Buffer tank

AlHeating

I still believe that any problems should be cause by over-sizing instead of the temperature. No matter what are the temperatures, a 20 degrees delta T is a 20 degrees delta T. With a these "low water content" units, you're circulating fast enough. I operated a few Knights for snowmelt with return temperatures sometimes as low as 50 degrees with NO problems, without any external controls.

When i face low load applications, i use the standard "scale firing" of the Knight to start the boiler slowly. ( 6 steps with adjustable times and firing rates ). So even if you have only one zone demanding, you have no cycling problems since it's sized CORRECTLY. It modulates so quickly that it match instantly any load in its range.

The Vitodens 200 is a real good unit. But the control tries to think too much by itself. It can be a problem in the lower range of the unit. The control is too complicated. And on the other side, the Vitodens 100 as no controls ! That why I install units with a complet and integrated control.

EMB

Buffer tank

Hi all, I'm putting together a proposal for a system with two (one high temp, one low) heating circuits. The problem I'm having is the tightness of the house means that at design temps the total heatloss will be less than 35,000 btu's.
The owner wants the best (Vitodens 200), however, I'm concerned that on anything other than design day, the boiler will short cycle itself to death.
I'm thinking of running everything (except dhw) through a buffer tank to avoid this possibility.
Anyone have any thoughts/ideas?
I'd also appreciate your thoughts on control strategies.
Thanks, EMB

Mike T., Swampeast MO

Mfgrs: Give US Smaller Mod-Cons!!!

Having carefully observed and monitored the operation of a Vitodens 200 6-24 for five years, I'm convinced that the engineers had one and only one view of how to achieve the highest possible efficiency--match boiler output to the real-time heat load using the lowest possible average temperature in the HX able to meet the load. I agree with this philosophy, but it can cause some difficulties.

Presuming this is a Manual-J heat loss of 35mbh, I'd say that a Vitodens 200 would rarely--if ever--be able to do what it's designed to do and thus never achieve the efficiency levels it is fully capable of producing. Instead, in most cases it will either act like a conventional on-off boiler (operating at minimum modulation) or it will repeatedly and frequently fire for just a few seconds at maximum modulation. In my house (100 mbh Manual-J heat loss) during a typical heating season the boiler spends nearly half its' time away from true modulation.

Mine is a gravity conversion system with the mains alone holding over 100 gallons. As I suspected, the mains act as a sort of buffer, but once load drops below some level (about 12 mbh in my system it seems) the buffer effect disappears. Even though I didn't need the low-loss header with the internal circulator driving the entire system, I have heard of similar operation in systems using the LLH when load is low.

To achieve the desired operation of the buffer it should be able to alternately appear either as a load to the boiler or as a heat source to the emitters. While this sounds simple, I have yet to come up with a piping strategy that does this without numerous expensive components and it would be further complicated by a two-temp system. Considering the cut-in/cut-out differential of the Vitodens is both variable and non-adjustable, use of a buffer is even more complicated.

While I highly respect the homeowner's desire for the "best", my personal opinion is that the smallest Vitodens 200 is far too large for this system and that the mod-con with smallest output (and greatest turndown ratio) be used.

If the homeowner insists on the Vitodens 200, here's an idea:

Use a dual-coil indirect as the ONLY load on the boiler. The lower coil will heat the water in the tank, the upper coil will transfer heat to the space heating (you'll set up a conventional system using the indirect as the boiler) with the water inside the tank used for DHW. BTW, Viessmann is the only mfgr. I've found who publishes very good data regarding heat transfer in their indirects.

With such a small space heating load, there shouldn't be any problem unless peak DHW demand occurs at the same time as setback recovery. This won't result in the highest possible efficiency from the Vitodens, but nothing else will either and at least it won't be operating in "pulse" mode most of the time.

EMB

Smaller mod-cons

Thanks Mike, yes, we do need smaller modcons. Maybe we should open a thread/petition that hopefully at least some of the manufacturers will take notice of. In the meantime though I agree entirely with your opinion of the Vitodens being too much boiler for this application. Even the new TT Solo is almost double the required load, but would be somewhat closer.
EMB

AlHeating

Smaller Mod-Con

Hi EMB.

I personally think that this person should better go with a Knight from Lochinvar. They have a wall-mount model as small as 50mbh with a 5:1 modulation ratio(10mbh minimum). I use it most of the time for this application. The boiler (especially the control) is great. It modulates perfectly even in the lower range.

Al

Unknown

Vitodens \"pulse mode\"

The Vito 200 is a great machine, but as mike has stated it's native control strategy will result in "pulse mode" operation under low load conditions. One consolation is that the machine was designed for this type of operation because it's sister the combi, makes dhw via an integrated plate heat exchanger and must be able to tolerate intensive cycling.

Mike, from my observations I believe that low supply temperatures are the crux of this issue. The ignition sequence of the boiler lights the burner at a relatively high output and then it throttles it according to load. I have observed that when target temperatures are below around 90 deg. the boiler reaches target during the ignition period and the burner shuts off before it even makes it thru the ignition period and gets to min modulation.

I have my suspicions that at very low temperature operating conditions there is no buffer strategy that will eliminate this cycling behavior. As I see it if return temperatures are only a little above indoor ambient ( say 70-75) the burner (at it's relatively high ignition sequence firing rate) very quickly raises this to a 90 target temperature.

