Vitodens, Outdoor Reset & Solar Influence

Mike T., Swampeast MO

Just heard from Viessmann engineer that I can "play" a bit.

Am going to have two outdoor temp sensors--one mounted as suggested on the North facade, a second on the South. Will connect them via a DPDT relay (gold-plated contacts at his recommendation) with switching a function of solar intensity, outdoor temp and hopefully wind. (Switch won't occur on very cold or cold/windy days.)

He said that the momentary loss of contact when the relay switches shouldn't cause a problem as input will be restored before the boiler can detect a fault. Also said that the built-in "outdoor damping" of the Comfortrol™ will deal very nicely with the situation should there be a large temp difference when the switch occurs.

Why, you ask?

My reset curve is based on the most under-radiated zone and the system isn't particularly well-balanced. The boiler target is, therefore, generally much higher than needed by most of the house. The radiant bath floors work off of this curve, but during periods of high solar influence and/or warmer weather really don't need much heat--nor do they give off much heat... If the kitchen cools a couple of degrees more than normal during the heat of the day, it won't bother me in the least.

I had no problem shutting down the old system completely when outdoor temp reached 55° or so but it's difficult with the Vitodens. I can disable the burner, but the circulator will still run--undesired as the heat in the radiators when the old system shut down was enough to keep things fine for hours. With the circulator running, the heat will keep moving and likely be given off in the coldest part of the house--the basement. Should be able to fool the boiler a bit this way and have it "think" that the outdoor temp has risen above setpoint.

jp_2

trv's

don't your TRV's take care of rooms overheating by the sun?
I thought they mount on the wall? I've only worked on floor heating so I really don't know much about radiator systems.

odd thing about the floor it can be -10F or 30F and I still get about delta 20 on return and out going water temps, the room will vary about 5F then. really like to get one of those hobo's.

I'd be curious to see a piping diagram your system?

Mike T., Swampeast MO

Only to a point.

The radiators are so oversized and operate at such low temps that even a slight increase in mean radiant temperature has a surprisingly large effect.

Today for example was beautiful and sunny with a pleasing SW wind. Outside temp (measured on the North topped at about 64 degrees late in the afternoon). Measured at the South, the temp topped 69 degrees by noon.

By late afternoon the South-facing rooms were fully 10 degrees above TRV setpoint and the North-facing 6 degrees above setpoint. EVERY rad in the house was cool (probably ambient) by that point.

The Vitodens continue to "pulse" at 5 very short firings per hour--it won't shut down until the outside temp exceeds the room temp setting. Since that happens at 69 degrees, it didn't shut down.

Previously (still TRVs, same reset curve and constant circulation but different boiler) I would shut down the entire system at 55 degrees outdoors and the indoor temp would only fall objectionably in very unusual circumstances.

This is the one problem I've found with the outdoor reset scheme of the Vitodens--it may however be rather specific to this installation as my radiation, while greatly oversized, is not well balanced and the curve has to supply the least over-radiated zone.

Were this the indoor reset scheme of the new Buderus, the boiler would have shut down. The problem would be finding a ideal location in the house for the indoor sensor...

I could provide an "external burner disable" connection to the Vitodens via my old WWSD control, but I believe the circulator will continue to run. Not a big deal in some conditions, but with long-running periods of outdoor temp near the shut-down point, I believe the "reserve" heat stored in the radiators would be moved to the basement where the air is coolest.

Kritz_3

Outdoor damped?

Hi Mike,
What is the 'Outdoor Damped' function of the Confortrol?

Steve K.

Mike T., Swampeast MO

As explained to me today, the controller essentially ignores rapid swings in outdoor temp assuming that the mass of the building will carry it through. No mention of the magnitude nor the time involved--my guess is that it's part of the learning function and is probably different in every structure.

Was told that the Comfortrol has nearly instant damping response so it could handle a very rapid sensed temperature change but that the Vitotronic (hope I got that right) couldn't.

