Domestic Hot Water Secondary Circulation

[edi454]

Hi,

I am a graduate mechanical engineer and would appreciate if someone could offer an explanation on the design of a DHW secondary circulation system.

I initially designed my system so that the pump on the return hot water pipe would pump towards the hot water calorifier. I have been told since that this is incorrect and that the circulating pump should be pumping in the opposite direction. Also, a non-return valve should be fitted on the return pipe before the circulating pump to prevent flow back to the calorifier. Does anyone have an explanation on why this is the case or even point me towards some information that is available on the design of a DHW secondary circulating systems.

Thank you

 

[JohnBreen]

http://www.engineeringtoolbox.com/hot-water-heating-systems-d_475.html

http://www.hvac-toolbox.com/heating-systems-t_38.html

http://www.maintenanceresources.com/bookstore/hvac/hvacpump.htm

http://www.engineeringtoolbox.com/design-hot-water-system-d_92.html

http://www.psdmagazine.com/ASPE_memberpdf/20070506/High-RisePlumbingDesignForHigh-RiseBuildings.pdf

http://aspe.org/xcart/product.php?productid=3&cat=1&bestseller

 

[edi454]

Thanks for all the links, but what I am looking at is a domestic hot water system. Where hot water is stored in a cylinder. The flow pipe from the cylinder would go to a tap. Due to the length of the pipe from the cylinder to the tap, it would be more than likely that a person would have to wait a long time for hot water to come through the tap. This is due to the water that is still in the pipe going cold when not in use. To prevent this problem a loop is made with a return hot water pipe added from the tap back to the cylinder. To circulate the hot water round a pump is added. With this the tap would have instant hot water on demand.

My question is where exactly should I position the circulating pump?

I have been told that the pump should be positioned on the return pipe and circulation should be from the return to the flow pipe. This would be opposite the flow of hot water. The explanation that I have been given for this set up is that when a tap is opened it will gain maximum flow both from the flow and the return pipe. If the pump circulate from flow to return then a portion of the flow will always circulate this way when a tap is opened. Thus, potentially reduced flow to the tap.

Is this a correct?

 

[hydtools]

Here is information from Taco, a pump supplier for hot water systems.

http://www.taco-hvac.com/en/product...irculation/products.html?current_category=360

Ted

 

[Ashveen]

The circulating pump can be installed in the return pipe, but the direction of flow should be from the flow pipe towards the calorifier.
It does not make sense to have opposite flow in a pipe as this creates noise, vibration and frition losses.
The direction should be towards the calorifier if installed in the return pipe.

 

[jk2000]

edi454 Did you eventually solve this problem & if so, how.
Thx.

 

[SAK9]

The convention is to install the pump in the pipe that returns water to the calorifier(pumping into the calorifier,to be precise!).But I do not see any reason why it can not be installed in the flow pipe before it starts branching into taps.The system is a closed loop anyway.

 

[BigInch]

I think that would tend to keep the tank empty and/or at lower pressure rather than full of water at higher pressure, allow air to collect in the tank and also pump water from a higer elevation, thus NPSH is reduced.

"I'm all in favor of keeping dangerous weapons out of the hands of fools. Let's start with typewriters."
- Frank Lloyd Wright (1868-1959)

 

[IRstuff]

One would think that if the pump was on the supply side of the faucet, you'd need to put in a bypass, since it's unlikely that you'd actually put in a pump capable of pumping 2+ gal/min at pressure, as that would appear to be a great waste energy in the pump, since 99.9% of the time, a substantially lower flow rate is needed. If the pump were size for simply maintaining water temperature, it would be inadequate for supplying normal usage flow.



TTFN

FAQ731-376