Outdoor Reset

[Dutchie84]

If you are using some kind of mixing device to control the temperature of the circulating fluid, knowing the outside temperature allows the controller to set the ideal temperature of the circulating fluid to have you house reach the desired temperature. The goal is to use the flow temperature to get your room t the desired temp not the t-stat. Below are examples of the curve which determines my flow temperature. I get to choose the curve (move it up or down) and adjust it (I can move the right hand end point out while leaving the left hand end point in place.

View attachment 195927

John, why would you have to change your curve for the time of year? Your emitter size didn't change, the insulation levels in your house didn't change, so your house's heat loss per degree F from inside temp to outside would be the same all year. Isn't that what ODR is for?

 

[JohnDolz]

John, why would you have to change your curve for the time of year? Your emitter size didn't change, the insulation levels in your house didn't change, so your house's heat loss per degree F from inside temp to outside would be the same all year. Isn't that what ODR is for?

I find that the curve I can use in the should season doesn't quite do it when it get really cold. I could choose a curve that meets my winter needs and leave it there but in the shoulder seasons it produces flow temp just a little hotter than I need (circs end up going on and off). Now that I think about it, I could probably play with the adjustment feature and maybe get to where I need. I have taken the lazy way and just tweak the curve once or twice a year (it literally takes 2 seconds).

 

[JohnDolz]

Or....use a mixing valve or similar to vary the coolant temperature based on indoor temperature: keep it constant...indoor temperature stabilizes with mix percentage at some point so that heat input = heat loss at the setpoint. Setpoint changes, mix percentage and coolant temperature changes until it reaches some steady state at the new setpoint. Would be better for setback recovery, plus you still have that constantly running pump you like.

I think maybe the attraction of ODR is for fossil fuel boilers to minimize losses, ie to run it at a lower temperature. With a tank, what you have is what you have-it has a certain amount of BTU's in it at any one point in time. You can use it in different ways for different emitters at the same time, as in fast setback recovery in baseboard, and maybe not so great a delta for radiant.

I wouldn't know where to buy this stuff though...I just dreamed it up, lol

You are over my head sir. I can only tell you that if you believe the concept of pulling as little hot water as possible from storage equals the most efficient use of stored BTU's the having ODR pick the lowest possible design temp to meet the need is the reason. You are correct on Recovery, I do not know how Sparky's ODR address it but with the Effecta I also have an Indoor sensor. This sensor keeps an eye on actual indoor temp vs are temp. When it is time for my Recovery to begin, this sensors sees that actual is wll below target so it overrides ODR. It then uses the Outdoor Temp and the delta of actual vs. target indoor temp and i chooses a desired design temp. As Indoor Temp approaches target this control is constantly adjusting flow temp until it finally gives control back to ODR. I know Mr. Caldwell's leverages inside temp as well as the Outdoor temp.

 

[maple1]

I think I have been here before. Maybe this time last year.

I have no doubt ODR gives a more constant smoother even heating operation through the complete range of storage supply temps, but I will still stick with my feelings that it won't extend your storage capacity or let you go longer between burns or run storage lower - all else being equal. At the bottom storage temps (as storage nears depletion), they are both doing the same - circulating the same lower temp water, constantly, until the house starts losing temp & you need to recharge.

That's my story & I'm sticking to it.

 

[nhtreehouse]

John, why would you have to change your curve for the time of year? Your emitter size didn't change, the insulation levels in your house didn't change, so your house's heat loss per degree F from inside temp to outside would be the same all year. Isn't that what ODR is for?

Very interesting discussion here...

While I don't have a boiler at the moment, I'm in the research phase and ODR is a very interesting aspect. What I think is really cool about the forum is we have all sorts of users reporting their actual experience which other members can then offer other insight into, and in the end, we all learn something.

So, on moving the reset curve based on season... In theory, you shouldn't have to. But it's important to remember that ODR is a first order correction. By that term, I mean that it provides a prediction of the correct supply temp to the emitters based on knowledge of the outside and (optionally) inside temperature.

What happens on days when the wind blows the "oysters off the rocks?" Well, the heat loss of the building increases and the circs will flow 100% of the time and not keep up. Or conversely, if the building has high external gain, from, say solar radiation, the circs will throttle down the heat distribution from the emitters to prevent overheating.

So I suspect JohnDolz's heating system has some external loss during mid winter which is not present in the shoulder seasons. Or, it would be just as valid to say that his system has more external gain during the shoulder seasons than in mid winter. Potato, Potatoe...

