Modulation Control

Uni R

Help me out for a second.


Why would a control system for a modulating boiler with continuous circulation that monitors the supply and return temps ever need an outdoor sensor?


I would think that the return temperature is already a function of the outdoor temperature, solar gain, occupancy etc. and that knowing the outdoor temp doesn't even need to be part of the logic. BTW, I fully believe in the value of outdoor reset for non-modulating boilers.

Floyd_5

Not sure about others...

but the WM Ultra, runs based on the supply and not the return.
Are you saying that if you could set up a boiler to monitor the return temp. and then respond to that... you may have a point. It would be interesting to know...
I have two Ultra boilers running off of a Tekmar 265 on a total radiant system. The 265 monitors supply, return, and outdoor and adjusts accordingly.
Pretty cool..... at 0*- 120* water, at 40*- 95* water. return water temp. varies and have not paid a whole lot of attention to it. maybe I should.
Of course this system is not constant circ. but I don't think the pumps run more than half time and in a totally radiant system with low mass boilers like I have, I don't see that it would gain me anything.

Floyd

Mark Hunt

You're right


You wouldn't need it.

Do I care what's going on outside when my customers are standing inside?

Not really.

Mark H

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jerry scharf

one thought

Uni,

without outdoor reset, running constant cisrculation through the zones would cook the people. If you have a high mass system, it's relatively easy to use that to smooth out the indoor temp swing between times when you need a burst of BTUs to keep the temp right. With a low mass system, you have to accept wider temp swings or cycling you or zone valves quite often.

If we assume that the BTU load is proportional to the delta T across the wall, then by varying the target temperature, you should be able to keep the system near constant circulation. I'm not ready to design a system without a thermostat yet (Mike T mused on this,) but it does give a totally different view of heat control.I may take the bait on Mike's idea, but I'd want a year of careful data comparing actual demand to predicted before I even think about it.

Without outdoor reset, you're taking that nice proportional (aka modulating) boiler and hooking to to a on-off control system. This is not the most efficient way to run the system and can reduce the comfort of the system as well.

You can tell I'm sold on the modulating/outdoor reset combination.

jerry

Uni R

Jerry

I can certainly tell...


If the controls monitor both the supply and return temps, it can modulate down as the DeltaT closes. It has to just look at the rate of closure and put itself ahead. What I'm talking about is the supply and return deltas determining the modulating rate.

jerry scharf

which question are you asking?

Uni,

You are right to say the outdoor reset does not impact the modulation of the boiler. Outdoor reset and modulation are solving different problems. Modulation solves the issue of matching the heat being added to the system to the heat being expended to the enclosure. As you said, this is a function of the delta T from supply to return. Outdoor reset shifts the target temperature to allow the control system to run in a proportional mode rather than on-off.

If you are looking for maximum efficiency and comfort, there can be a significant advantage to pairing them up. The cooler the target water temperature, the cooler the return water temperature. The cooler the return temperature, the more condensing action and the higher the boiler efficiency. Also, with the low mass high emission panels now available, the ability to keep the tubes flowing can make a perceptible difference in comfort as well.

If you're asking why you need the outdoor reset to be part of the boiler controller, the answer is that to mdulate, you have to know your target temperature. To vary that temperature with outdoor reset, you have to have some way to tell the boiler to change it's target. I've not seen on that has a nice external computer port to allow anyone to do as they wish with the temperature, so you're stuck with buying the package.

Or maybe you're saying something else completely and I'm just dense.

jerry

Floyd_5

Your making sense to me.....

This system is doing exactly as you have described...
The return temps. of the water are in the high 70's to low 80's all the time.... supplies are varied according to outdoor temps. from the 120's to the high 80's..... vent temps. on the boilers are running below 110 most of the time and the modulation of the boilers is below 50% most of the time.....
I would have a tough time imagining a system that will get much more eff. than this.
2-WM Ulra 155 and Tekamr 265 to control them.

Dave_22

Control....

