When to use a Hydronic buffer tank.

bob eck

Contractor will be installing a small combi boiler. There will be three small radiant zones plus one Hydronic unit heater in the garage. Heat loss is being done now. Most combi boiler have 10 to 1 turn down. Let’s say 100,000 BTU input lowest firing rate 10,000 BTU input. The radiant zones can be less than the lowest firing rate of 10,000 BTU input. I was looking at including a 20 or 30 gallon buffer tank to give the system some water volume to help the boiler from short cycling. Is using a buffer tank on this type of job the correct thing to do. With the buffer tank there is no need for primary secondary piping kit. What Caleffi Idronics book talks about when to use a buffer tank.

STEVEusaPA

Why do you think you need 100000 btu's

Gordy

Since a combi is being used that’s about as low as you can go, if that most are 125k. To get any dhw output.

I’d do a buffer. The zones at design are quite different than when warmer outdoor temps are seen.

Buffer, hydraulic separation, air removal in one unit.

psb75

Consider allowing the boiler to condense as much as it can in the main part of the winter, so that it operates at peak efficiency (+95%). A buffer tank could possibly work at cross purposes to this goal.
I was going to use a buffer tank on a small 60K Rinnai modcon install and I "scrapped" the idea. Its now spending most of its time...condensing while heating a radiant slab.

hot_rod

I have both a 110 and a 120 combi at my place, I doubt you could get by with much smaller for any reasonable DHW output.

He is a simulation from the buffer tank module in Siggys HDS.

It allows you to define all the loads and toggle them on and off to see results, change operating temperature, tank size, ambient temperature, tank dimension.

A 30 gallon tank, 140-130 operating temperature, 10,000 boiler input with a 1000 btu/hr. load.

With a 10° delta on the tank a 16 minute run, 149 minute off cycle.

With a 20 delta on the tank 33 minute run, 298 burner off cycle