Buffer tank questions???

Jason_13

I have been thinking about this whole buffer tank idea and some questions come up.

1. Is it best to place on supply or return?  I saw one placed on return just to add water volume no aquastat.

2. If on supply would it not be better to now have an aquastat on the tank as it will make ODR harder since the aquastat only will have one setting.

3. If the boiler does not have WWSD the tank would maintain all summer.

4. Wire the aquastat in series with the end switch of a multi-zone relay.

Thanks in advance

Brad White

Some thoughts, IMHO

1. Is it best to place on supply or return?  I saw one placed on return just to add water volume no aquastat.



A. It depends. I had one on my non-modulating Monitor MZ some years ago, to forestall short cycling, mounted on the return but in parallel to the return line, not in series. In this way, I could bring the buffer volume on-line or off-line as needed. When the return temperature came back elevated (120F or above), a circulator started to pass this to the tank, forcing the presumably colder tank water towards the boiler to prolong condensing. Cut my cycling about in half, depending. (More details to it, but that is the idea for discussion.) But the emitters (radiators) always saw hot water first without delay. I call this the back-end buffer. This is pretty much what you describe.



If on the supply side, the buffer tank acts as your primary set point location and is typically kept hot, adding volume. If no set point in the tank then you will wait to get heat to your space, not a good thing, In a supply-side application, the buffer tank is, in my opinion, a very good hydraulic separator in addition to a volume reservoir.



2. If on supply would it not be better to now have an aquastat on the

tank as it will make ODR harder since the aquastat only will have one

setting.



A. If you do have ODR, the aquastat setting in the tank would really be a sensor and set the target temperature of the ODR output. Constant circulation is your friend here.



3. If the boiler does not have WWSD the tank would maintain all summer.



A. Not if your ODR target temperature is the tank temperature. But if you use an indirect, you of course would depart for the indirect after the boiler but before the buffer tank.







4. Wire the aquastat in series with the end switch of a multi-zone relay.



A. If you use limited on-off control, sure. But I would prefer use of the tank as an ODR target location IF using it on the supply. But the best benefit as I saw, was on the return side. The net effect I called "a thermal pole vault". Once the tank was fully charged and that heating cycle depleted, the tank would also be depleted until the return temperature dropped again (crude, I admit), but that stored capacity allowed the burner to rest and the system to coast for a while, maybe another ten minutes.  I admit I put the buffer tank in on the fly, and would do it differently if today.



My $0.02 anyway!

NRT_Rob

Many ways to skin a cat.

I prefer tank on the outlet of the heat source. the idea is to avoid satisfying the load before you finish charging the tank. As long as your flow rate is adequate the boiler will settle into a modulation range until you heat the tank, get the hot water out to your load, and then return elevated temperatures to the boiler. This should also keep the return colder just a long as the mass on the return... you can't cheat physics, energy is conserved.



I don't like maintaining buffer tanks. I use them like fat pieces of pipe, totally passive.



Usually this means either the boiler is still doing its own sensing, or I have a sensor on the 'hot out' of the buffer tank.