Buderus Logamatic 2107 burner differential?

DaveGateway

Today was the first really cold morning (15F) for my Buderus system.

I have a Logamatic 2107 control with outdoor reset and use a standard setback thermostat (63F night, 68F day). I also run the 2107 in continuous day mode (no night time setbacks programmed in the Logamatic).

When the thermostat was continuously calling for heat this morning (circulator continuously on), I noticed that the burner was cycling on at 180F and shutting off at 187F (187F is the Boiler Max Temp Setting). It appeared that the burner differential was only ~7 degrees F relative to the boiler max temp, which is pretty small.

It also seems like the burner differential is shrinking as the outside temperature goes down. Will the differential continue to get smaller as the boiler target temperature approaches the boiler max temp setting, or is there a minimum burner differential built in to the 2107 prevent too frequent cycling? Is there someway to program/maintain a larger burner differential to reduce the frequency of cycling on really cold days?

DaveGateway

minimum differential

The answer is 7 degrees F.

Lowered the max boiler temp setting and the R2107 still maintained a differential of 7 degrees F. It looks like the R2107 will maintain a minimum of a 7 degree differential relative to the high limit setting. The differential is normally greater (~14 degrees F) when it's not shutting down on the high limit.

dconnors

in the manual

On page 4 of the service manual is listed the heating curve along with the burner and room sensor compensation differentials. Hope this helps.

Paul_28

Constant day mode

I have the Buderus gas boiler with the 2107 controller. I use the day-night setbacks in the 2107 with on/off setback thermostats. Staying in the Constant Day Mode may be better.
On an 18* morning the boiler temp. was 153*. I need to raise this number with a new set point or a curve adjustment. What is the set point you use?

Rudy

Was looking for minimum spec'd differential

Dave,

These curves really don't describe what happens to the differential when the burner is shutting down on the high limit. It shows a nominal differential of +/- 14 degrees around the heating curve when not high limiting. I am looking for the specification that describes what the minimum differential would be when high limiting. On really cold mornings the curves will intersect the high limit point and I want to be sure that some reasonable burner differential is still applied.

I'm not sure that my experiment is totally conclusive, but with the high limit (max boiler temp in Buderus terms) set to 187F, the burner was coming on at 180F. When I lowered the high limit to 185F, the burner would come on at 178F. From this it looks like the R2107 is imposing a minimum burner differential of 7 degrees F for the case when the heating curves are being limited by the high limit setting.

A differential of 7 degrees F seems pretty small and results in a fair amount of burner cycling when the heating system is trying to recover room temperature during long periods of constant circulator operation (like first thing in the morning on cold days). During this time the burner will run for 1-2 minutes to make up the 7 degree differential and then will stop. The heating loop (baseboard) will loose the 7 degrees over the next few minutes, and the cycle will start again. It would be really nice to have a larger differential to work with ... say 20 degrees F.

Rudy

Settings

Mine is an oil-fired boiler.

I use a Honeywell Chronotherm IV setback thermostat programmed for 63F night and 68F day and have IntelligentLogic mode disabled (used just in timer mode).

My R2107 is set as follows -

Constant day mode,
Room Temp = 74F,
Setback mode,
Boiler Max temp = 187F,
Reference temp = 170F,
Offset = +2

On a day that was 18F, my boiler is ~180F

I found that these settings gave me better response overall, but particularly on mild days (45-55F outside). My wife's schedule is rather erratic and she complains if she has to wait too long after bumping the thermostat before feeling comfortable.

Ron Schroeder_3

Hi psd,

I agree with you. My Buderus likes a minimum differential of about 20 degrees with the thermal mass of my piping.

Could you set a higher hi limit and a lower design temp on your heating curve so it would never bump into your hi limit and keep a larger differential?

Ron

Rudy

Yes, but then I would sacrifice heating system response time.
My wife is used to our old system that would produce heat as soon as she adjusted the thermostat ... she's not very patient and isn't content with knowing that the boiler is "Buderus blue" and the technology is state-of-the-art. As she simply states ... "heat is heat and I want it now!"

Do you also find that the R2107 will limit with a minimum differential to about 7 degrees F? I'm concern that it may get worse on colder days and that would really be bad for short cycling.

