As of late, I have been asked what size reservoir is required for a system. As a TLDR; whatever you can fit inside your case or externally.
The size of the reservoir generally isn't critical, but having a larger reservoir will increase the heat capacity of your loop. Imagine putting a cast iron pan on a stove at scorching heat and putting a spoonful of water on it. It'll immediately evaporate as the water will absorb huge amounts of energy, flashing to boiling point instantly.
Now imagine heating up an Olympic swimming pool with the same setup. It'll take forever to raise the temperature of a pool a few degrees. As a larger reservoir is able to hold more liquid, the loop as a whole will be able to absorb more heat before transferring it to a radiator.
Larger reservoirs aren't without their disadvantages though. The increased capacity means more energy is required to maintain flow. A larger loop also requires more coolant, which can become expensive for those service intervals. Not to mention, more liquid also means more weight, if you do need to move your rig around often for any reason.
On the other hand, whilst smaller reservoirs may address these issues, it may be more difficult to fill up a loop. Since you have a smaller buffer, you'll need to quickly top up the loop during priming, so as not to run the pump dry and risk damage to it.
What else do I need to look out for in a reservoir?
A reservoir doesn't have to be super complicated, but there are some nice to have features that can make big difference in your experience.
- Materials: Glass tubing provides that extra bit of clarity in your loop and makes cleaning much easier. These are typically made from borosilicate glass, which is the same stuff used in laboratories. Highly chemical and temperature resistant, glass tubes reservoirs will basically last forever.
- Ports: Having enough ports is also important. At the bare minimum, a reservoir will have an inlet and an outlet. Having an additional fill port will allow you to top up your reservoir with out having to take apart your loop. You can also add an exhaust valve in case you're having pressurisation issues in your loop.
Some reservoirs also come with metal ports. These are much more resistant to overtightening so you won't risk cracking or damaged your reservoir.
- RGBs: Whilst it might not be to everyone's taste, lighting up your reservoir to check for any growth or debris is always a good idea every now and then. Whilst most coolants will recommend a one year service interval, you may be able to extend that just a bit more if it seems like your loop is running clear.
- Pump mounts: Some reservoirs come with mounts to attach either a D5 or a DDC pump. Some are also pure reservoirs without any pump mounts. These can be ideal for space restricted systems where a pump needs to be mounted elsewhere instead.