Outdoor reset and circulator delta T

doughess

On a system with outdoor reset I monitored the delta T for months. After the circulator runs for a few minutes the delta T was remarkable stable around 20F over an outside temperature ranging from 10F to 50F. It is obvious why and reflects a benefit of ODR’s that can reduce the need for a delta T pump. Would be interested in comments on this.

Ironman

A lot of it has to do with how well the circ is sized to the needed flow rate and how well the boiler is sized to the load.

It sounds like yours is very close. Many aren't.

Gordy

Ironman hit it. There is no substitution for the math if you want stellar results. I would also add emitter saturation has an effect also. To little contributes to a wider delta and a lot contributes to narrower deltas. It's a combination of things.

doughess

I agree it is by the numbers. That was my “duh” moment after I first noticed the stable numbers. With a constant gpm, the water temp varying from 140F to 180F should be a very stabilizing factor on the delta T in spite of major BTU load variations. Without ODR the delta T would vary more.

hot_rod

and largely how well the emitters are sized to the boiler output. Connected heat emitters drive the systems thermal equilibrium.

icesailor

I read an article in the November issue of P&M by Siggy that explained most of the unknown unknowns for me. It was about "System Equilibrium".

You are reaching system equilibrium.

hot_rod

Our latest idronics 16 tech journal may help clear up some of the questions.

It clears up some of the questions regarding flow rationed heat transfer.

http://www.caleffi.com/sites/default/files/coll_attach_file/idronics_16_na_0.pdf

Gordy

Hatterasguy said:

Gentlemen,

I suspect that some data is flawed.

If DT remains identical, and flow rate remains identical across the entire reset curve, we have a situation where the BTU's delivered are identical. Since we know that is not the situation because the SWT is varying, there is a piece of data missing or in error.

The boiler is irrelevant to the discussion. It will simply cycle more or less depending on its size.

The only window that I see as a possibility would be the circulator flowing slightly less at lower SWT's but this is relatively insignificant as compared to the reset curve.

So your saying that delta t HAS to vary at a fixed flow rate unless gpm varies?

I have a similar scenerio with out ODR. I always have rock solid deltas no matter the outdoor temp. The water temp varies with how long the boiler runs. Radiant system with constant circulation.

icesailor

Gordy said:

Hatterasguy said:

Gentlemen,

I suspect that some data is flawed.

If DT remains identical, and flow rate remains identical across the entire reset curve, we have a situation where the BTU's delivered are identical. Since we know that is not the situation because the SWT is varying, there is a piece of data missing or in error.

The boiler is irrelevant to the discussion. It will simply cycle more or less depending on its size.

The only window that I see as a possibility would be the circulator flowing slightly less at lower SWT's but this is relatively insignificant as compared to the reset curve.

So your saying that delta t HAS to vary at a fixed flow rate unless gpm varies?

I have a similar scenerio with out ODR. I always have rock solid deltas no matter the outdoor temp. The water temp varies with how long the boiler runs. Radiant system with constant circulation.

FWIW, that's my observation. Unless I misunderstand the issue, the Delta T remains the same, between the SWT and RWT. The differential setting determines the swings. And can make the Delta T appear off. Thermal lag with the controls has to be taken in to account. You see more swings through thermal lag in the boiler than anywhere else. If you don't understand the thermal lag issue, you can get confused.

Think like a child on a swing. If the child never swings his feet out, sooner or later, they stop. Swing the feet, and the arc increases. When the arc gets too big, you stop swing your feet out. ODR won't stop it because of thermal Lag.

Or why some with OCD and their Delta T's and condensing boilers are driving themselves into a dilemma.

Gordy

Your forgetting Time. Or length of heat call.

Gordy

But then I think there is little information here other than the OPs observation to draw any stark conclusions.

icesailor

FWIW, what I was trying to say is that it was my observation that regardless of how it was being controlled. the Delta T always stayed the same. If it was 20 degrees, the delta T stayed the same as the controls overrode the high limit and dropped below the high limit, due to the differentials set in the controllers. If the High was set to 180 degrees, then the low was 160 degrees. If the high over rode to 190. the return was 170. When the burner came back on, the SWT might be at 175 degrees, the RWT was 155 but climbing. Any differentiating was in the thermal lag of the controls. If it is P/S and the ODT is the secondary, the boiler will cycle. The controller will control the secondary. If the burner cycles, it is only adding needed energy. IMO.

I did this a few times. What else do you do at the end of a day, you're all picked up, and its too late to start another project?

Go learn.

icesailor

You just don't understand the concept of averages.

Jean-David Beyer

"If the flow rate is constant and the DT is constant, the output from the emitters must be constant. There isn't any other option."

I have never understood this, and I still do not. I would think this is true only for a constant supply temperature, such as a boiler without outdoor reset.

The part of my system that is baseboard seems to be greatly overpumped. Except when it is extremely cold, the DT is so small as to be unmeasurable (i.e., less than 1F). But with constant pumping rate and essentially the same delta T, the heat output varies greatly depending if I am putting 110F water or 125 F water into the baseboard. If I go even higher, say to 145), I do get measurable delta-T values, and even more heat output.