I understand what your saying and it makes sense to me. Here's the only problem I see with using it with constant recirc.- If you had a "fixed" target temp. you may be fine on a real cold day controlling like that. But, on a warmer day, your boiler might not be able to modulate down far enough to keep you where you need to be. Now you will either overheat the place, or shut the boiler down on high limit. This is why the outdoor reset control works by changing that target temp... Whattya think???

Uni R

Communications 101

Can you tell I never took the course? ;-)


My question is simply this. Why can't a control system just reset the target temps based on the closure of the delta-t when it is able to monitor the supply and return temps? Maybe they already can. I just cannot understand what outdoor monitoring adds when to me the best information is the delta-T change rate when determining the target temps and therefore the corresponding modulation ratio for modulating boilers. Nothing is "fixed". The boiler modulates up and down based on the delta-T so that it continues running. If the delta-T starts to narrow too fast, the modulation backs down. If it can modulate down far enough, it switches the burner off.

Brian

Outdoor Reset

Uni
Seeing as your up in Hazel land (took me awhile to figure that out).You will probably be looking at the "Trinity".It comes with outdoor reset and I've been hooking it up.What it does is reduces the load on the boiler and reduces the chance of over heating in your zones.Although an arguement can be made to run it without.I figure if it comes with the system why not use it.

Good Luck
Brian

Wayco Wayne_2

The simple answer

(hopefully) is that outdoor re-set gives you multiple set ponts so you can modulate to lower temps and get more savings outside temps allowing. When it gets colder out you will have to increase the set point automatically to meet the load and not be as efficient, but you may as well get the condensing savings while you can by lowering the set points when it's mild out. I hope this makes sense to you. WW


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R. Kalia

\"function of...\"

> Why would a

> control system for a modulating boiler with

> continuous circulation that monitors the supply

> and return temps ever need an outdoor sensor?

> I would think that the return temperature is

> already a function of the outdoor temperature,

> solar gain, occupancy etc. and that knowing the

> outdoor temp doesn't even need to be part of the

> logic.


Indeed the return temperature is a function of heat loss, but you don't know what the function is. It depends on the house, the radiation, the piping, etc.

With outdoor reset, it is relatively easy to find an initial approximation to the function (i.e. reset curve), starting with a reset curve that goes from 70 at 70 to design water temperature at design outdoor temperature. It ignores a lot of smaller variables, so it needs a thermostat to police it, but it more-or-less works. It gets better as the homeowner tunes the reset curve based on experience until the ciculator is running close to 24 hrs/day.

Relating the delta-T to water temp needed is not quite so reliable. Let's say you manage to do the measurements necessary to find the T-vs-delta-T reset curve for a particular house, which can take some time...then someone turns a radiator valve, and that function changes. The outdoor reset curve does not have this problem.

To be specific, consider a system like you are proposing, where the function has been tuned for the house and works perfectly with all radiators wide open. Now turn one radiator off because the occupant wants that room cool. Now there is a lower heat load so the delta-T gets smaller. Your control system detects this and concludes that less heat is needed by the house (presumably because it has gotten warmer outside), and so it reduces the water temperature. But it has NOT gotten warmer outside; the rooms with radiators open still need the same heat (maybe they need a bit more because of the cold room next to them) and so they need the same water temp or a little higher. But what they get is a lower water temperature...

Hope this makes sense. (PS I don't think everyone understood your question. You do want water temperature reset, but you want to use a reset curve that instead of relating outdoor T to the water T necessary, relates the delta-T to the water T necessary.)

