I don't understand the merit of a Delta T circulator in my system

satwar

I had a Vitodens 100 Combi-plus (I believe it has it's own circulator) installed several years ago with a low loss header feeding an old gravity fed cast iron radiator heating system. A Delta T secondary circuit circulator is on the return line to the low loss header (this is not the best because it is pumping to the PONC and I plan on moving the circulator to the supply line from the low loss header). The house is warm but I am having a hard time realizing any gas savings, so I'm looking to fine tune the system performance.

The installer left the heating curve at factory default, which had the radiators smoking hot in winter weather. I decided to change the heating curve to lower the water supply temperature from the boiler, with a target of 150-160 F @ -20 F outdoors, but am not sure yet whether this water temperature will keep the house warm in the dead of winter. But at least I seem to b on the right track, and I can see that a savings in gas can be realized here.

And then there is this pesky Delta T circulator. My delta T is always 20 F, and gets higher as the boiler water temperature climbs. When the delta T gets above the set point the circulator speeds up and return water starts short circuiting in the low loss header to the system supply line, which lowers the delta T. The delta T is under control, but I'm puzzled as to how this is of any benefit to me ?

Firecontrol933

If your sensors for the DT circulator are mounted correctly then the 20° DT is what is considered "normal" for the DT across the boiler. Once you play with the settings and get the outlet temperature down to that magic lowest number and you home still heats fine at the coldest temperature the benefits will become obvious.

Say you can get the supply temp down around 140 or so, that will get your return down around 120 on the coldest day and a lot cooler on the warmer days.

Think about stretching that top temperature down even a little further if those living in the home can put up with a small drop in temperature over night that rebounds during the day. I say this because most "coldest temps" happen at night and as soon as the sun comes up the temperature quickly starts to rise and as soon as that happens the house will catch up.

Went to a Vitoden factory school a long time ago, one of the most asked questions by those attending was where is the thermostat connection on the boiler. After avoiding the question for most of the morning the instructor finally said "If you design the system correctly and set up the boiler correctly you don't need a thermostat. The boiler will always be putting exactly the amount of heat into the home as it's loosing. You want the home warmer, you just increase the curve temps." I guess I've never trusted my math, the builders, my customers and the possibility of remodeling and other factors that time always throws into the mix to try a system without a thermostat.

Paul48

On a gravity conversion, placing the supply and return sensors anywhere on the piping does not guarantee a fixed DT through each radiator. In fact, you can wind up with zero flow through some circuits.

Rich_49

I would like to see some pictures of this LLH and the circ , is that possible ?

satwar

Here she is:

As I said earlier, I'm moving the circulator from the system return line to the system
supply line feeding the radiator system (left pointing flow arrow)

satwar

In case it's not very clear, the circulator temperature sensors are on the system return line before the circulator intake and on the system supply line from the LLH.

Paul48

You need to have working valves on all your radiators, so you can balance the flow. I don't know how that balance would be affected with an ever-changing pump speed.

satwar

Yes, my valves are all working and the temperature is very comfortable throughout the house. After balancing l never have to touch them.

Eliminating evening thermostat setback made a huge improvement.

satwar

I may be seeing some light here, regarding my understanding in the use of a delta T circulator. Increasing Delta T indicates a consumption of heat, which is met by increased water flow once the delta T setpoint is reached. If the set point is too low the return water temperature will be higher than necessary and leads to lower boiler efficiency. If the set point is too high you may not have sufficient flow to heat the house.

satwar

Well I studied the operation of the delta T circulator and ODReset this past heating season and found the results somewhat confusing, and would appreciate any comments.

(1) I found that setting the delta T at 40 degF, the circulator runs at the lowest speed possible, all of the time, and has no problem heating the house. I assume that the system piping is so large from being designed originally as a gravity feed system, that the hot water circulates virtually on it's own.

(2) The minimum outdoor temperature only got to -13F this winter, with the ODR calling for a supply temperature @ 166F. The circulator was starting to run more frequently as the delta T exceeded 40F at the beginning of a heating cycle but settled down to a delta T of 30F. The furnace still had room to heat the supply temperature to 176F and ramp up the circulator speed at outdoor temperature less than -13F.

(3) I was using a ODR heating curve that achieved maximum 176F supply temperature @ -22F outdoor temp, versus the factory default of achieving 176F supply temperature @ 0F outdoor temp.

