Buffer tank control

njtommy

Finally getting around to install my buffer tank in the next two weeks.

My biggest question is what the best way to control the tank. Currently I'm just running hydro air and a fan coil in the garage using my boiler odr max 160 swt at 0 degrees from a Navien combi unit. I really don't need the water this hot, but I like the Boost mode for when we do night time set back 2 degrees.

I'm hoping by the the fall I will be starting my radiant floor project for my entry way, kitchen, and Laundry room. ( would also like to add panel rads in future) I'm not sure yet what my temps will be, but radiant will be under floor plates for floor warming.
Being that I'm using ODR I was wondering if my best option was a Taco 701 control to us as ODR and install the supply water temp in the buffer tank it's self. I just don't want to run the boiler if I don't have to.

I'm sure there are a few ways to do this, but I would like your guys input. At this rate I wish I installed The Lochinvar boiler do to having the ability to run ODR, supply water temps sensor, and indirect sensor.

Thanks for the help
Tommy.

SWEI

I'm not familiar with the 701, but an SR501-OR-4 is enough to control a buffer tank. The PC700-2 is essentially a rebadged Tekmar 256 AFAIK.

ISTR you were running a Navien there, so you should be able to use the onboard ODR for the tank and then mix down for the radiant with something like an iSeries-R valve or an injection pump. Or maybe I misunderstood the question.

njtommy

Yes sorry PC700-2 for some reason I was thinking 701.

Yes for the Radaint floor I was planning using a Taco I valve.

My main thing is I'm trying not to run the boiler for every single call for heat. If the buffer tank can handle the load I would like to let it pull all the btus it can out of it before refiring the boiler.

SWEI

If you set the curve on the iSeries-R just below the curve on the Navien and let the Navien read the system temp off a thermowell in the buffer tank, that should get you pretty close.

If you really want to maximize the run time out of the buffer, you can use an outboard ΔT control measuring input (from the tank) and output (to the floor zone) temps across the iSeries-R. Set it so it closes at 2°F and reopens at 50°F and wire it to the boiler TT terminals. The boiler ODR curve can still limit the tank temp to less than that.

njtommy

Not sure if I could pull the supply water sensor out of the unit to install it in the dry well of the buffer tank.

hot_rod

Many of the newer boilers have an external sensor included or as an optional part. I would think the sensor supplied on the boiler could be pulled and moved to the tank?

Are you piping the tank with the two pipe method?

njtommy

Yes two pipe seems to be the better way to with using fan coil units and hydro air.

I'm a little reluctant to pull the supply sensor out of the boiler because I'm not sure if it will effect the DHW side of it do to the fact it's a combi boiler. I will give Navien a call and see.

Boon

@njtommy how did you end up controlling the buffer?

I have the system sensor in the buffer tank and the boiler [KHN] never shuts off - runs 24 hours a day. I think I'm ok with this, but the system & supply temperatures swing like a pendulum (see chart attached). For example, a setpoint of 120 might see a buffer tank temperature as low as 117 and as high as 125. I set a generous differential offset to compensate for the overshoot, which I've seen as high as 12° under certain conditions and when recovering the buffer after a DHW call.

I'm not sure I really need to do anything about this. A better design would have a more direct relationship between the system sensor and the boiler supply temperature. Maybe a real solution is remove the buffer.

I think the pump speeds is a good part of my 'problem.' The 15-58 is on medium and the Alpha is on Auto, though it runs at 43 watts almost all the time. I don't know if controlling both/either pump speed is practical.

Experimenting, I disconnected the system sensor from the buffer so the boiler produces a consistent output @ setpoint temp but then the differences in pump speed + mixing out of the buffer gives me, it seems, a system temperature that is lower than I'd like to see, it is difficult to know what is going on in the buffer.

Thoughts on this situation appreciated.

David

NY_Rob

Is that a Boiler Buddy buffer tank?
If so, their install schematic is a bit different than your setup...
With their recommended setup- would you even need to relocate the system sensor? Seems like you could leave it as is and let the boiler control the CH water temps because you have hydraulic separation.