Uni R

Chuckles

Your PS is very correct. I'm thinking I should shut my mouth about this and quietly file a patent, but I'm allergic to wealth. :-) Yes, to me the best system would be one that factors in the rate of change for the delta-t and modulates from there. Using the derivitive to me makes sense because as the house becomes "saturated" with heat, the rate change in the delta-T could be used to predict on a continuous basis what the exact modulation should be. This system isn't going to run at max right up to a high temp limit because it will quickly see the decrease in the delta-t and project from the rate it is dropping what the equilibrium level of modulation should be and be mapped to level down to it. For this the outdoor reset does not seem to be needed. The outdoor reset won't know if windows were left open, or if you have 5 times the occupancy rate or if there is a fire roaring in the woodstove, but these factors are always going to be revealed within the delta-t.

jerry scharf

I think I got it

Uni,

Let's see if this is right: You have your system with only two temp inputs, supply water and return water.

So let's look at the case where you are running zone valves or zone circulators controlled by thermostats. So every time a zone reaches heat, the amount of flow changes and thus the demand on the system. The temperature drops because the boiler now sees less load. How long before you get to the case where your coldest zone gets it's water temerature below the temp required to heat that zone? Your 200 guests make it such that the largest loss zones never call for heat, and so you end up with a cold upstairs bathroom when the party's over. Maybe you need to add a flow sensor as well, but that would limit the plumbing choices (no p/s, no deltaP bypass.)

Or there's the other case, where you're running without zones and just balancing all the loops. You walk into the upstairs bathroom after all 200 guests have left, and cold as ice again. You've dropped the temp because of all the heat happening in the entertaining area, and the chosen heat is not enough to meet the design temp in the other areas. Dynamic flow balancing across loops is far beyond what is considered affordable for current control systems.

Outdoor reset attempts to set the water temp based on an approximation of the normal system needs. When you have unusual situations, the water temp stays the same, and the zone controls block out use from areas that are not needing as much heat, but the rest of the system is left unchanged.

IMO, you are trying to do too much controlling with not enough feedback in what you are proposing. The boiler control is part of the larger system control, and I don't see your solution playing well in that larger context. As chuckles said, the system you are controlling is poorly characterized. It is unlikely the logic you could build into your component would be as stable and managable across the range of possible installation and living situations that can be expected.

I can't tell you all the ideas I have tossed out after finding a corner case that blows the control system. My experience is that once I find a couple cases, I get real suspicious and start looking hard at control stability.

my opinion is worth all you paid for it.

jerry

Uni R

Opinions

Your opinion is greatly appreciated on this Jerry, along with everyone else's. Thanks.

Uni R

Thanks Wayne

I currently have outdoor reset on a cast iron boiler, but I am grapling with whether or not controller logic could omit the use of outdoor reset for modulated boilers that have supply and return sensors. This is just a mental exercise.

Mike T., Swampeast MO

When I've played with flowcharts...

...for hypothetical modulation controls I tried to get it to "work" with nothing but delta-t for input. After all--heating requirement and delta-t have a direct relationship...

But, if you want to say that X° temperature inside equates with Y° delta-t AT MAINTENANCE STATE you are assuming that the heat output in the house is nearly linear with changing supply temperature. The only systems that would seem to "qualify" are large radiant panels and extremely oversized standing iron.

If you simplify this further and just say that a steady delta-t means you have achieved maintenance, I can buy that--BUT there is a problem--maintenance doesn't have to be steady maintenance--it can also be maintenance of a constant rise or fall in inside temperature. So, you think, "that's what the thermostat is for."

Maybe so, but not a conventional thermostat. A digital (on-off) thermostat only lets you know "I'm OVER satisfied," or "I'm UNDER satisfied," NEVER "I'm EXACTLY satisfied." Remember--they ALL have hysteresis. With only delta-t as your sensed/controlled variable you would have to intentionally over- and under-fire until you had an idea of the true delta-t needed to maintain a [presumed] steady inside temp. This might work eventually as the boiler "learns" but only if you never changed the thermostat setting. Change the t-stat and the BOILER WON'T KNOW YOU HAVE CHANGED IT!!! Add the complication of multiple thermostats in a zoned system and the poor confused burner will NEVER be able to come up with a STEADY modulation state...

