I would like to know if someone can help me better understand a 2 and 4 pipe heating and cooling system in a commercial application such as a school. I am a maintenance technician that could better his workmanship with a little better knowledge.
2 pipe - 1 supply and 1 return (one coil)
Hot water and chill water use the same pipes. You either have heating or cooling, not both.
4 pipe - 1 each chill and hot water supply and return.(two coils)
Each system is independent of the other.
Two pipe systems supposedly result in a lower initial installed cost since there are only one pair of supply/return lines (2-pipe), as opposed to two pairs (4-pipe) and a single coil/control valve in each unit. In reality, the system can become much more complicated control wise.
There needs to be a central valved changeover system to select hot or chilled water flow throughout the building. This can be manual or automatic. There needs to be limit controls that prevent changeover until the loop temperature rises above/falls below a safe value to allow dumping cool system water into a boiler or warm system water into a chiller.
Additionally, when you have heating only coils (convectors, unit heaters, radiators) there normally is a valve to close off flow to these particular devices to prevent condensation in summer.
Finally, the control valve at each unit is normally sized for the larger (chilled water) flow, so is way oversized in heating mode and may cause control problems.
There is a section in the Honeywell Engineering Manual that gives a thumbnail description of 2-pipe systems also.
Click the first available link, points to document
77-E1100 - Engineering Manual of Automatic Control
It's a huge document (8 Megabytes) and will take forever to download at dialup speeds. Check book pages 367 and 368.
And then there was the 3 pipe system
A compromise was the 3 pipe system where variable temperature hot water and chilled water were both supplied seperately and mixed with a valve at the coil to provide desired air temperature. The return water was then divided between the boiler and chiller. If not properly balanced and controlled it was a nightmare.
Thanks for the info and link.
Originally Posted by hanknerd