My view is that if the pumping rate is "too high" and there is no measurable delta-T, that is giving out the most possible heat for that supply temperature, since the entire baseboard is at the same temperature. If the circulator was downsized to give a delta-T of 20F, that would mean the average temperature of the baseboard would be 10F less than the supply temperature, so of course less heat would be delivered.

There seem to me to be only three reasons not to over pump a system.

1.) Overpumping can be too noisy.
2.) Overpumping can waste electricity to run an oversize pump (even though mine are Taco 007-IFC).
3.) Overpumping can wear out the pipes, they tell me, especially where change of direction occurs, such as elbows, Ts, etc.

Gordy

Hatterasguy said:

Gordy said:

Your forgetting Time. Or length of heat call.

All of the analysis is based upon steady state conditions at equilibrium. I can forget time and length of call.

If you're not at equilibrium, none of your data matters in the slightest as it's a moving target.

Equilibrium is reached when the thermostat is satisfied, and the boiler shuts down no?

Maybe I just have a perfect system. No thanks to me, but to the dead guy who designed, and installed it.

I have a parallel piped ceiling, and floor radiant system.

I have I/O thermometers on the main supply after the mix valve, and on the main return from the system. I also have thermometers on supply, and return out at the ends of the mains at the other end of the house about 50 ' away. I also have thermometers at the supply return for the radiant floors.

Minimum maximum thermometers are nice for this you just reset them and the high on the supply minus the high on the return is delta t at equilibrium. I can also tell what the lowest system water temp goes down to when the heat call kicks in.

I can tell you I have watched countless heat calls. Usual run times are 15 min. On a heat call. My boiler never cycles of high limit. High limit 155. Supply Mix temp 108 return temp 93.

Delta t reaches 15 on the ceilings and 8 on the floors 2/3 into the heat calls. And maintains.

hot_rod

Jean-David Beyer said:

"If the flow rate is constant and the DT is constant, the output from the emitters must be constant. There isn't any other option."

I have never understood this, and I still do not. I would think this is true only for a constant supply temperature, such as a boiler without outdoor reset.

The part of my system that is baseboard seems to be greatly overpumped. Except when it is extremely cold, the DT is so small as to be unmeasurable (i.e., less than 1F). But with constant pumping rate and essentially the same delta T, the heat output varies greatly depending if I am putting 110F water or 125 F water into the baseboard. If I go even higher, say to 145), I do get measurable delta-T values, and even more heat output.

My view is that if the pumping rate is "too high" and there is no measurable delta-T, that is giving out the most possible heat for that supply temperature, since the entire baseboard is at the same temperature. If the circulator was downsized to give a delta-T of 20F, that would mean the average temperature of the baseboard would be 10F less than the supply temperature, so of course less heat would be delivered.

There seem to me to be only three reasons not to over pump a system.

1.) Overpumping can be too noisy.
2.) Overpumping can waste electricity to run an oversize pump (even though mine are Taco 007-IFC).
3.) Overpumping can wear out the pipes, they tell me, especially where change of direction occurs, such as elbows, Ts, etc.

What would constitute over-pumping?
To my way of thinking the indicator of over-pumping would be the flow velocity rate. Stay within that 4-5 fps range and all should be fine, no noise and no excessive component wear, but good turbulent flow..

Furthermore if you can pump that max flow for the best emitter output, with an ECM consuming 40W or less, that seems like the best method of heat transfer at the highest distribution efficiency.
Highest AWT translate into highest "mean" temperature of the radiator, coil, floor circuit, or baseboard which is it's peak output.
On typical heating days I hear my HTP Phoenix fire MAYBE once an hour, only on a large DHW load does it even modulate to high fire.

Ideally the boiler modulates output and/ or a high mass boiler or buffer tank and there should be no short cycling.

TRV control, are better yet a non-electric proportional temperature regulator matched to a ∆P circulator.

Harvey Ramer

Hatterasguy said:

Jean-David Beyer said:

"If the flow rate is constant and the DT is constant, the output from the emitters must be constant. There isn't any other option."

I have never understood this, and I still do not. I would think this is true only for a constant supply temperature, such as a boiler without outdoor reset.

It is true for any supply temperature. The only variables that affect BTU's delivered by the emitters are flow rate and DT. If you vary the SWT, you will deliver less BTU's (as that is your goal) and the DT must, by definition, get smaller (assuming constant flow rate).

Close but not quite. As water temp increases the density decreases along with the specific heat of the fluid. Therefore, if the flow rate and DT remain constant and the water temp is varied by ODR. The BTU output will change dependent upon the water temp.

