Is there any advantage to multi-pass over single pass? When should I use series storage over parallel?
Stratified, Single Pass Storage vs Mixed, Multi-Pass Storage:
The only advantage we have found in a multi-pass system is the response from the contractor when reviewing the drawings. Single-Pass systems are not often understood at first glance because contractors are typically used to installing boiler systems in multi-pass configuration.
Stratified storage carries the potential of increasing storage capacity by over 500%. You can extract a lot more BTU’s out of stratified storage because the majority of your storage system can drop down to 50°F. While supplying 140°F water to the mixing valve. Since your entire tank temperature is equal to your delivery temperature in mixed storage, your minimum tank temperature can only drop to about 120°F before you start to deliver unacceptable water temperatures to the building.
Stratified Storage in Parallel vs Series:
There are two things to consider when choosing between storage in parallel or series:
- In-Series tanks will have a higher effective storage capacity because the stratification layer is only in one tank. That means all of the tanks downstream are 100% full of 140°F water.
- Each tank in series will see the full building DHW flow*. If two tanks are in parallel, each tank connection will only see half the building’s DHW flow. This may save on piping and custom tank connections.
*The flow through the tanks is calculated as: (tempered DHW flow) x (Hot water ratio between tempered and storage temperatures). For example, 10gpm max building demand @ 120°F will be a mixture of 2.2 gallons of 50°F water and 7.8 gallons of 140°F water. That means each tank connection in series will see a max flow rate of 7.8 gpm. If there are three tanks in parallel, each tank connection will see 2.6 gpm flow.