So, you say, let's try this with a proportional system, e.g. TRVs. The problem is that TRVs aren't really a proportional temperature device--they are a proportional FLOW device. They will do their best to achieve steady space temperature regardless of the supply temperature or delta-t. While a given supply temperature WILL equate with a given delta-t IN A SPECIFIC STATE there are INFINITE combinations of supply temp/delta-t that will satisfy at any given temperature setting and they are incapable of communicating the minimum temperature required back to the system because delta-t will always be constant at their maintenance level--and again maintenance doesn't necessarily mean STEADY maintenance...

All of these problems can be solved by adding some form of temperature feedback--be it indoors or outdoors it doesn't really matter. Indoor temperature feedback has some problems though:

1) If the output device is particularly massive (e.g. a concrete slab) the system really needs to start changing the supply temperature BEFORE it is actually needed in the space. If the outside temp changes rapidly it will be too late once the inside temperature changes enough to detect a need for a different modulation state and the space temp will over- and under-shoot like mad forcing the burner to STOP modulating and go into "digital mode".

2) Where do you put the inside sensor? Unless it is a perfectly balanced, single zone system there really is no place to give a true reading.

3) How do you "set" the inside sensor? Just knowing inside temp alone isn't enough--you have to know the inside temp AND ITS RELATION TO THE DESIRED INDOOR TEMP. While I suppose that vector-driven thermostats exist in the big-time commercial world, you would still have a problem in a multi-zone system. Why go to the trouble (and expense) when (unless you're talking about an enormous, tall building) you can assume that the temperature outdoors IS the temperature outdoors--EVERYWHERE around the structure.

-------------------------

A system using proportional flow control could be set up to modulate without actual outdoor/indoor temp feedback. How? Via delta-p (change in pressure). The problem (I've been told) is that such sensors are too inaccurate and/or too expensive and/or too trouble-prone.

-------------------------

This isn't to say that such a control scheme is impossible--VERY little in this universe is truly so. These are just some of the problems I encountered when trying to find a way to do it. What I found was that without some way of determining an AIR temperature VECTOR I could never get the burner out of digital mode...

--------------------------

What is NOT needed however is an indoor thermostat in the conventional sense! IF you "tell" a modulating boiler that it is NEVER satisfied BUT you give it an upper limit via a reset curve linked to outdoor temperature the burner WILL modulate fully using nothing more than delta-t. As long as the system is reasonably balanced and massive the end result will be extremely low temperature swing in the space. But, you ask, "How do you change the comfort setting?" Merely by "fooling" the system into thinking that the outside temperature is a bit different than actual...

eleft(retired)_5

Great stuff!

Thank you guy's..for one of the most interesting posts on the wall this year.

al

Mike T., Swampeast MO

T-stat-less installation

If I'm right you don't have to engineer too much... Make an educated guess at the reset ratio required by your system (using an accurate heat loss calculation and ratio appropriate to the output device of course), set the curve and jump the thermostat connection. I'm not sure about other modulating boilers but a Munchkin WILL modulate in this circumstance.

If too warm shift the entire curve down--opposite if too cool.

If my kooky views of radiation are anywhere near correct the system will AUTOMATICALLY compensate for occupancy loading up to the amount of heat delivered to the space via radiation AND the boiler will AUTOMATICALLY become as efficient as its control program allows... Why, you ask? Because the IDEAL state of a modulating burner is one nth AWAY from satisfying the comfort state desired.

jalcoplumb_2

Wish I had time...

to reply.

hydronicsmike

Modulating Boiler Control with Reset

I wish I had more time to reply to this in detail as well. Mind you, I've read some pretty good answers to this already.

At the end of the day, the modulating output of a control will allow the modulating Boiler to maintain it's target based on the load. This is instead of turning the boiler on and off and minimizes fuel consumption - thats all. MAINTAIN STEADY SUPPLY TARGET TEMPERATURE.