Also, there is really no way to practically prevent bumping against the high limit in New Hampshire where the outdoor morning temperatures will get to -10F to -20F. In this case the R2107 really needs to maintain a reasonable burner differential to prevent frequent cycling

Rudy

Ron,

I'm attempting some different settings that push the heating curves down a bit. This should move the outdoor temperature point at which high limit is bumped, thus allowing a greater burner differential more of the time. With these settings high limit will still be crossed, but I'm hoping that the R2107 retains more of its differential as long as it can reach the heating curve before high limit is detected. I wish I knew more about how the burner differential works when operating at/near the high limit!

My questions for Buderus are -

Given that a +/-14 degree burner differential is normal when not bumping the high limit, what happens in the following cases:

What is the differential when the high limit is bumped after reaching the heating curve, but before reaching the upper threshold of the differential range? Is it the remainder of the normal +/-14F degree differential? Or is it half the normal differential (14F degrees total)?

The more important question is, what is the differential when the high limit is bumped before reaching the heating curve? Again, is it the remainder of the normal differential, or is it some fixed minimum value, or is it the larger of the two? If it is strictly the remainder of the normal differential, then some really bad things occur, so I really doubt anyone would design it like that!

My new settings are -

Day temp = 74,
Max Blr Temp = 190,
Ref temp 155F,
Offset 5

And my heating curve now looks like this -

131F @ 50F, 154F @ 32F, 178F @ 14F (after change)

135F @ 50F, 165F @ 32F, 192F @ 14F (before change)

I'm finding that I really need increased boiler temperature for the mild days (~50-60F) for reasonable performance, so I need to keep the bottom half of the curve elevated. The only way I see to do that while not bumping a reasonable high limit on cold days, is to flatten the curve and then offset it. This means decreasing Ref Temp and then adding positive Offset.

Ron Schroeder_2

Hi psd,

I don't have a Buderus control on my Buderus boiler so I was just speaking generally. I think that if you program a lower design temperature AND a higher maximum temperature, it would keep the same slope of reset curve, just extend it. That way, it would still have the same boiler temperature at your true design temperature but since it would not be at the high limit at that point, it would have a larger differential. Since the reset curve would still be the same, the night setback recovery should still be the same.

Ron


Ron

Ron Schroeder_2

Hi psd,

I was just looking at the 2107 manual. It looks like what you need to do is just set MAX TEMP 1 higher. That won't change the heating curve, it will just extend it and move the point of only 7 degree differential to a colder outdoor temperature.

If you don't get quick enough recovery in cold weather, increase the REF TEMP.

If you don't get quick enough recovery in cool or cold weather, adjust the OFFSET up.


Ron

psd_3

Ron,

Yes ... I have read the manuals exhaustively a number of times and understand the parameters that can be changed.

I do not want to push max temp much higher than where I am now (190F) as this leads to other issues (more heating noise due to pipe expansion, greater stress on components, etc).

Given this, I'm sort of stuck. I either have to give up low end boiler temp which I need for better performance on mild days, or I have to live with a very small differential when bumping the high limit.

The solution I've posted in my last message is sort of a compromise, but I don't know how well it will work. Lowering the Ref Temp flattens (reduces the slope of) the heating curve so that I can retain higher numbers at the low end without having extremely high numbers at the high end. Then increasing the Offset allows vertical movement (shifting) of the whole curve so that I can tune both ends together.

The settings I now have seem reasonable at both ends 131F @ 50F and 178F at 14F, but I'm not sure how well it will perform in the middle of the curve. Also, the upper end setting will still not prevent bumping the high limit on really cold mornings, so understanding how small the differential can go in the R2107 is still important. Hopefully someone from Buderus will explain how the differentials work around the high limit.

Ron Schroeder_2

Hi psd,

My thought is that if you have the right slope, the boiler will never hit the high limit unless the outdoor temperature is colder than the design minimum temperature so even if you set the MAX TEMP 1 at 500 degrees, it will never get there unless you put the outdoor sensor in liquid nitrogen. I would plot the curve of boiler temperature down to your design temperature and the set MAX TEMP 1 about 10 degrees higher

psd_3

Ron,

Perhaps true with a very shallow slope and significant offset. But at this point I would be significantly deviating from Buderus's recommendation for baseboard heating. I would need to run a very low Reference Temp (140F or lower) and significant +Offset (likely beyond the +/-9 capability of the R2107).

