I can't figure out why one uses the outdoor temperature to modulate the water temperature in a radiant system given how poorly correlated to the actual heat load of a structure outdoor temperature really is.

For example on a cold sunny day it is possible that a structure might have a load of near or even zero whereas on a warmer one with complete cloud cover and wind the load could be significant. And the load of a structure will be considerably different at the same outside temp depending on if it is day or night and if it is night whether it is cloudy or clear (or even if drapes, if any, are drawn or not).

Given that it is really pretty straight forward to measure the actual heating load directly why use a proxy at all, let alone one that is at best an approximation and at worst wrong?

Is it just a matter of long standing custom from a time before there were dirt cheap electronics that could calculate the real load so the best one could do was hook up a thermostat to vary the water temp?

Perhaps I am missing something but I just don't get it? Can anybody help set me straight?

I agree with most of what you said and that is why they have applied indoor temp feed back and PID to the better controls..Eric

A standard t-stat is nothing more than an on/off switch (sort of). Even with indoor feedback it's still not perfect, but yes I ageee it's helpful. Outdoor reset is much better than nothing at all, and if your outdoor reset control has the ability to add indoor feedback then by all means...

But why use outdoor reset at all? If you know the load then just adjust the water temp accordingly.

It just seems so darn simple to vary the water temp based on the actual load which is what one is trying to do anyway that I am having trouble understanding why use such a poor stand in as outside temp.

What am I missing? Somebody help me here as it seems reasonable yet no one does it.

Why not vary the water temp based on the actual load of the building? An outdoor reset is supposed to do that, but just doesn't do it very well, so why not just change the input to one that is always correlated regardless out outside conditions?

All the rest of the system stays exactly the same. Its only that the water temp would now track load far better regardless of outside condtions.

More random thoughts.

Assuming one agrees with Siegenthaler that continuous flow is a great idea then one could vary the water temp by simply adjusting it such that the system (or the zone calling for the most heat if zoned) ran nearly all the time, say 90%.

This would be the minimum temp. If the load decreased then the system would reduce its on tme this would be a signal to reduce the water temp. Like wise if it increased then this is the signal to increase water temp.

No information about the outside temp would seem to be needed or even desirable. As the load changes, the water temp changes regardless of the reason the load changed.

Two notes.

I would think that one would also need an input from the heat source since there would likely be conditions under very light load in which it would be preferable to let the the water temp rise rather than let the system short cycle.

In a zoned system whichever one is calling for the most heat ie the cycle time closest to the target (in my example 90%) is the one that is controlling the system. Basically a wired OR which is pretty trivial.

From what I have read. It seems to me outdoor reset, more than anything gains the ability to "anticipate"the load according to outdoor temps. thus reducing the "flywheel" effect. Acting like a cruise control for the system. It is more benificial to high mass radiant than low mass, but both systems do benefit, and see more efficiancy.

I agree that input from an indoor sensor would make it much better.....But how can an indoor input sense adverse temp changes out side? It would always be lagging what ever is happening on the outside of the dwelling since Btu loss/gain is not an instantanious thing as specialy with an abrupt drastic swing in outdoor temps which is where outdoor reset really shines.

You could lash up all kinds of sensors.............some examples wind speed/direction,cloudy/sunny,night/day for information to be processed.....but how much better would all the extra hardware really make it? I don't think all the extra information would enhance efficiency a great deal,and someone has probably already seen it that way.

There is alot more technically skilled individuals here than I who can better phrase it for you.
Gordy

[This message has been edited by Gordon Kaske (edited 03-01-2005).]

I think that the need for outdoor reset is greater when:

In your climate, you are subject to frequent large outdoor temperature swings.

You have high mass (slab or gypcrete) heat delivery.

You have less than optimal insulation, leaky envelope, and/or lots of glass.

And, of course, the need is less or non-existant with stable outdoor temps, low mass delivery, and a well sealed, insulated envelope.

So one extreme would be a house with a beautiful view of the mountains,say, the Denver area, with cathedral ceilings, lots of big windows for the view, and slab delivery system, and no provision for supplemental or shoulder season heat. Or an old farmhouse in the middle of North Dakota.

The other extreme would be, say, the Seattle, Portland, Vancouver - type climate with a small, well designed and insulated house with an above-floor plated sandwich delivery.

