Differential Controller Theory

[EricV]

Hi All

I am into year 3 of my system and just love the thing and love reading about everyone else's systems.

I am trying to make up a good diagram of it to post as I got the auto reversing coils working great and would like to share that and get feedback as well.

I have determined I need a differential controller to turn off the pump to the boiler when the tank is warmer than the boiler. I think when the pump continues to run after the fire dies out it is then bringing heat back to the boiler from the tank and wasting it.

My question is where to install the sensor for the tank. My gut says at the top of the tank where with proper stratification is the warmest layer.

Is my thinking correct?

I am now looking at the different brands out there, any recommendations from experience?

Thanks in advance.

 

[DaveBP]

I have a Tarm Solo 40 also. Do you have the installation instructions with the extra pages that explain some changes for systems that use storage? It's just a few extra pages inserted into the regular manual.
Those substitute instructions have you put the circulator aquastat into a well in the chamber at the top of the fire tubes instead of in the boiler water. Thus the circulator comes on when the fire first starts going and stops when the fire has burned down to flue temps close to 200F. Is that how you have yours installed?Is it still circulating too long?

 

[EricV]

Odd, I don't think so. I'll look when I get home tonight. My circ runs from the switch on the front panel, either on or off, no temp control at all.

Thanks, I'll investigate that when I get home.

 

[Nofossil]

Hi All

I am into year 3 of my system and just love the thing and love reading about everyone else's systems.

I am trying to make up a good diagram of it to post as I got the auto reversing coils working great and would like to share that and get feedback as well.

I have determined I need a differential controller to turn off the pump to the boiler when the tank is warmer than the boiler. I think when the pump continues to run after the fire dies out it is then bringing heat back to the boiler from the tank and wasting it.

My question is where to install the sensor for the tank. My gut says at the top of the tank where with proper stratification is the warmest layer.

Is my thinking correct?

I am now looking at the different brands out there, any recommendations from experience?

Thanks in advance.

In my estimation, this is an area of compromise that's unavoidable unless you have a variable speed boiler circulator.

Towards the end of a burn, storage is hot and the boiler's heat output is dropping. If you maintain constant flow, then you'll see a drop in boiler outlet temp.

As long as the boiler outlet temp is hotter than the top of storage, it's a no-brainer. However, in a well stratified system you start to get into a gray area as the boiler outlet drops to the same or slightly lower than the top of storage. You're still extracting heat from the boiler, but you're starting to lose stratification.

Technically, as long as the water leaving the boiler is warmer than the water entering the boiler, you're still extracting heat. Practically, you'll want to stop circulating some time before that happens. In a really well stratified tank, you could have the top of storage at 180 and the bottom at 120. If the boiler outlet is 160, you're still getting a lot of heat out of it.

At the end of the day, it's a tradeoff between maximizing stratification (and therefore useful heat) vs. maximizing heat recovery from the boiler.

With a variable speed circ, you can lower the flow rate through the boiler to maintain a higher outlet temp. Otherwise, you have to decide where the point of diminishing returns is. I don't think there's a hard-and-fast answer, though I think it's safe to say that a good rule of thumb would be to stop circulating if either of these conditions is met:

1) The heat gain through the boiler drops below some threshold - perhaps 5 or 10 degrees.
2) The boiler outlet is less than 5 or 10 degrees above your 'minimum useful temperature'.

There may be additional rules that make sense in any specific installation.

 

[kabbott]

Hi All

I am into year 3 of my system and just love the thing and love reading about everyone else's systems.

I am trying to make up a good diagram of it to post as I got the auto reversing coils working great and would like to share that and get feedback as well.

I have determined I need a differential controller to turn off the pump to the boiler when the tank is warmer than the boiler. I think when the pump continues to run after the fire dies out it is then bringing heat back to the boiler from the tank and wasting it.

My question is where to install the sensor for the tank. My gut says at the top of the tank where with proper stratification is the warmest layer.

Is my thinking correct?

I am now looking at the different brands out there, any recommendations from experience?

Thanks in advance.

Nofo has much more experience then myself but I think in general:

#1 as long as boiler is warmer than tank top---charge the tank.
#2 as long as boiler is warmer than tank middle and the same as tank top.....charge the tank.
#3 as long as boiler water is returning colder than the supply.....charge the tank

Number 1 and 2 will not result in de-stratification, number three may result in de-stratification but will still add to total stored heat.

I have sensors at the top, middle , and bottom of my tank.
Judging from my own storage I would say with a single sensor I would put it at 60% to 80% of the tank height and charge the tank
when boiler is a few degrees warmer.

 

[DaveBP]

Excuse me, Eric. One of the hazards of working 2nd shift is that I can be pretty fuzzy brained in the morning. The probe that goes in the flue gas is the control for the draft fan, not the circulator as I stated. The circulator is controlled by one of the other aquastats in a well in the top of the boiler water.

One of the differences in the "Alternate Instructions" for use with heat storage is the Lo-Limit Thermostat (inside the control panel, looking down from top on mine). It is set to90C(190F) instead of the 60C(140F) that is used with systems without storage. Check with Bioheat's tech support people but that might make a difference if yours came set for no storage.

EDIT: Trying to read both manuals and type at the same time, I muffed the centigrade figures. I corrected them above.

 

[EricV]

Excuse me, Eric. One of the hazards of working 2nd shift is that I can be pretty fuzzy brained in the morning. The probe that goes in the flue gas is the control for the draft fan, not the circulator as I stated. The circulator is controlled by one of the other aquastats in a well in the top of the boiler water.

One of the differences in the "Alternate Instructions" for use with heat storage is the Lo-Limit Thermostat (inside the control panel, looking down from top on mine). It is set to 60C(190F) instead of the 60C(140F) that is used with systems without storage. Check with Bioheat's tech support people but that might make a difference if yours came set for no storage.

I was almost sure that is what you meant but wanted to check first. I've been up all night with one of our Goldens having puppies so I know the fuzzy brain syndrome.

Thanks