If it works like I want, solar gain will be both anticipated and allowed to "run out" so in most circumstances the temp difference between the two sensors won't be too great to begin with. Will lock out the feature when the weather is much below freezing and the burner fully modulating anyway... Even in the worst case where the South sensor gets selected during bitter cold weather, I have at least 30 degrees of outdoor temperature tolerance before there's the slightest chance of the least over-radiated area getting too cold. My least over-radiated area (the kitchen/laundry) maintains about 59 degrees with an average radiator temp of only 64 degrees or so in 34 degree weather and no occupancy gains...

jp_2

partly cloudy??

would the system start bouncing on days that are party cloudy somewhat cold?

oops you answer that already.

if the "reset" has a real curve, why can't you adjust curvature at one end?

I was thinking too, it you are running constant circ. and your boiler short cycling 5x per hour, doesn't that sound like piping losses in the house?

also seems like your house is not insulated real well?
judging by the inside/outside temps you gave? just a guess.

Mike T., Swampeast MO

Curve of the Vitodens is a true curve--not a perfectly straight line. A bit steeper at warm outside temps and a bit flatter at very cold. You can only adjust slope and shift however--you can't change the shape of the curve--it's built-in.

Yes, the majority of the heat during those short, little burns is just being given off by the piping--gravity conversion system so there's LOTS of it. There's also the radiant floors that just run "wild" off the system, so they're receiving and emitting heat as well.

Since there's shift in the reset curve, the boiler always calculates a setpoint above the outside temp--even when the outside temp is at the room temp setting of 68°.

If the outside temp is say 50°, I still get the very short "pulsing" burns--in between longer burns that vary from about 1 - 2 hours. While it's a modulating burner, it can only modulate down to about 22 mbh output so if the house isn't loosing 22 mbh it can't fire continually.

Insulation/infiltration is reasonable for a 1903 house with 47 large windows (plus 5 in the basement) and 14 doors to the outside (4 of them single-glazed traditional French doors with narrow muntins). While it does seem to heat rapidly with the sun, it also holds heat reasonably well. Actually, I kind of thought that turning off the old system above 55° outdoors with an objectionable drop inside only in rare circumstances showed decent insulation.

Am going to hook up my old warm-weather shutdown control to the Vitodens in just a minute via the "external burner disable" connections. It won't stop the circulator, but it will stop the burner. Have the new sensor on order, but probably won't have time to set up the system before the next heating season.

Mike T., Swampeast MO

Forgot the piping diagram, sorry.

Really too simple for a drawing. Two-pipe, direct return gravity system with two main pairs. Supply/return pairs brought together near the boiler and the boiler just connects via a differential pressure bypass valve.

TRVs on all of the radiators and four very short radiant floor heat loops serving bathrooms. The radiant loops just connect to the mains so they receive constant circulation. Only form of control for the radiant is the temp of the reset curve.

If you're familiar with Vitodens piping diagrams, it's the simplest one: system type 000 with no DHW.

Kritz_3

Thanks Mike, simple and to the point.

Steve K.

jp_2

radiant floors

sounds like the floors dictate system temps if there are no valves?? don't they get too warm on real cold days?

David Woycio

What if.....

instead use a differential temperature controller (like a Tekmar 155) and mount the collector sensor on the south side of the building and mount the storage sensor on the north side of the building. The outputs of this control could be used to "read" these 2 sensor locations. You may need an additional DPDT relay. Keeping your Vitodens sensors in place (south and north) the diff. control would automatically switch between sensors dependent upon temperature differences between south and north.
For example; if you set the controller to activate when the south side of the house is 20deg warmer than the north side of the house the south sensor would be read by the boiler. A setting of 5 deg. warmer could be set to switch back to north sensor reading.

Mike T., Swampeast MO

Floors came after and were engineered to work with the available supply temperature for the least over-radiated area (kitchen). Heat transfer mechanism varies depending on the construction of the panel.

Highest temp I've measured on the intentionally warmest floor (a shower) is 87°. The rest top out around 83°.