My 2 cents...

 

[nhtreehouse]

I think I have been here before. Maybe this time last year.

I have no doubt ODR gives a more constant smoother even heating operation through the complete range of storage supply temps, but I will still stick with my feelings that it won't extend your storage capacity or let you go longer between burns or run storage lower - all else being equal. At the bottom storage temps (as storage nears depletion), they are both doing the same - circulating the same lower temp water, constantly, until the house starts losing temp & you need to recharge.

That's my story & I'm sticking to it.

Hey Maple,

That was what I was thinking when I first read Sparky's and JohnDolz's posts to the effect that ODR gives longer run time from storage. But I got to thinking about it and I believe there may be some second order effects which are not at all obvious. I'll throw out a couple of theoreticals which support the ability to run longer from storage when mixing down the tank supply with ODR:

1. While I agree 100% that the energy in water is a fixed and calculate-able number, it's the available energy to the supply system which we are trying to figure out. Themo-heads would call this Exergy, and it is most definitely not equal to the energy in the tank. The Biomass Training sticky has some discussion on this.
2. On Exergy, it seems to me that mixing down the tank actually increases the available energy in the tank. The reason why is the flow through the tank supply/return ports is lower using ODR than with a straight supply from the tank to the system plumbing. The reason why is system flow is made up of system return flow and supply from the tank. As we are turning over the tank less, stratification is promoted, Exergy is higher.
3. Loss in system piping is less as the heat loss from system supply piping is proportional to some delta-T. Now, you can argue that all system piping loss is inside the building envelope, and there is some merit to that. It all depends on the system piping. If that piping is inside conditioned space, then, yes, this is the case. As soon as you put a supply pipe in a wall, that pipe looses heat both to the outside and inside the building envelope. It gets complicated.
4. The last and quite hard to quantify effect might be that the heat emitters work more efficiently with constant flow. No experience here, but obviusly if the emitters are more efficient, then the effective Exergy of the system increases.

I'm planning my system to have storage in an outside building. Mixing down system supply in that building before shoving it over to the house in underground pex will certainly lower the system piping loss.

On running with a lower supply temp at the end of the storage interval, I am truly stumped. Maybe I need to get my "I violate the laws of physics" card punched?

Everything in this post is strictly theoretical and as always, your mileage may vary...

 

[Dutchie84]

Hey Maple,

That was what I was thinking when I first read Sparky's an JohnDolz's posts to the effect that ODR gives longer run time from storage. But I got to thinking about it and I believe there may be some second order effects which are not at all obvious. I'll throw out a couple of theoreticals which support the ability to run longer from storage when mixing down the tank supply with ODR:

1. While I agree 100% that the energy in water is a fixed and calculate-able number, it's the available energy to the supply system which we are trying to figure out. Themo-heads would call this Exergy, and it is most definitely not equal to the energy in the tank. The Biomass Training sticky has some discussion on this.
2. On Exergy, it seems to me that mixing down the tank actually increases the available energy in the tank. The reason why is the flow through the tank supply/return ports is lower using ODR than with a straight supply from the tank to the system plumbing. The reason why is system flow is made up of system return flow and supply from the tank. As we are turning over the tank less, stratification is promoted, Exergy is higher.
3. Loss in system piping is less as the heat loss from system supply piping is proportional to some delta-T. Now, you can argue that all system piping loss is inside the building envelope, and there is some merit to that. It all depends on the system piping. If that piping is inside conditioned space, then, yes, this is the case. As soon as you put a supply pipe in a wall, that pipe looses heat both to the outside and inside the building envelope. It gets complicated.
4. The last and quite hard to quantify effect might be that the heat emitters work more efficiently with constant flow. No experience here, but obviusly if the emitters are more efficient, then the Exergy of the system increases.

I'm planning my system to have storage in an outside building. Mixing down system supply in that building before shoving it over to the house in underground pex will certainly lower the system piping loss.

On running with a lower supply temp at the end of the storage interval, I am truly stumped. Maybe I need to get my "I violate the laws of physics" card punched?

Everything in this post is strictly theoretical and as always, your mileage may vary...

Tree house. When are you planning on doing you system? I am planning a similar system in my new house. I hope you will keep us up to date with lots of pictures [emoji6]


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[nhtreehouse]

Hey Dutchie,

Assuming a fair and following wind, this year sometime. With the amount of work I have to do just to prep for the install, it's going to be a miracle if I'm firing next winter. We shall see!