Where you fall between the extremes and what similar installations around you have successfully used would better determine whether you should invest in outdoor reset or not.

At least that's my take on it.

not really accurate. Using *some* kind of dynamic mixing has benefits in practically all situations, as it allows you to NOT set the system to the highest water temperature it will ever need and run it there year round. It will lower the temperature as it can, which is good in most cases.

Indoor feedback is powerful stuff, but gordon hit the nail on the head; it is by definition a "step behind" outdoor reset. It would be suitable for low mass applications by itself, perhaps, but high mass needs "eyes on the outside" to see where things will be going later.

Outdoor temperature is not a perfect correlation to indoor temperature. But it is pretty good, and we have at least thermostats as a high limit because it's not perfect; if it were, we wouldn't need indoor control at all! When it's cold it generally takes more heat to stay warm though, and that's a truism outdoor reset takes advantage of simply and easily.



------------------
Northeast Radiant Technology, LLC
-=RFH Design, Supply and Consultation=-
RPA certified Radiant Designer
http://www.NRTradiant.com
rob@NRTradiant.com

To be honest I'm not seeing too many problems with outdoor reset, with some room sensors in the "funny room" (the ones with solar gain etc.)

Ah, now I get it. The case for the outdoor reset is about lag. Got it.

Glad to hear it seems to work pretty well for that, but frankly I am a bit surprised as outdoor temp doesn't change very quickly. I would think that if you wanted to anticipate instantanious load changes one would do better with a clock and a calender. What better way to predict the swing from solar gain (day) to loss (night) with extreme precision hours ahead?

Add a solar cell if you want even better data.

Your point about the type of structure brings the point home and is much appreciated. I am so wrapped up in low loss buildings I forget that not all homes are as low loss as I am used to. Likewise I am only working with low mass systems.

This thread has really helped to make it all come together for me and I thank everyone for their help. Given the my house is very low loss and low mass I am going to try going with a control system that measures the actual load of the house to set the water temp. Think of this as setting the system wide upper limit on available heat to all zones. Idea will be to have the zone calling for the most heat running most of the time consistant with some lower bound on the water temp. This makes the system look more or less like constant flow.

Seperately I am going to consider the issue of lag. If I don't have any problems of over or undershooting using a thermostat (with some sort of reasonable algorithm for prediction) then I am done. However if I do then I think the issue of lag should be dealt with at the zone level as rooms will have wildly different need for acceleration depending on the amount of glazing.

So I am thinking that if each room knows what time and day it is, the outside temp and the amount of sun (pretty simple inputs) then it can make a decision as to how much heat to provide based not only the current situation but what is likely to happen in the future. So if it is a bright sunny day but nightfall is coming each rooms thermostat can decide to ramp up early based on learned experience.

What that means is the thermostat has a setting. It then attempts to maintain that setting and when it fails (either over or under heating) it can look at the inputs and adjust its behavior automatically for next time. This is pretty simple to do.

The water temp system wide rather than being regulated by an outdoor reset is determined by each zone calling for heat.

Nice thing about this is the only thing one needs is a thermostat with a clock and calender plus a little bit of brains in the coding. The rest of the system then takes its cue from each room.

Anyway should be fun. And thanks again everyone.

Rob,

I was thinking some more about your post and it raised a question.

If the reason to have an outdoor reset is to reduce lag, ponder this.

The reset ups the water temp. No information is given to the thermostat in an individual zone. So the temp of the water goes up, but the thermostat, still ignorant of the temp change outside does nothing at least until the room begins to cool. The lag is still there!

Now once the room cools the ability to warm it again is greater than without the reset because the acceleration is greater. But think about it wouldn't it have been a lot better to have the thermostat calling for heat immediately when the change outside occured?

Seems to me that if the problem the reset is trying to solve is lag, then a better answer is to use the information to immediately call for heat (or call off the heat) rather than waiting.

So unless I am missing something (someone set me straight please) outdoor resets that control water temp do not remove lag. Rather a system with outdoor reset controling water temp is able to compensate for a thermostat that is subject to lag better than one that is not.

Perhaps this is just an artifact of when thermostats were mechanical. Given that I can buy a clock with an RF link to an outside thermostat for about $10 wouldn't it be better to send the outside temp information to the thermostat? That way the room can call for or reduce heat immediately rather than having to wait for the lag?

Anyway enough ramblings.....