The outdoor reset portion of the modulating Control will adjust the boilers target based on outdoor temperature (as usual). The heatloss (load) of a building is always related to outdoor temperature. At the coldest day of the year, you may need (for argument purposes) 180°F, where on a milder day you may only need 150°F water temperature to maintain the indoor at 70°F. So when the load of the building decreases, the water temperature is decreased, as typical outdoor reset does. FLOATING SUPPLY TARGET TEMPERATURE.

If you didn't have an outdoor reset modulating boiler control, the modulating boiler would maintain the same fixed supply water temperature at a milder day. This would cause room temperature swings and expansion noises, as the thermostat(s) would only call for a short period of time and then stop calling for a longer period of time. That’s the problem, if the same water temperature is supplied that it needs at the worst case scenario (outdoor design day).
Outdoor Reset increases the SYSTEM EFFICIENCY due to lower distribution and stand-by losses.

I hope this makes sense.


Mike

hydronicsmike

Modulating Boiler Control with Reset

I wish I had more time to reply to this in detail as well. Mind you, I've read some pretty good answers to this already.

At the end of the day, the modulating output of a control will allow the modulating Boiler to maintain it's target based on the load. This is instead of turning the boiler on and off and minimizes fuel consumption - thats all. MAINTAIN STEADY SUPPLY TARGET TEMPERATURE.

The outdoor reset portion of the modulating Control will adjust the boilers target based on outdoor temperature (as usual). The heatloss (load) of a building is always related to outdoor temperature. At the coldest day of the year, you may need (for argument purposes) 180°F, where on a milder day you may only need 150°F water temperature to maintain the indoor at 70°F. So when the load of the building decreases, the water temperature is decreased, as typical outdoor reset does. FLOATING SUPPLY TARGET TEMPERATURE.

If you didn't have an outdoor reset modulating boiler control, the modulating boiler would maintain the same fixed supply water temperature at a milder day. This would cause room temperature swings and expansion noises, as the thermostat(s) would only call for a short period of time and then stop calling for a longer period of time. That’s the problem, if the same water temperature is supplied that it needs at the worst case scenario (outdoor design day).
Outdoor Reset increases the SYSTEM EFFICIENCY due to lower distribution and stand-by losses.

I hope this makes sense.


Mike

Uni R

Modulation Control

It totally makes sense, because that is the most common way it gets done now and it does work. I'm just trying to think of how it could work better. In the past, controls really have been simple. Thermostat says either to generate more heat or not. The boiler burner is on or off. Boiler is or isn't at its high temperature point, etc. Reset added in a scale for the boiler temps to better match it to load and help smooth out all the on or off simple binary logic; an excellent move trying to match the heat to the expected load. However, and this is my quandary, if you know the delta-T the controller will actually know with 100% certainty how much heat the house is absorbing from the heating system which is reflective of how well the system is satisfying the heat loss.


The tough part is using this information and I appreciate the questions raised by everyone about the impact of zones since they in essence have their own logic and are just looking to have enough heat in their supply to satisfy their own part of the system and this has been the rationale for using the outdoor reset to set temperatures so they indeed get the heat they need. The one thing I see with this is that in many ways the zones are almost acting like on off circuits and that makes it tough to calculate a modulation ratio.


Maybe something like this would work. Each zone is on a thermostat (actually a true sensor - and nearer the outer edge of the space as opposed to thermometers normally being central to the space) and updates the controller with both its desired temperature and its current temperature. Also the controller knows the "weight" of each zone, since all zones have different heat loads. It could either be entered through a set up function or the controller could calculate it through responsiveness when it is the only active zone or some algorithm with 2 or more. Using a weighted balance of the heating demands, it can look at the current delta-T, which shows the heat it is currently supplying and set the modulation from this. The tough part with this is making sure that the zone that was really starving for BTUs actually gets them in a timely manner without excessive ramping up and down of the burner modulation - a true autopilot but flying a formation instead of one aircraft. Thus, all zones would have to be controlled by the central controller in order for this to work.


