Do I need a buffer tank?

quadplumbing

I recently took out 3 cast iron boilers piped in series and installed 3- 90plus boilers in a primary / secondary piping configuration. Now the system pumps are not performing up to standards. Did the old cast iron boilers act as buffer tanks ?

Ironman

Yes, the old boilers certainly added a lot of mass which you now don't have. But I don't see how that would effect the system pumps if your p/s piping is correct.

Can you post some pics?

hot_rod

Both mass and water content have been removed with the old boilers.

The new boilers should have modulation, which should help overcome some of the issues with them lower mass and waster content. With 3 modulation boilers you should expect a wide turndown ratio. proper piping is critical.

What are the performance concerns?

Snowmelt

do you have it piped P/S, do you have 1 pump per boiler and 1 for the system?

quadplumbing

We don't have issues with water temps from the boilers, we are running a 180 degree supply temp, but our zone return temps are 100 to 120 degree. There is 2 main supplies to different areas, each has its own pump, and each area has 10 zones of 3/4" baseboard, with zone valves. When we turn 1 zone off the zone left on will return 140 degree water. All mains are 2 inch copper piping. The system pumps are Taco VR 3452.

GroundUp

So what's the problem?

hot_rod

When you pipe with primary secondary there is some temperature blending going on across those tees. If the loads are piped from a series primary loop each set of tees sees a lower temperature.

80° would be a high delta even under design conditions, typically 20- 30° is a design, 10- 15° on radiant slabs.

If the end of the fin tube loops are only seeing 100- 120°, don't expect much output.

How did the temperatures, S&R run before the change out?
Ideally with high efficiency condensing boilers you want to run the lowest possible supply temperature to get the job done.

If the building can be heated with 140 SWT the boilers will run more efficiently.

A picture or drawing of the piping may shed some light.

Gyrosexa

Yes will need a buffer tank.

Hydronicolic

Definitely NOT a major Air issue? If not, I'd go with a large Low Loss Header before a buffer tank $$$$.