Mike T., Swampeast MO

Thanks! Like the idea of an off-the-shelf controller. Will definitely look into it.

Ted_5

Turn down your sun dial to 59F

and then do a pluss shift to get the curve back as needed. The boiler will shut on WWSD at 59F. If yo disable the boiler you can have the pump shut off, but you will loose DHW if you have it. You could also add a room temp feedback sensor to the Vitodens. It will then ajust its set point based on room temp. Viessmann does not reccomend this in high mass systems.

Ted
Viessmann rep.

Mike T., Swampeast MO

Thanks! May just try that Ted, but it did give me another idea.

Could I use my WWSD control as the trigger for reduced temperature mode via the "external changeover" function?

No DHW at present, so that's not a concern.

Ted_5

Yes, that will work

take the jumper wire out of the X4 1&2 and put your N/O dry contact wires on there. Then change code address 011:001 and it will switch.

Ted

Mike T., Swampeast MO

Done just before your message.

Reduced setting counteracts positive shift of 11°C

Initial changeover point 48°F (9°C)

Ratio 0.8

Will be interesting to see how long it takes to consume the buffer.

Mike T., Swampeast MO

Interesting Results

"Telephone Prog." (or something similar) appears in display when the contacts closed at around 52° outside.

Circulator stops running with target temperature of 32°.

Turned up the reduced mode temp dial considerably and waited a few minutes. No change in target temperature and circulator did not re-start.

Is this normal?

Ted_5

look at coding

OC8: are you closing or opening the contact? Do you have a timed setback programed?

Ted

Mike T., Swampeast MO

It's the OC8 programming. Knew I'd seen that before and sort of forgotten and sort of confused.

The statement "continuous space heating ON" mean" sounds like another way to say "external call for heat" where it heats to the 0A2 setting...

Am changing it right now as outside temp is right at the crossover point.

Mike T., Swampeast MO

Had done that before during initial setup (for the electric TRV setback) but had problem and returned everything to factory settings.

Spent more time looking and figured out the problem. With contact closed, it's running at the NORMAL mode regardless of timer.

So, to get it to switch to REDUCED mode when I want, I have to set the boiler for continual reduced mode operation and use the normally closed contacts of the WWSD control--that way it reverts to the time program (continual reduced mode) any time the outside is below my switch point.

Am doing that now. Does it sound right?

Mike T., Swampeast MO

Success. All timer programs removed so the reduced temp mode is in effect continually. WWSD gives the mode change command to normal mode whenever outside temp is below the setpoint.

It also appears that anytime the calculated target is below the room temp setting, that the target is "adjusted" to 32° with the circulator disabled.

Sound right?

If so, have finally simulated complete WWSD.

Unknown

I don't understand this at all. High mass systems are exactly when one would WANT indoor feedback more than any other time, right? How does viessmann handle, say, a slab radiant with internal solar gain during the day? As far as I can tell, without an indoor sensor the boiler is completely blind to potential temperature overshooting. Is this accurate? Is it really relying ONLY on outdoor temperature and differential between return water and its outgoing calculated water temperature?

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Mike T., Swampeast MO

Outdoor temperature, calculated setpoint and boiler supply temp--don't believe it even senses return.

As with any boiler a slab with a lot of solar gain will be a problem. That's probably why they say that the indoor sensor with feedback should only be used with low-mass systems. By the time it detects a problem, it's already too late

You'll have to control the solar gain, monitor the slab, etc.

The "ideal" curve for the Vitodens [seems] to be one with either zero or negative shift--no problem with high mass systems like a slab.

My problem is imbalance in the radiation (inherited) and probably made worse by my radiant bath floors.

With a positive shift of the curve, the boiler always wants to produce heat because the calculated setpoint is ALWAYS above room temperature and there's no accounting for solar gain.

It would be nice if there were an option for a "sticky" curve with the very bottom always staying at the room temp setting regardless of shift with an adjustable "pivot point". Believe that Tekmars have similar settings for their curves.