Adding a buffer tank to a mod/con boiler

[Homemachineshop]

Hello everyone, this is my first time posting on this wonderful forum, and I have a question about heating the 30x3 garage I am building.

I'm installing a Navien mod/con combi boiler in the shop, the smallest one they make, which can turn itself down to 12'000 btu's, which is still quite a bit for my shop. Max heat loss is estimated at 25'000, so the boiler may be short cycling quite a bit in the spring and fall, when the heat demand may only be 6-8000.

I've got about 950 feet of 1/2 pex running through a 6" slab

After researching online I came across guys adding a small hot water tank to the system to act as a buffer tank, so the boiler will get longer cycle times.

Any heating experts out there think this is needed? Or will the slab thickness and large thermal mass help me out?

 

[Firebrick43]

Have you purchased the boiler yet?

If not consider a htp Phoenix light duty. It's basically a mod/con in a buffer tank. It's available thru home cheapo branded as a Westinghouse.

I used one for exactly the reason your considering a buffer tank. It's 76k btus and I still have it down to 65% running my 1450sqft icf house.

You can also set a higher differential on your slab thermostat to prolong cycle times. Many thermostats have a two degree differential but many programable ones can be programmed differently. Having a 3 or 4 degree differential will have longer run times. .

Also are you planning on a mini split for cooling? Getting one with heat maybe an advantage in the shoulder seasons and shut the floor heat down.

 

[Homemachineshop]

Boiler has already been purchased, but I'll look into the programable differential thermostats, thanks.

 

[chinboys]

I have a modulating and condensing boiler too and I use the outdoor thermometer reading to inform my boiler on how hot to make the water to feed by in-floor PEX and baseboard emitters. The efficiency of the boiler is directly related to the return water temperature. So on those really cold days, my boiler makes hotter water but it is fed cooler water until the returning water is within 5- 10 degrees of the supply water temp. Then the boiler shuts down but the circulator pumps move the water until such time as the return water temp is >10 degrees of the design water supply temperature and the boiler fires up again.
So unless you need the buffer tank to heat up domesticate hot water, you ought to just get a thermostat and allow your boiler to do it thing. It will cycle per design and based on its heat load.

 

[Jackfre]

There is definitely a case to be made for buffer tanks. I just did a system similar to yours for a friend, but we decided to for go the BT. The boiler has a modulating pump so that even if the unit must shut down, the pump will maximize the heat delivery prior to restarting the burner. You live in one of the nicest places in NA. The climate is very moderate. I think I would run it for a while and see how the system behaves prior to going to the expense of the BT. Configure your piping to accomodate it.