Cold water to primary circuit of Storage Type Calorifier

[OriginalTexasRanger]

Hi all,

The company I work for is a MEP subcontractor, we are working on the installation of a small hotel (30 storey) where open vent storage type calorifiers are used in combination with heat pumps (helps provide Chilled Water for A/C system) to preheat potable water before it is fed to gas heaters and then to hot water taps.

The current design feeds cold water to primary side (tube) [15°C, in 32°C out] of the calorifier while the heat pump hot water feeds the secondary side (storage) [45°C in, 40°C out].

I've done some reading up online, and it seems that most descriptions have the hot water source to feed to the primary side, while the cold water are fed to the storage side.

I've spoken with the consultants, and they have said that the reason they have designed it in this way is because the calorifiers & heat pumps are located @ 5/F, while the hot water has to feed to the upper floors, in which case, the cold feed has to be pumped as well. To prevent water escaping from the calorifier air vent, they have decided to put the cold feed to the primary side.

Are there any particular difference whether the cold water is fed to primary or the secondary side?

My intuition is that, putting the air vent issue aside, it would make more sense to have the cold water on the secondary side, since that would mean a reserve of hot water on demand for the water taps, since the chilled water demand can fluctuate, therefore the heat pump hot water source is not a "reliable" source of heating. And without some kind of mixer, when the instantaneous demand peaks, the hot water temperature around the tubes may be locally lower than the bulk of the water in the storage tank, reducing the heating capacity of the tube. All taken together, it would seem that the gas heaters would just have to work harder than it needs to because the calorifier isn't as efficient as it can be.

Anyway, just my thoughts, it would be nice if someone can educate me on this subject. Big thanks!

Regards,
KH

[I am actually not from or anywhere near Texas, just that as a child, I used to watch Chuck as the Ranger!]

 

[jspath2339]

OTR,

This may not be helpful, as I have not worked on any systems like this, but here are some general thoughts:

-Typically domestic hot water systems are required to be capable of operating regardless of whether or not other systems are operating. The manner I usually see this is in terms of solar tubes being used for DHW. It can be used for energy savings, but there must be a back-up water heater in case the sun is not shining and people need to wash their hands.
-Water source heat pumps generally are connected to cooling towers & boilers. The reason for that is, that it's not certain whether or not the system will be in heating/cooling mode and whether it will drive the condenser water temperature up or down. You can drop the condenser water into freezing temperatures if you are not careful. Even more restrictive than that: most water source heat pumps aren't rated to accept an incoming condenser water temperature of less than 60 deg. F.
-For the temperatures you are discussing (all in Celsius so I had to convert), keep in mind any code about legionella. I'm not a plumbing engineer, but I think the requirement is that all standing hot water pipe be kept at 120 Fahrenheit or higher(this could be wrong, might be 140?) to mitigate legionella production. From there, thermostatic mixing valves are implemented at each fixture to reduce the temperature. The "out temperature" you showed is 32 C = 90 deg. F. It's worth asking a question.

I know that none of this answers your post, but hopefully can help you ask some good questions.