DAE,

Low loss dwellings, actually are good canidates for outdoor reset. The "flywheel effect gets a little wilder with a low loss dwelling. When outdoor temp swings are to the cold side the low loss dwelling shoulders through a little longer, When the swings are from cold to warmer the dwelling shoulders through this period as well. Everything lags even more from an internal perspective.

The pre determined reset curve takes into account the dwellings specific heatloss, and with input from the outdoor temp sensor, anticipates the heatload, or lack of as needed, and adjusts the water temp accordingly. The t-stat calls for heat and the reset curve determines the right water temp for the conditions. The two still work together.

Gordy

[This message has been edited by Gordon Kaske (edited 03-02-2005).]

Someone mentioned that outside temps don't change quickly. I have to disagree with that. I'm exaggerating a little, but there are times of the year that we need heat at night, and A/C in the afternoon. Granted this probably depends on where you live. I guess it all depends on what you think is a "quick change".

You're looking for sophistication in controls that isn't quite there yet. Not a lot of stuff out there actually learns how the space reacts, and even if it did, such learning would still be at the mercy of varying conditions inside and outside the home. Sunny vs cloudy days, wind chill, cooking/shower frequency, lights, occupancy, etc.

outdoor reset allows you to use cooler water when you can, indoor feedback allows it to further fine-tune itself even further for the greatest off-the-shelf precision available today. The big worry with low loss homes and high mass heat is overshooting. They lose heat slowly, so even a little extra heat can mean noticeable temperature rises. With low mass or higher loss this is *generally* less of a concern.

It sounds like you're thinking of tying into a PC and coding some kind of routine yourself though? That would be a really cool experimental method if I'm not misunderstanding what you're planning.

------------------
Northeast Radiant Technology, LLC
-=RFH Design, Supply and Consultation=-
RPA certified Radiant Designer
http://www.NRTradiant.com
rob@NRTradiant.com

DAE,
I think Rob is on to something here. It sounds that your system would benefit from a fuzzy logic approach which can be implemented with a standard microcontroller such as Motorola's 68HC12. The instruction set includes fuzzy control. With this type of control there is no need for expert knowledge of the plant, which as you have indicated as a moving target. For current hydronic controllers (PID) to work efficiently they rely on this expert knowledge which can only be found through empirical modelling and we all know this is rarely the case.

DAE,

I'm roughly in the same place you are--studying what's available, so I can get what's best for my own home (at least, that sounds like what you're doing ). My initial take of the control systems I was seeing was that they were rather primitive, and I too felt that "outdoor reset" was a crutch because they didn't know how to build a decent control loop around the inside temperature. (And where does this corny "reset" term come from anyway?--what they really mean is "outside feedforward." )

Then I discovered tekmar. (They spell it with a lower case "t".) Their current generation of controls is quite sophisticated, using loops that take into account both the inside and outside temperatures so that they can set the water temperature as low as is possible while still achieving the desired air temperature. They also take into account the rate at which these temperatures are changing, so that they can, for example, temporarily increase the water temperature to get a zone up to temperature when changing from "unoccupied" to "occupied."

I'd suggest that you take a look at their 363 "Universal Reset Control," and at their 369 "Zone Control." I suspect that you'll find a lot of what you're looking for in them. Also, their new "tekmarNet 4" system appears to have it all, but info on it is a bit sparse at this point.

Sometimes technology is a double edge sword. Overkill comes to mind here. Outdoor Reset has been around as early as the mid 30's. The Sarcotherm control "primitive form". I do believe that over the last 70 years alot of dollars have been spent on research/development.

Between the boiler,controls, and tubing manufactures. I'm quite sure that they have figured out what the "cost effective" solutions are to tame this wild beast radiant heat.

I say keep it as simple as possible. why add more electronics/sensors than are necessary. My system " beast or primative" is simple. One pump,boiler,t-stat, and the radiant. Four system components to check if there is a problem. The boiler has more sub systems but yet basic in a sense.

Now I could add all the bells, and whistles to my system zoning, reset, sensors for this, and that but why? Will the added cost save me money? "Think diminishing returns on investment" make me more comfortable than I already am? It makes me very comfortable now,its economical to run, its SIMPLE. The only things that have been replaced is a boiler after 40 years of service and the circulator.