I live in New Hampshire where morning temperatures go below -10F regularly in January and February. If you take a curve with a "reasonably" shallow slope (say 1.2 boiler_temp/outside_temp) and a "reasonably" large offset to provide adequete mild temp response, I think you will see that hitting the high limit is still an issue on cold NH mornings. Also, high limit can only be programmed to a max of 210F.

Remember that the upper differential threshold goes above the curve by 14 degrees F, so you need to factor that in when you look at hitting the high limit. That's why I'm asking Buderus to explain the differential behavior under a number of scenarios around high limit.

From experience, a reasonable curve for my system is one that is roughly 130F @ 50F and 170F @ 14 (a slope of 1.1). This can be roughly achieved with a Ref Temp of ~150F and an Offset of +8. At -10F the heating curve is at +200F and the upper differential is +214F (above the high limit).

In any case, I think it's possible to reduce the days that have an issue with small differential, but I'm not sure that it's entirely avoidable.

Ron Schroeder_2

Is -10 F your design temperature? What boiler temperature do you need to heat the house at that temperature?

Ron

psd_3

The design temperature for my area and heating system is actually -20F.

The boiler temperature needed is somewhat subjective. The boiler temperature to maintain house temperature, as opposed to raise house temperature in a reasonable time period can be quite different. I would say that an average temp of 170-175F would easily maintain, but an average of 180-185F may be needed to provide reasonable recovery performance. To complicate this further, you need to consider what's happening at the other end of the curve. I still need to maintain a reasonable boiler temperature (~130F) on a day that is 50F for reasonable setback recovery.

Ron Schroeder_2

Ok, I will see if I can come up with a curve that will work for you. It will probably be monday before I have a chance.

Ron

psd_3

Ron,

Thanks

Over the weekend I'm trying the following settings:

Day Temp = 75F (adds ~ +4 degrees of equivalent offset)

Ref Temp = 140F

Offset = +9F

The combination of the Day Temp adjustment and Offset, raise the curve by about 32 degrees F.

The data points I see reported from the R2107 are:

133F @ 50F; 153F @ 32F; 172F @ 14F

Extrapolating the curve, it now appears to cross the high limit of 195F at ~-10F. The upper differential crosses earlier (just before 0F). I'm hoping and speculating that the burner differential is not severely limited, or fixed to a small value, until the actual curve crosses the high limit point, or better yet, until the residual differential errodes to 7 degrees. This could still cause some frequent cycling issues on a -20F day, but those are far and few between. We won't know this until Buderus responds with some details.

Bill Nye_2

You guys !

> Ron,

>

> Thanks

>

> Over the weekend I'm trying

> the following settings:

>

> Day Temp = 75F (adds ~

> +4 degrees of equivalent offset)

>

> Ref Temp =

> 140F

>

> Offset = +9F

>

> The combination of the

> Day Temp adjustment and Offset, raise the curve

> by about 32 degrees F.

>

> The data points I see

> reported from the R2107 are:

>

> 133F @ 50F; 153F

> @ 32F; 172F @ 14F

>

> Extrapolating the curve, it

> now appears to cross the high limit of 195F at

> ~-10F. The upper differential crosses earlier

> (just before 0F). I'm hoping and speculating that

> the burner differential is not severely limited,

> or fixed to a small value, until the actual curve

> crosses the high limit point, or better yet,

> until the residual differential errodes to 7

> degrees. This could still cause some frequent

> cycling issues on a -20F day, but those are far

> and few between. We won't know this until Buderus

> responds with some details.

Bill Nye_2

You guys !

You guys are making me crazy. I think you are missing the whole concept of what the control is trying to do for you. 190° is nuts, my house was built in 1910 and my water temp has never gone above 165° on a minus zero day.

Forget the setback t-stats, you don't need it. You are already running outdoor reset so you are saving money. Let the system run constant circulation at the lowered water temp. You will be much more comfortable and you will not have the recovery problems. You will not need 190° water to warm the system back up from cold start.

I have not had a thermostat in my house since 2000. I run constant circ with baseboard and in-floor [staple up] and my house is either 69° or 70°, always! Unless of course we have a lot of solar gain and the daytime high may be 72-74.

If I am not mistaken your control will reset the water temp. for your night setback. You can reset the boiler temp. there by reset your room temp.