Boon

It is a boiler buddy. Because this was my first rodeo I originally perceived some space constraints and opted for a 2-pipe installation. I also thought 2-pipe install would reduce mixing in the buffer and I'm still under that impression.

I don't understand how 4-pipe changes [improves] anything. I just re-read idronics 17 on stratification and it says [paraphrase] (a) mixing in the buffer should be avoided; (b) 2-pipe avoids mixing [preserves stratification]. But it also says, "the timing and rate at which water is added and removed from the tank," affects stratification. So I think my control issue lies with circulator speeds more so than 2 or 4-pipe arrangement.

With a primary circ pushing more water than the secondary circ, I'm adding heat to the buffer through the supply. With my secondary circ pushing more water than the primary, I'm actually adding heat to the buffer tank through the return water. The later seems backwards, when satisfying the buffer the boiler input temp can sometimes exceed the set point, and while the buffer tank is satisfied the boiler's output temperature dips below set point, which also seems wrong.

At the end of the day, though, I have better heat than I ever thought I'd have and my boiler runs 24 hours a day.... there are worse problems. The more threads I read on this topic, the more I think my configuration is acceptable, yet something seems wrong about it.

njtommy

I was finally able to pipe in my buffer tank in late October. I ended up 4 piping a 22gallon Heat Flo buffer tank. It's a short fat tank. From what I understand I would of been better off with a taller tank to drive stratification. I may repipe it this summer if I have time. I've just been too busy with work and the family to do hard wood floors and radiant floor heat as well.

I have the boiler set for a 20f offset from supply water set point right now. I have to buy a Johnson A419 control or a Taco odr control not sure which yet. Yet again just haven't had the time to figure it out which way is best.

Brewbeer

What I've noticed with my system (Loch WHN055 and Boiler Buddy 30 in 4-pipe configuration) is that the buffer makes the boiler "think" that the temps coming out of the boiler are lower than they actually are, which causes the boiler to ramp-up when it's not necessary to meet the heating demand load. I'm wondering if it wouldn't be better to move the boiler's system sensor out of the buffer tank and into the piping that connects the boiler to the buffer.

NY_Rob

^ or move the sensor back into the boiler again in the OEM configuration?

hot_rod

Boon said:

It is a boiler buddy. Because this was my first rodeo I originally perceived some space constraints and opted for a 2-pipe installation. I also thought 2-pipe install would reduce mixing in the buffer and I'm still under that impression.

I don't understand how 4-pipe changes [improves] anything. I just re-read idronics 17 on stratification and it says [paraphrase] (a) mixing in the buffer should be avoided; (b) 2-pipe avoids mixing [preserves stratification]. But it also says, "the timing and rate at which water is added and removed from the tank," affects stratification. So I think my control issue lies with circulator speeds more so than 2 or 4-pipe arrangement.

With a primary circ pushing more water than the secondary circ, I'm adding heat to the buffer through the supply. With my secondary circ pushing more water than the primary, I'm actually adding heat to the buffer tank through the return water. The later seems backwards, when satisfying the buffer the boiler input temp can sometimes exceed the set point, and while the buffer tank is satisfied the boiler's output temperature dips below set point, which also seems wrong.

At the end of the day, though, I have better heat than I ever thought I'd have and my boiler runs 24 hours a day.... there are worse problems. The more threads I read on this topic, the more I think my configuration is acceptable, yet something seems wrong about it.

With a two pipe, you don't mix the tank as much as with 4 pipe. So you can leverage the stratification and exergy (usefulness) of the tank.

With 4 pipe, unless both sides are the exact same gpm flow rate there is always mixing going on.

With a two pipe if the boiler gpm out and the load gpm are exactly the same nothing flows in an out of the tank. No need to have a buffer, or flow through it when load and boiler out match exactly.

Of course that is a rare condition in multi zoned hydronics.

With a 4 pipe buffer, similar to a hydro sep, you can move the sensor to get the best results. Some installers prefer the sensor on the supply piping, downstream from the separator.