Harvey Ramer

And the specific heat of the fluid delivering the BTU's. The Universal Hydronic Formula is not entirely accurate.
Investigate the 500 # that is used in this formula. It's based on assumed averages. Understand, we aren't talking huge differences with straight water within a 40° range however plug in 40° water/glycol mix and see what happens.

doughess

When I used the phrase "remarkable stable around 20F" I thinking of 20F ± a few degrees. My basic point was it was not varying widely. I felt the ODR was a strong factor in that. I had been thinking about buying a delta T pump and saw not need for it after seeing the data.

doughess

I did not keep a written log of readings. Today at 50F outside, boiler at 150F delta was 22F. The Tekmar 256 ODR is set for 150F min with 5 deg delta. There is a PID controller that cuts off circulator below 140F and burner over 190F

This is main upper level zone of a split level home. Piping is 180 ft of 1" copper with diverter tee's for 11 convectors. Other levels run on a different circulator and piping. Pump is a B&G Series 100 running at .56 amps (motor label says 1.7 amp full load)

Boiler is a 60 year old Weil McLain Model 57A rated at 1.80 gph oil with a Beckett AFG firing at .85 gph.

With the basic hyrdronic equation based on gpm / load / delta-T the ODR has to be a stabilizing factor.

I put this house data in the US Energy Star calculator and got 9+ out of 10.

doughess

Thanks for the comments and analysis. The DT is a Fluke 52. I have not checked the calibration recently but that is easy to do. The ODR and PID also display boiler temp.

The zone is actually split into 2 parallel zones connected to the same 1" lines. At .56 amp I was surprised at how lightly loaded the pump was. Years back I worked for company that make very large industrial centrifical pumps. Motor current was a key factor in measuring load.

Other data: zone thermostat has 1 deg delta. Outside walls are urethane foam insulated. Interior walls are 1" thick plaster over sheet rock lath and slow to change temperature. Use a Honeywell "smart response" thermostat to bring temp up in the mornings and have a PID low temp circulator cut off.

Since no real problem here I will take more readings next season.

doughess

No speed controls, just on off. Will try a test and slowly close off the loop isolation valve when it is running and watch the amp meter.

njtommy

So your saying pick one or the other ODR or Dealt T pump. If you have multiable zones off of one pump your better off having a Delta T pump to maintain 20 delta T any way. ODR is just a added benefit to save fuel cost of not firing to 180 on a 50 degree day. our Delta Ts are based off of design heating days. So yeah our now 50 degree day you could see a 10 degree delta t. Not really a bad thing for a cast iron boiler depending on your temp setting and tank capacity you could run and satisfy a smaller zones with out even running the boiler which is a win in my book. Once you get into some of these mod cons with higher turn down ratios you could be looking at short cycling problems do to a bad install, no buffer tank, and over sizing. You want a little longer run time on the boiler any way to get your efficiency for the system to equalize and cruse right along on a call for heat.

SWEI

There's nothing wrong with using a ΔT pump on an ODR-controlled system. It likely won't make quite the difference it will on a fixed temperature system, but it will reduce the electrical consumption of the system, quite possibly a lot (for an ECM pump.)

Kakashi

What kind of system do you have? A picture would be great.

SWEI

Hatterasguy said:

What is also very interesting from the OP's measurements is the power consumption from the B&G. At 67 watts, it's dramatically lower than expected from such a large pump

Is that true Watts or VA? The power factor on those things is pretty ugly IIRC.

SWEI

I have the OEM specs for the motor (well one of them -- there have been a number over the years) and I don't see how it could ever draw that little. Full load efficiency is 40.6%, 3/4 load is 35.1%, and 1/2 load is 27.2%.

SWEI

The efficiency times the output power (1/12 HP or ~62W) should allow a translation of some sort. Most failing motors draw more power and not less IME. I can imagine a failure mode where one or two of the windings failed open (effectively derating the motor) though I've never personally seen it in an AC induction motor.

bob_46

A flow of a huge amount of water should draw a huge amount of current . I suspect his head is high and the flow is small .

SWEI

In which case the Series 100 is really not the right pump for the job. Given the motor's plummeting efficiency at part loads that tiny amp draw would have to mean the pump was barely moving water at all.

doughess

The Fluke measures ΔT. A digital clamp on meter measured motor current.

doughess

I activated the B&G with the zone valve closed. Motor was drawing 1.67 amps. Sort of worst case operation, max load or call it whatever.
Another factor in light load is the zone is fed with 80 ft of 1" to a tee where 2 loops controlled by one thermostat split off (5 convectors on side and 6 on the other). The returns joint at a tee for another 25 ft to the pump.

My basic point in starting this was that ODR can be a positive and stabilizing factor on ΔT.

Gordy

Seems like a lot of delta O on the delta t topic for such a small in measurable tolerance. Let human comfort be the instrument of success.

Just my Delta O

doughess

While the focus of ODR is usually on savings, a secondary benefit, more even ΔT, seemed to be worth mentioning. I had been thinking about a variable speed pump but see no need for one now. My wife, who gets up hours before I do, often gives me a status report in the morning. Even with the lowest temperatures we have ever had, she has been satisfied.

Gordy

And that is what counts