They heat homes in Europe with 140° water! I use Viessmann but is very similar to Buderus. Did I confuse your 2107 with the 2109? If so, you should have bought the better one.

Your set back t-stat doesn't help you one bit. Set it at 72° and leave it. Set your curve so your house never reaches 72. During the spring and fall you may have to move the parallel shift up or down a little. That's easy enough.

In extreme cold , night set back is not your friend and pipes could freeze while your house is cooling down. Daytime setback, while everyone is at work or school, may make more sense. Do I make any sense at all?

Paul_28

Constant circulation

I think I under stand what you’re saying. If I want a day time temp of 70* and a night time temp of 65* then the 2107 should be set for those values. I would set the thermostat for 72* and leave it there. The pumps run constantly because the thermostat setting can not be reached.

Ron Schroeder_3

Hi Bill,

As much of an energy nut as I am, I don't use night setback for savings, I do it for comfort. If psd is doing the same, I can see the problem of balancing the reset curve with recovery rate. One possable solution might be to use the day/night mode to fake out the curve by setting the day mode to just the hour or two of the recovery time. Then the room monitor feedback might boost the boiler temperature to allow the house to recover faster but the thermostats would prevent overshoot.

If psd's house was designed with an actual heat loss, the radiant may very well have been designed for 180~190 deg at the design temperature. A lot of houses have much more radiant than they need so they of course could use lower temperatures.

Ron

steve_68

don't know the answer to the burner diff question, but

are you also using a BFU for adjustment to the curve based on indoor conditions?

psd_3

No BFU, just outdoor sensor for outdoor reset control.

As stated previously, lower boiler temperatures will maintain house temp well, even on the coldest days. The issue is strictly with morning and afternoon recovery times.

Yes, using 2 curves, one for sustaining/maintaining periods and one for catch-up/transition periods would help with the recovery issue. On the other hand, it doesn't solve the loss of burner differential when bumping high limit ... the fundamental issue that started this whole thread.

As for maintaining constant temp day and night (no setback), or using boiler setback with constant circulation, I understand these concepts well. We just don't like the constant circulator humming noise, it's a lot more comfortable sleeping in cooler temps, and the dryness isn't as noticable (less morning headaches).

I'm also a firm believer that setbacks save money as long as the setback duration is reasonable (say 8 hours or more). Heat loss is driven by temperature differential. The greater the differential, the more the loss. Also, a boiler that is running for a sustained period of time to catch up is on average running more efficiently than one that is just running periodically to maintain temperature. This is especially true after a sustained off period as is the case with cold start.

I have a 2600 sq ft home and typically use between 500 and 600 gallons of fuel for heat and hot water. That's with keeping the house at 68-70 during the day and 63 at night, and with 2 teenage daughters who enjoy 45 minute showers (ugh! that's another issue). When I first installed the new system, I tried running constant temp (68F) with a reduced heating curve, using minor adjustments for comfort. I didn't like what I was seeing for fuel usage (comparatively speaking).

My home is well insulated and the pipes are well protected, so I'm not at all concerned about freeze-ups with a ~6 degree setback.

In retrospect, I should probably switch to a 2 curve approach ... one for sustaining and one for catch-up. This would save more energy by reducing boiler standby losses during sustaining periods. I would need to synchronize these curves with my setback thermostat(s) for proper behavior/performance. I think I would also require more state transitions than the R2107 allows to be programmed. Six states are required to support 2 setback periods per day. This is where a BFU (room sensor) would be handy. Used with the R2107, it would automatically boast boiler temp to deal with recovery performance during transition periods.

Mellow_2

psd

Try buderus.com they have a forum. You can ask direct.. The room sensor will give you faster responce and if you run constant circ on the largest zone you can setback the whole system. good luck

m_3

bill nye

I agree with Bill Nye here, this conyrol is not designed for such deep setbacks. Allow the control to go into normal operation with its day/night settings and either lose the set back at the thermostats, or keep their setback to 2-3 deg range.

psd_2

I understand that the control is very flexible. I understand that the control allows curve switching based on day/night heating requirements, and that this could result in constant circulation when operating on a lower curve if a thermostat setback is not also used.

A 5-7F degree setback is not deep. Moreover, even with a 2-3F degree setback, high limit (the programmed max boiler temp) could be bumped on really cold days.