My goal in this study was to see if I could increase the efficiency of the boiler. To my surprise, when I plotted Gas Consumption versus Heating Degree Days, I could see no improvement when I ran a higher circulator delta T or a lower ODR curve. I clearly save some electricity by setting a higher circulator delta T, because the circulator runs slower most of the time, but I can't see any gas savings.

What am I missing ?

Gordy

What you are missing is your delta t circulator saves energy cost in circulator power consumption due to the fact it is ECM, and can very its speed based on delta t which is a moving target. When it's cold it will run faster than when it is warm out this you have seen.

As far as gas consumption it has to do with getting the boiler down in condensing range to see higher advertised efficiencies. If you don't have enough radiation to run lower supply temps then that is it.

I would have to ask if your ODR is actually properly Setup. If it is the thermostat is no more than a high limit, and the ODR runs the show.

Gordy

The proper way to setup your ODR to use the lowest possible SWT is to set the thermostat above your desired setpoint. If you like 70 set it to 80. Now you want the reset curve set so it maintains 70 degrees with out the thermostats intervention.
This takes time but once accomplished it will guarantee that you are using the lowest SWT needed to heat the space to your desired setpoint. In doing this however you can not use any setbacks since the system will not recover due to the SWT being low.

The thermostat then becomes a high limit device, only intervening when solar, cooking, wood burning, or high amounts of people elevate the temps in the space.

satwar

Thanks for offering your advice.

With the current ODR heating curve in use (176F SWT @ -22F) at low outdoor temperatures (less than 23F) the thermostat is shutting off the boiler (~12 on/off cycles per day). The house is very comfortable, but it seems that the SWT is hotter than necessary.

At warmer outdoor temperatures (greater than 23F), the SWT operates within a narrow range of 110-120F and the boiler will run intermittently but the thermostat can be calling for heat almost all day (1-3 on/off cycles per day).

Lowering the ODR heating curve further would seem to be exceeding the capacity of heating system to deliver sufficient heat to the house. Perhaps lowering the circulator delta T (currently 40 F) will increase the average flow rate and hence allow more heat transfer at lower SWT. Unfortunately lower circulator delta T will raise the RWT and hence decrease boiler efficiency.

Gordy

I think you could lower your curve. Ultimately on design day the boiler would never shut off IF it is properly sized to the design day load.

By the way how often did you hit the -23 F this year

Gman66

This has been discussed here before, often with lots of confusion. You are confusing the importance of the RWT of the secondary or system loop with that of the primary or boiler loop. You really want lower RWT on the primary loop, that is what determines if you are condensing or not. Running large (or small) DT on the secondary/system has little to no impact on the RWT of the primary/boiler loop. You would be well served to lower the primary/boiler flow as much as possible which would give you the lowest RWT on the primary. Now, how low a flow can you go? You want the primary flow to be the greater of 1. minimum boiler flow (see your install manual) and 2.the lowest system flow you can tolerate (if you can tolerate a 30 degree DT on design day, that is your lowest system flow). Matching the system and boiler flow is what you are after; with a single zone like you have things are quite a bit easier.

EBEBRATT-Ed

I wouldn't bother moving the circulator. The resistance of the old gravity system is so low it won't change anything

Gordy

Gman66 said:

This has been discaussed here before, often with lots of confusion. You are confusing the importance of the RWT of the secondary or system loop with that of the primary or boiler loop. You really want lower RWT on the primary loop, that is what determines if you are condensing or not. Running large (or small) DT on the secondary/system has little to no impact on the RWT of the primary/boiler loop. You would be well served to lower the primary/boiler flow as much as possible which would give you the lowest RWT on the primary. Now, how low a flow can you go? You want the primary flow to be the greater of 1. minimum boiler flow (see your install manual) and 2.the lowest system flow you can tolerate (if you can tolerate a 30 degree DT on design day, that is your lowest system flow). Matching the system and boiler flow is what you are after; with a single zone like you have things are quite a bit easier.

Not sure if you miss worded your post however.

In a p/s piped system, or one using a low loss header as this one.
System flow must be equal to, or greater than boiler flow.

It's a vitodens 100 boiler with a water tube not a firetube HX. With that being said it might have been possible to direct pipe with a gravity system. However that is not how it was installed.

To the op what is the boiler side delta?

satwar

Gordy,

On the coldest heating day this winter (~-12F) the boiler SWT & RWT were running at ~166 F & 134 F respectively. The secondary loop was running with supply of 161F and return of 129F.

Gordy

It hurts nothing to take the curve to the edge. You can always pull back if it's to much. Make small adjustments, and let the setting equalize a couple days.