In this scenario it is modulating to heatloss and not to outdoor temperature. The heatloss encompasses envelope losses that are a function of the outdoor temperature and more importantly also factor in other heat sources and losses from within, such as occupancy levels, open windows, solar gain, and everything else.


I'm just trying to think outside the box here...

PS
And do you know what? If the house has 16" supersized concrete mass within the building envelope and has some infinitely small heat loss, essentially an oil tanker powered by a 5hp outboard, I'd add outdoor reset to the equation and even try and lead the changes by plugging in to the Weather channel. L

hydronicsmike

I definitely like what I read.

The delta T of the system could be used to see whats going on in the space, no question.

A simple control system would use the outdoor temperature to calculate an approximate target temperature and the delta T across supply and return would determine whether the target supply temperature has to be shifted down or up. The Outdoor Temperature is mostly used to get a starting point. Without it, where would you start? Basically, you would somewhat have a fixed delta T. If the delta T in the system increases, the load in the building is bigger and the target temperature is increased (shifted up). If the delta T in the system decreases, the load of the building is smaller and the target temperature is decreased (shifted down). Its a great concept and with what you wrote, I see that you fully understand it.
This is a Control method already used. The Europeans have done it like that for years. But is it practical?

I say somewhat. It makes simple outdoor reset "a little" better, but more control logic is involved. IMO, the benefit of adding that to current control systems is not enough to make it worth while. Outdoor Rest does what we need. In Europe, thats a different deal. Most systems there are with continuous circulation gained through balancing the flow to address the different loads/zones. In North America, we turn zones on and off and the load and the flow changes all of the time, which makes it harder for us to control a fixed delta T. Delta T control has been used in snowmelting controls for years, because there the load and flowrates are steady.

Again, the delta T would change the target, where the modulation would be used to maintain whichever target is to be met.

Since in North America we turn zones on and off, rather than trying to balance a system to gain continuous circulation across the whole system, I think it is much more advantageous to get sensor feedback information from the individual zones. Then we know what the load for each individual zone is and can comepensate by changing the supply temperature based on that info. If Indoor Feedback (as it is called) would not be the better way to address the North American Market and North American Systems, there would be Controls that operate based on delta T only.

I may be right or wrong. Regardless,it is my personal opinion. I prefer Outdoor Reset combined with Indoor Feedback over strickt delta T Control.

But then I also think that we are more advanced than the Europeans when it comes to individual zoning and temperature control.

I hope this makes sense. Please excuse my english writing skills. I am sure I have plenty of mistakes in most everything I write. English grammar and spelling is not my strongest field, as you can hopefully see.

jerry scharf

this would work much better

Uni,

The key here is that you've added a feedback loop independent of the boiler, so you can do much more.

If you like control systems like I do and want to play with this, here's how I would do it. Use a controller with outdoor reset capability. Instead of hooking up the thermistor, hook up a digitally controlled resistor ladder (half of what makes a D/A converter work) across it. Then use this to have the computer tell the controller what the temp is outside, and since you programmed the reset curve, you get to select the target temperature.

I'm not sure 8 bits would do it, but you can check out the dallas semi/maxim 1-wire stuff for a start. Here's the link for their 8 bit potentiometer DS2890 You can't beat their temperature sensors for cost and ease of use.

enjoy,
jerry

jerry scharf

this would work much better

Uni,

The key here is that you've added a feedback loop independent of the boiler, so you can do much more.

If you like control systems like I do and want to play with this, here's how I would do it. Use a controller with outdoor reset capability. Instead of hooking up the thermistor, hook up a digitally controlled resistor ladder (half of what makes a D/A converter work) across it. Then use this to have the computer tell the controller what the temp is outside, and since you programmed the reset curve, you get to select the target temperature.

I'm not sure 8 bits would do it, but you can check out the dallas semi/maxim 1-wire stuff for a start. Here's the link for their 8 bit potentiometer DS2890 You can't beat their temperature sensors for cost and ease of use.

enjoy,
jerry