Buffer Tank Questions

Domark68

Will be installing a much oversized (for the application) Lochinvar mod-con, KHB-155/Propane.

Issue - Preventing short cycling, even though there are programs available on this boiler such as ramp delay, after reading many similar scenarios the general consensus from all the great contributors on this forum is the use of a buffer tank, just rehabbed my boiler room and now have space for one.

1,100 sq' ranch over a finished walkout, 4 different types of heat emitters, fan coil, heats entire main floor, fin tube base board, (heats half of basement, family room, laundry room and a mud room entrance, a 30"x30" Runtal panel (used as a point of use towel warmer) and radiant under 3/4" engineered hardwood over Warmboard, this is in the most recently finished second half of the basement, a small gym 12'x'22'. The radiant, 250' of Pex-Al-Pex has the lowest estimated figure of 7.5k btu/hr. This boiler has a 10-1 turndown (15.5K), sizing with a run time of 20 minutes comes up with a tank size of 16 gals, 10 minute run time yeilds 8 gallons. SupplyHouse offers two Zilmet Buffer tanks, one at 14 gals and another at 7 gals.

Questions:

1 - With the goal of eliminating short cycling, which tank (7 or 14 gals) do you think would be a better fit with relation to the varied emitter array? I'll be utilizing all three available loop temperature inputs highest at 155f and lowest at 110f

2 - 2 or 4 pipe configurations? Idronics #17 gives a great overview of the two scenarios especially with respect to liquid flow rate and path through the tank. Would the two pipe config be a better fit with the three different types of heat emitters? I don't think a delay in heated supply is a concern in such a small dwelling. The fan coil gets the most use during the heating season, fan coil target temp will be at 155f, I'm thinking the 4 pipe might give a lower return temp?

Some points: 1. Domestic hot water will be from a Lochinvar Squire SIT-040 pumped direct, not through buffer. 2. 3 variable speed circs will be used on the system loops (radiant and towel warmer will share a circ utilizing two separate zone valves. 3. Total estimated capacity of all zones and equipment including boiler and indirect is approximately 17.2 gals, the buffers at 7 and 14 would bring it to 22.72 and 29.72 respectively.

Install will follow the Lochinvar manual; the manual doesn't address buffers directly but does list closely spaced tees or a low loss header, the manual depicts placement of the system return temp sensor just before the low loss header, I'm assuming this sensor obviously would now be installed in the center thermowell of the buffer tank, and the boiler firing rate and primary pump speed will react to the rising and falling thermocline?

The boiler includes a Grundfos UPMS primary loop pump, this will be wired to and controlled by the boilers 0-10v output.

Sorry in advance if a little long winded.😒

leonz

Having only 7 or 14 gallons is very small amount of buffer tank for the storage and use of thermal mass in the scheme of things.

hot_rod

Here is the math to answer the question. Basically the lowest turndown which should be around 15,000 for that boiler, the manual will tell you.

Against the smallest load (zone) it could see.

Shoot for at least a 10 minute run time.

I like the 3 pipe buffer piping. The Zilmet tanks work nicely, good connection size and location, well insulated with a wall mount bracket. I like everything except their piping diagram :)

Direct to load, 3 pipe buffer piping bottom right here.