That's a decent boiler delta at -12 outdoor air. So now if you would adjust the curve to get the return down below 130* on the boiler loop you could start seeing some fueli savings. Real condensing doesn't start until below 130* if your not condensing your mid upper 80's efficiency.

hot_rod

Remember with low loss headers or hydraulic separators, one of 3 conditions is present.

If the flow rate on the boiler exactly matches the distribution, the RWT from distribution will equal boiler return.

Most often one of the other two conditions exists.

And in your case with VS on the distribution, that mix is always changing.

Does that model Viessmann have a VS circulator inside? That would make the RWT prediction or calculation even trickier.

Two goals would be best efficiency with lowest possible RWT, and also long run cycles, don't trade one for the other. With a modulating boiler properly sized, and high mass radiation you should not see a lot of cycling?

satwar

Well there's not much winter weather left, but I will keep an eye out when the outside temperaature drops below 32 F. That's where the heating system seems to struggle. Once the ODR gets significant with lower outside temperature, the boiler runs very smoothly.

Thanks for the help.

hot_rod

You mentioned in the first post that the home stays warm?

Or does it struggle to stay warm below 32F?

Was the boiler sized to a heat load calculation? Any changes to the radiators, like removing some?

If the home heated well with the old boiler, the rads are probably adequate, so the problem may be pumping, piping, sizing or control logic related. Or a combination of those

Yeah, it may take another winter to sort it out and get it operating as desired.

satwar

Sorry for the confusion, but there is plenty of heat available to keep the house comfortable at low outdoor temperatures with ODR fully in control. My concern occurs when ODR is not acting very strongly at outdoor temperatures of ~23F or above. The SWT drifts around 110-120F and the thermostat never quite reaches setpoint, with the burner turning off & on periodically.

Perhaps this is a good thing, with the thermostat calling for heat all day, but it makes me nervous.

hot_rod

satwar said:

Sorry for the confusion, but there is plenty of heat available to keep the house comfortable at low outdoor temperatures with ODR fully in control. My concern occurs when ODR is not acting very strongly at outdoor temperatures of ~23F or above. The SWT drifts around 110-120F and the thermostat never quite reaches setpoint, with the burner turning off & on periodically.

Perhaps this is a good thing, with the thermostat calling for heat all day, but it makes me nervous.

In an ideal condition on a design day, the boiler would run 24/7. Assuming the boiler output exactly meets the loss of the home. It sounds like you need to make more tweaks on the ODR. I'm not familiar with that control brand or how much adjustability it has. Can you shift the curve and make other % adjustments?

Be interesting to put the pump in a fixed speed position and see if that changes the "comfort" We have seen other cases come across the list where the ∆T circulator and the boiler modulation are at odds with one another, different response times and they end not working in harmony.

Personally I don't see a problem with the delta T in the distribution moving around a bit from the design #. Let the burner modulation (SWT change) handle the load adjustments.

satwar

The Vitodens 100 has a simple dial that shifts the ODR heating curve, but does not allow any change to the shape of the curve other than what is specified by the manufacturer.

I find that at modest heating requirements (outdoor temperature above 23 F) the SWT drifts between 110-120F (I don't know why since the boiler's ODR is 70-120F at these outdoor temperatures). The burner is cycled on/off by the boiler. The thermostat rarely shuts down the boiler.

Once the outdoor temperature drops below 23F, the SWT follows the ODR (120-176F) and the thermostat shuts the boiler off as required to maintain a constant temperature in the house.

By shifting to a lower heating curve I would expect the boiler to control the burner operation at a lower outdoor temperature before the thermostat starts tripping boiler operation. Hopefully lower RWTs are my reward.

Gordy

The 100 was designed to compete in the u.s market. The 200 logic is more detailed. There are some wallies very intimate with the vitodens that could direct you on a more precise level of control.

@Paul Pollets is one.

satwar

Thanks Gordy,

There don't appear to be any tricks with the Vitodens 100 heating curves. You just keep adjusting the heating dial down until the boiler can no longer maintain desired room temperature.

It seems that you must test each heating dial setting over a wide range of outdoor temperatures. At modest outdoor temperatures the ODR calculated SWT is insufficient to heat the house too quickly. The pumps stay running and the burner cycles on & off until eventually the house thermostat trips the boiler (burner and circulator). On the other hand as the outdoor temperature gets colder the ODR calculated SWT heats the house more quickly and gets sufficient to trip the boiler with the house thermostat, with no burner cycling.

I have no idea how the boiler decides what SWT to use, because it does not correlate with the heating curves published in the owners manual.