Forget all this, and go back to the initial question ... how does the burner differential work when operating at/near the programmed "Max Temp"? How small can the differential get? How best to program the control to maintain reasonable differentials knowing that it may not be possible to avoid operation at/near "Max Temp" on some days? Then factor in the desire to use a 5-6F degree setback and ask the same set of questions.

Case in point -

The Buderus service manual suggests a "Max Temp" of 185F. With a reference temperature of 167F (the default for baseboard perimeter heating), and the normal +/- 14F differential swing around the curve, Max Temp would be bumped on a +5F to +10F degree day. On a -10F to -20F degree day, the default curve for a Ref Temp of 167F will significantly exceed "Max Temp". If the differential gets really small under these conditions, there could be issues.
Upward adjustments to "Day Temp", or adding +Offset, will add to the issue as these shift the curve vertically in the direction of the high limit. This is all independent of the use of setbacks!

Ron Schroeder_2

Hi psd,

It's my understanding from reading the manual and looking at the graphs that the differential is both above and below the target temperature of the point that you are on the heating curve. The differential is plus 7 and minus 7 degrees for a total of 14 degrees. As you get close to the high temp limit, you start taking away from the plus 7 part of the differential untill only the minus 7 part is left. The control never lets the differential go below 7 degrees.

So........If you want to keep the 14 degree total differential, you need to set the hi limit to 7 degrees above the average boiler temperature needed at the coldest design outdoor temperature. And you need to adjust the boiler temperature setting for 14 degrees and the offset to get the curve to hit that target average temperature at the lowest design outdoor temperature.

Hopefully Joe@Buderus will jump in here and confirm my guesses.

Ron

dconnors

You go Ron

Thats pretty well spoken. Good to know some do read the manuals.
Congrats.

psd_3

Ron,

That was my first impression after reading the manual the first time. However, it didn't work that way in real life. I was observing a +/-14 degree swing around the target curve. I went back to the manuals with a straight edge and looked at the curves and sure enough they also showed a +/-14 degree swing (indicated by the hash marks around the solid curve).

Further emperical analysis showed that the differential would degrade to ~7 degrees when the target curve exceeds the high limit. On the otherhand if only the upper threshold exceeds the high limit, then the differential degrades to about 18 degrees. I would really like to know the algorithm used because I bet it's really not that simple. As stated in previous posting, and as you speculated in your last post, I thought the differential would also degrade gradually when the target curve is below the high limit. To prove this I started with a target curve that was below the high limit at +5F, but whos upper differential was above the high limit. I then reduced the high limit by 5 degrees. In both cases I observed an 18 degree differential. Go figure ... the manual isn't the proper source for these details.

I found a set of parameters that seems to work well to about -10F. I observed the behavior to about 0F and it seems predictable. I don't really know what will happen below -10F, but I'm hoping that the differential doesn't degrade beyond the ~7 degrees. I'm still hoping that Joe or Dave from Buderus will jump in with the theory behind the control so we can base this on the actual design intent rather than a few test points and a heap of speculation .

Ron Schroeder_2

Hi psd,

Your right about the +/-14 degrees. I was looking at the manual while waiting at a light in my truck so I missed the scale a little.

Change my reccomendation to set the high limit to 14 degrees above the target at your coldest point and you should keep the full 28 degrees differential. But realistically, 7 degrees wouldn't make much differance in your oil consumption since your outdoor temp won't be at -20 very much.

Ron

psd_3

Ron thanks -

Seems that our thinking is converging -

Setting the high limit +14 over the target curve for the coldest day (-20F) is really not my preference as this would put it at the limit (210F). My current solution is as follows:

Max Temp = 195,
Ref Temp = 140,
Day Temp = 74,
Offset = +9

The combination of Day Temp being +4 over the default and an Offset of +9, gives an effective Offset of +13. This translates to a vertical shift of ~+29F.

This gives me a fairly shallow curve with the following points:

129F @ 50F, 149F @ 32F, 169F @ 14F

The upper threshold crosses the high limit at ~0F, at which point an 18 degree differential was observed. The target curve crosses the high limit at about -10F, and I suspect that I will retain an 18 degree differential at this point. The differential will degrade to about 7 degrees (and I'm hoping that's as small as it goes) somewhere above this point.