacket loss of another standing tank water heater and the second standing tank water heater loses by a mile. On demands should almost never be used in a typical residential setting with regular occupancy in the home... you can get the same efficiency in a tank with a lot less maintenance issues and better hot water delivery (with a few exceptions for long draw users). One service call on an on demand kills your efficiency savings, if there was one at all, for at least a couple years.
Rockport Mechanical<br>RockportMechanical.com
NRT.RobUser is Offline
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25 Nov 2014 08:40 AM
Also the efficiency differential of a tank at 120 vs a tank at 130 is far less than the additional jacket loss of a second tank heater.
Rockport Mechanical<br>RockportMechanical.com
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