Just thought I would stir the pot a bit for discussion. I don't want anything sticking to the bottom )

Gordy

A PID system is typically more complex than a Fuzzy system. These new (fuzzy) controls (which are really not that new) simply have not been introduced into the mainstream of hydronics...

We now use microprocessor based controllers (fuzzy friendly) in place of analog electronics (PID friendly) in many control applications.

If anyone knows of a fuzzy based controller commercially available please post a reply. I would like to research it and perform some comparisons.

I do agree with Gordy as far as keeping it simple. There is a time and place for more or less controls.

We should not however become stagnant just because something has worked in the past. The world is changing at a very rapid pace - we can either embrace it and learn from it or let it pass us by!

Mike

yeah, those complex modulating gas valves are really unnecessary

just yanking the chain gordon. Time and place for all things, definitely. We just find that for most whole-house systems, a few hundred dollars on a better control system is *usually* money well spent.

------------------
Northeast Radiant Technology, LLC
-=RFH Design, Supply and Consultation=-
RPA certified Radiant Designer
http://www.NRTradiant.com
rob@NRTradiant.com

Well, first off thanks again to everyone for the time and thought they have put into these thoughts of mine. And since a number of questions have been raised, I think it would be worthwhile for me to address them.

BTW just for background, I am an engineer. No experience in HVAC (which explains my dumb questions) but a lot with computers and systems.

Gordon, good point about low loss building being more subject to problems of over and undershoot. Maybe that is why in their hasn't been quite the demand in the past for controls to get smarter.

Guest, what I meant by slowly was the the load in a structure due to changes in temp happens far more slowly than say a change from sunny to cloudy or day to night. The former can literally happen in seconds. The result can be wild swings in load that would not be sensed by monitoring just the outside temp.

Rob, your right, controls aren't there yet, but by trying to figure out how I think they ought to work, then I have a measure against to evaluate the various controls that are out there.

However I think you are generalizing when you say that the controls are at the mercy of the conditions you listed. I would suggest that one divide changes into those that have a significant effect on load vs those that do not, those that happen quickly vs those that happen slowly and (to a lesser extend) those that increase load vs those that decrease it (since in some systems the time to add heat is not symetric with the time to shed it).

If an effect can quickly and significantly change the load on a building then I suggest it is wise to consider monitoring that effect and adjusting the heat to the structure directly rather just monitoring the inside temp and waiting for something to happen.

Perfect example, morning on a sunny cold day. Don't wait for the temp to increase since risk overshooting. If you have reason to believe the load is going to dramatically down, make adjustments when the event has occurred.

Likewise if the input will only have a small effect or the effect happens slowly don't worry about it since the system will be able to track up off inside temp alone.

Predictive and adaptive systems are mature and well developed. It isn't rocket science to incorporate them into a controller, but somebody has to do it and shake the bugs out.

Mike, my goal at this point is not to code my own thermostats, but rather to make sure my topology (which is tough to change later) will accomodate them. As we all know anything with a microprocessor becomes cheaper and more powerful at a rapid rate. Maybe not today but at some point someone will come out with a stat that does the job right. I just want to make sure I am set up to yank whatever I have off the wall and put it in when they do. Or if it doesn't work throw it out and go back to a stupid one.

Solar, thanks I will take another look at Tekmar and see what they are offering.

Gordon, I couldn't agree more. First off any complexity has to be hidden. So if a stat uses a solar cell, or a clock or whatever I shouldn't have to worry or even know about it. Best stat of all from my perspective would be one that doesn't even have numbers on it. I set it to some level I find comfortable and it uses whatever inputs the designer deemed necessary to keep the room comfortable. Heck he might even change the temp on me thinking that most people dress in PJs in the am and clothes later on.

In the computer world we do this all the time. The interface is simple but there is a lot going on behind the scenes. My impression is that the controls for radiant haven't gotten to this point yet. The complexity (think the way a set back works) is all too obvious.

And of course it has to be bullet proof. Otherwise I'm with you. A dumb thermostat and something to fiddle the water temp sound great if they work.

So what I take away from all this is to keep my options open. Every room will have the ability to be a seperate zone. All my loops drop into a conditioned crawl so my manifolds are easy to get to and even they could be replaced if I needed to. I've got wires installed for thermostats in each room so I won't have to be fishing lines for that.

Next step is to lay in the piping and start thinking about heat sources.

Again thanks to everyone. If you're not already bored to tears, I'll keep you posted.