Buffer Tank Questions

[velvetfoot]

I ran my system with the oil boiler, no pellet boiler. I have some questions based upon what I saw, and copied this from my install thread for more visibility, if that's okay.

I started up the oil boiler a little while ago and had it charge up the tank. Interesting notes: the top tank sensor got warm, 115f, while the boiler temp was 130, just when the Caleffi is supposed to start opening. The return line was indeed felt cool that whole time. Somehow the water was circulating.

Another interesting thing: with the oil boiler aquastat set at 190 or so, based on the boiler temp gauge which is new (the aquastat did not agree), the tank got to 186 top and 172 bottom when the boiler turned off. The call for heat continued since both top an bottom were set at 175, and therefore the pump kept running. I let the pump keep running for a while and there was no change in temperature, except the boiler temperature went down, mostly slowly. I changed the bottom setpoint to 170 and the heat call and pump went off. I'm thinking that the boiler aquastat is 15 degrees or so, so the pump might run a long time and the field controls oil flue damper would stay open, losing heat. I wonder how the pellet boiler will react and whether I should do anything about it, especially since the max temp of the pellet boiler is 176.

edit: Now after a shower, the top sensor is 182 and the bottom sensor is 112.

 

[ewdudley]

The return line was indeed felt cool that whole time. Somehow the water was circulating.

Return line at the boiler or return line feeding into the mixing valve?

I changed the bottom setpoint to 170 and the heat call and pump went off. I'm thinking that the boiler aquastat is 15 degrees or so, so the pump might run a long time and the field controls oil flue damper would stay open, losing heat. I wonder how the pellet boiler will react and whether I should do anything about it, especially since the max temp of the pellet boiler is 176.

(Shared flue?) The flue damper should activate and prove according to call-for-fire, not call-for-heat.

 

[velvetfoot]

Here's a rough one. The supply pipe to the tank now goes to the top.

 

[ewdudley]

I wonder how the pellet boiler will react and whether I should do anything about it, especially since the max temp of the pellet boiler is 176.

Since the pellet boiler is a modulating boiler it should use its supply temperature to control the rate of fire. The pellet boiler circulator should run continuously whenever the pellet boiler is firing, and the buffer tank will help average out the demand on the boiler so it can run at a more constant rate of fire.

The oil boiler should see a call-for-heat go active whenever the top of the tank drops below top-of-tank setpoint (e.g., 180 degF), and the call-for-heat should stay active until the bottom the tank rises above bottom-of-tank setpoint (e.g., 175 degF). I think you will need a latched relay arrangement to implement the logic.

 

[velvetfoot]

Since the pellet boiler is a modulating boiler it should use its supply temperature to control the rate of fire. The pellet boiler circulator should run continuously whenever the pellet boiler is firing, and the buffer tank will help average out the demand on the boiler so it can run at a more constant rate of fire.

That's the idea, I guess. This pellet boiler should have a lower firing rate than the oil boiler as well should help. I monitored temperatures and it came out like this:
Oil Boiler Temp Top Bot
190 182 170

So, there's like a 10 degree difference between the boiler temperature, as measured by the dial thermometer in the boiler, and the top of the tank, as measured by a surface mounted sensor another ten degrees less than that. These differences continued as the pump continued to run since the lower sensor wasn't satisfied (yes they are latched, as per your response in another thread, and as acknowledged in my install thread!), and all it seemed to do was warm the basement, until the aquastat got to 170 and the burner started again.

A part of the issue is the differential of the oil boiler's aquastat. It seems to be around 20 degrees. So, it gets stuck in never never land which just causes the boiler pump to run and run since the controller is latched and it has to satisfy both top and bottom sensors before terminating the call for heat. It took 27 minutes of heating the basement via the uninsulated pipes to get the boiler down to 170 again, and it got to roughly the same temps as noted above before turning off again.

I just kept the top sensor at 175, lowered the bottom sensor setpoint to 165, both with 20 degree differentials, and called it good for now. Will have to dial the pellet boiler in when that gets started up,tweak the setpoints, but it will be lower than the oil boiler, and when the oil boiler is running, which hopefully won't be much, it will be dialed back as well. The goal being a well charged buffer tank to the maximum of the pellet boiler's ability.

 

[ewdudley]

I just kept the top sensor at 175, lowered the bottom sensor setpoint to 165, both with 20 degree differentials, and called it good for now.

I guess I was assuming that the oil boiler high limit was set high enough such that the burner would keep running for as long as the tank lower setpoint was not satisfied. Sounds like you've got it right, the tank lower setpoint has to be low enough that the oil boiler can keep burning without going off on high limit.

You could run the numbers on estimated gpm through the oil boiler, and deltaT through the oil boiler given the net output of the oil boiler, and that would give you and idea of what the maximum tank lower setpoint can be relative to the oil boiler high limit.

If there is a 15 degF rise through the boiler then the tank lower break-on-rise setpoint has to be at least 15 degF less than the oil boiler high limit.

 

[velvetfoot]

You could run the numbers on estimated gpm through the oil boiler and deltaT through the oil boiler given the net output of the oil boiler and that would give you and idea of what the maximum tank lower setpoint can be relative to the oil boiler high limit.

I used the HSS program, but it only uses the load to produce a boiler size, which can't be adjusted independently from load. HSS says 5.9 gpm and 5.7 of head. I have the Ecocirc maxed out. I can look at the pump curve and get actual gpm I guess.

Anyway, what I'm saying is, how do I do that?

 

[ewdudley]

I used the HSS program, but it only uses the load to produce a boiler size, which can't be adjusted independently from load. HSS says 5.9 gpm and 5.7 of head. I have the Ecocirc maxed out. I can look at the pump curve and get actual gpm I guess.

Anyway, what I'm saying is, how do I do that?

A WAG would be fine. With your B&G circ and those short fat pipes the head is pretty low, glancing a the pump curve call it 15 gpm. Divide net boiler output of (for instance) 125,000 btu per hour by 15 times 500 predicts temperature rise through the boiler of 17 degF.

So tank lower setpoint has to be at least 17 degF below oil boiler high limit. So try 20 degF and if that's not low enough tweak it down a bit.

 

[velvetfoot]

Thanks very much.