Open Storage Limitations

[Nofossil]

I've had several conversations here and elsewhere about open vs. pressurized storage. My conclusion is that the big limitation of open storage is that you have a heat exchanger in between the boiler and storage, and that heat exchanger transfers less heat per minute as storage heats up. Eventually, you can't transfer the full output of the boiler and you start to idle.

Here's a screen shot from this evening. I reloaded at about 7:30, and the combustion temp climbed quickly to my setpoint of 1200 (divided by 10 here to fit on the screen). The combustion fan does a nice job of holding it right there - the spikes are purge cycles.

After a while both the top and bottom of storage have risen to the point that the boiler outlet reaches 190 - my 'overheat' point. I drop the target combustion temp to 700 and dump heat to the main floor for a few minutes until the outlet drops to 185. This will repeat for the rest of the burn.

With pressurized storage and decent stratification, I'd see nice low return temps until storage was almost completely full.

 

[tom in maine]

Certainly no dispute with what you posted. I guess the fact of the matter is what is the design and performance of the heat exchanger.
Since hx work on temperature differential, the only option is to use as efficient hx as possible and operate at a slightly higher temperature than a pressure tank. From what I hear from our customers, we can fully charge a tank to the desired temperature with minimal idling.

In my case, I do not use a heat exchanger. My boiler is all stainless and we pump tank water through it. I usually heat to 180F and have bumped to 190 when not watching. We circulate tank water through our radiant system.
Ferrous boilers need a heat exchanger.

The heat exchanger can be the weak point. Good design and sizing make the difference.

 

[Nofossil]

Yeah - mine is a copper coil with about 1/4 the surface area I wish I had. My neighbor has a ginormous flat plate that works really well.