Buffer tank control strategy

@hot_rod OK thank you. So it sounds like 3-pipe would be best. That would also help in the one loop of high mass gravity conversion that has a temperature drop of 30+ F with a flow rate of about 6 gpm. Because of the high mass in that loop and relatively low flow rate, response time is long, and a 4-pipe buffer could prolong the response time if the boiler was recharging the tank when that zone called. In the 3-pipe config, boiler output can go directly into that loop for fastest response, and the remainder of the flow rate through boiler will go either through bypass to maintain return at 130, or into the tank.

I will take another try at a schematic using 3-pipe and stratified tank. What about the variable speed circ to get higher water temps at the top of the tank as needed with ODR? And is my thermostatic bypass set up correctly?

@hot_rod Thanks, that is an impressive range of storage tanks.

We met with the HVAC contractor this afternoon, and he understood the situation at my sister's house. He said he's seen a lot of older houses in her town with original gravity conversion systems that have been hacked up over the years and Frankensteined with baseboards and too many zones. He did agree that a buffer tank on the existing boiler would help, but ultimately I agreed with him that a new mod-con was the best solution. He installs Lochinvar and Viessman, and the GC who was there too said he has a Lochinvar and is happy with it. Sister also had a Lochinvar in the previous house. So it will probably be a Lochinvar, with a smaller buffer tank than we would have put on the existing boiler, but still useful for the small zones. It will be more $$ up front, but probably best to rip the Band Aid off now and do it right.

@hot_rod Thanks, I tried to convince him that a 110 would be sufficient, but he was more comfortable with a 155 with a buffer tank. I figured it wasn't worth arguing.

@hot_rod That's what I figured too. A small buffer tank won't add that much cost to the install and will be useful anyway. Still a win.

@hot_rod OK thanks. I didn't get into piping details with him, but he mentioned using a hydraulic separator and running only the baseboard loops off the buffer tank. The gravity conversion loop would run direct off the boiler. I guess that's almost the same as a 3-pipe arrangement.

I think he will propose a Knight. Sister had that in previous house, but it didn't appear to have ODR. I explained ODR to her and said we should make sure it gets set up right. Not sure why previous installer didn't do ODR, but this guy seems very competent so I expect ODR will be in the proposal.

@hot_rod OK, thanks, I'll put my system designer hat on (with your assistance) and give him a piping and control diagram. Here's my attempt at drawing up your schematic. Per Siegenthaler pg. 744 I included a check valve to prevent reverse thermosiphon, and added a purge inlet valve. If we used the Lochinvar Knight, the internal circulator would run the boiler loop, so the external circ in my diagram would go away. I assume there's no special control diagram I need to add, just an ODR sensor that comes with the Lochinvar Knight? So we would re-use the existing Taco 6-zone controller and probably add another zone controller because we will have to add a circ to the one cast iron radiator that was repiped in 3/4" and turn that into its own additional zone.

@hot_rod Great, thank you! I will drop the diagram off at the HVAC contractor this afternoon.

So if I understand, the boiler would run on ODR using a Lochinvar-provided outdoor sensor, plus another sensor that goes in the buffer tank to provide water temperature input to the ODR. And all the ODR logic is inside the Lochinvar Knight, so no separate ODR controller is needed.

Then the Taco zone controller only runs the circs, with no control over the boiler. Boiler only runs when the buffer temp drops below threshold, correct?

I also added a dirt separator before the ECM, since I've read here that iron crud inside gravity conversion systems can jams ECM's.

@hot_rod Great, thank you. I read the Lochinvar boiler manual last night trying to understand the programming for ODR, etc. I think I got the main points. It may take us a while to dial in ODR setpoints in the first few months. I'll be curious to see what water temp is needed on design day.

@hot_rod Ok thanks. I just got the proposal from the HVAC company. I won't quote prices but it was probably close to 3x materials, which I guess is reasonable around here. He didn't include a piping diagram, and I suspect he had already done the pricing by the time he got my proposed piping diagram, so needless to say it looks like he didn't take your good idea for a 3-pipe buffer. From the BOM, this is my best guess at his piping. I guess it will work, but it's not as elegant as the 3-pipe buffer because it forces all supplies through the 30-gallon buffer instead of allowing supplies to go direct to load. It's probably acceptable, but I'm posting it anyway just in case there's anything glaringly wrong. Again, this is my best guess based on BOM.

@hot_rod and @EBEBRATT-Ed Thanks for your inputs. I just found Siegenthaler's free online piping software and produced the following better rendering of Bob's 3-pipe buffer tank setup. Is it worth sending this back to the HVAC guy and asking him to re-price based in this schematic? I expect it will be more $$ because of larger pipe diameters and additional circs, but if this is the best way to pipe it, then $1k or so extra isn't a deal-breaker.

@hot_rod OK great. I will remove the extra check and send this over to the heating company. Many thanks for your help. I'll post a picture after the install.

@hot_rod will know the answer to the question above. I too still don't understand how the Knight is supposed to be wired/programmed so that it runs only in response to the SYSTEM SENSOR in the buffer tank. That's after I've read the Lochinvar manual several times. Lochinvar should have made that clearer, as you would think that's a control scheme many setups will want to use.