My own house has a very low design load (less than the Vito's minimum firing rate.) It's mostly exposed slab and supply temps are below 90 most of the season. I couldn't stand the short cycling. I have a unique work around that has performed very well utilizing a ODR based interval timer (Tekmar steam control 269).

I'm sure mike is correct in his assumptions about the intentions of the german designers, this pulse mode does not adversely effect performance, it may actually be highly efficient. Still it's my feeling that that in certain low load low temperature applications this is a shortcoming. If the control had the ability to expand it's differential under these conditions this would do a lot to help the problem. I have a feeling that if the boiler was being controlled by a Tekmar tn4 system this short cycle would not be as pronounced. If you have natural gas consider the vito 100, from what I'v heard it lights on low fire. It's also much easer to integrate with 3 party controls. I have a feeling that an automatic differential feature and the different ignition sequence would probably largly eliminate this issue, even in lower load homes.

Mike T., Swampeast MO

I've operated my Vitodens under a number of different heating curves in an attempt to reduce/eliminate the "pulses". In my system at least (no LLH, fully TRVd), raising the curve significantly will only stop the pulsing until the system stabilizes. Regardless of the supply curve, when load is low the target temp will be exceeded in the few seconds of high fire before it modulates down.

To my complete surprise the "pulse" mode appears to be quite efficient and to my expectation the entire boiler (especially the ignition system) is built with this mode in mind. The direct spark ignition lights the burner with the very first spark (unless you've worked on the gas lines and have some air or do something stupid like forget to turn on the gas supply after annual cleaning...)

When truly modulating at or near minimum input (in my system such occurs about 25% of the time at about 100F target using a barely adequate curve), I've calculated actual boiler efficiency above 99.5%. Granted I lack the ability to test the actual BTU content of my gas, but using very fair assumptions, the equations provided in the Viessmann White Paper titled Condensing Technology plus comparison to higher modulation levels, and highly accurate real-time condensate production levels, I'm reasonably confident that under optimal conditions the Vitodens 200 can come so close to 100% efficiency (in its 5th season) that standard rounding will find 100%...

It was the the surprising (to me at least) effectiveness of even a very short (about 6') horizontal, concentric flue at warming the incoming air that pushed efficiency to levels that I previously thought impossible. While the required Viessmann venting is very expensive, it is an integral part of the system that makes a significant and free contribution to efficiency. BTW, if you know my writing style, I often use quotation marks around a term like "free". No need in this case.

That wall-hung 50mbh Knight with 5-1 turndown ratio and SS HX deserves HIGH consideration in this system. BTW, it seems to be very new and I can't find much info even at the Lochinvar website.

AlHeating

That efficiency is not a surprise : the HEX of the Vitodens is without any doubt one of the best in the world. With cold water return and the lowest modulation, it almost get to 100%. I sometimes see 100% on a system start-up (when everything's cold).

The Knight wall hung is "hidden" with the standard models. (in boilers sections, Knight) It's new (less than one year) but it's the same thing that the floor standing models which is 3 years old now. We're installing a LOT of them around here. It's Cheaper than a Vitodens200, got a real smart control, nice SS hex (thanks to gianonni), but maybe less reliable than a Viessmann for long term ...

Unknown

A 20 deg. delta on a snow melt should be easy to achieve assuming the boiler is big enough. 20 deg. delta for a modest structure under low loads is a different beast.

I don't think 20 degree deltas are readily achievable in residential situations where heating curves are calling for 90 deg. supply temps. In these situations i think much narrower deltas are the norm.

I'm oversimplifying when I reduce the situation to low temperature operating conditions, obviously load and temperature are connected. What I'm saying is that I'v never seen this (short cycle) behavior at higher temps.

I believe that in a highly radiated situation like my own and perhaps Mikes as well (oversized cast iron). That low operating temps are all that is frequently required and under these conditions the Vito goes into "pulse" mode. It's not the short burns that bother me so much but the frequency of these burns. If the control had the ability to permit some degree of target overshoot, it would create a longer off cycle and reduce the frequency of this cycling.

Mike Because your system is TRV'd raising the curve tells you very little, if your system had no TRV's I'm prety sure that your room temps would go up and your boiler would stop short cycling. Night time setback was the real pulse mode problem time for me, shoulder seasons also bad.

The fact that the vito 100 has no controls is an asset as far as I'm concerned. This permits the installer to pair it with the control system that they favor for a given application, no waste. I think a 100 paired to a Tekmar modulating boiler control with automatic differential would give excellent performance with minimal cycling. I wonder when the newer generation of matrix-x burner machines will be making it to our shores. I have a suspicion that the Vito 200 may be reaching the end of it's production period.

Unknown

How are you guy's measuring efficiency? Seems to me that the only way too gauge this would be with a BTU meter, and gas consumption.

Measuring the condensate volume against fuel consumption is an interesting idea. Condensate production gives a good indication of efficiency, the steady and strong drip drip I see at minimum modulation in my system gives me a good feeling. I have observed other two pipe pvc vented boilers in-which more condensate is being produced in the vent than in the boiler, not encouraging, and a good argument for coaxial vent systems that have a chance of recovering some of this latent heat.

I think it's a reasonable guess that the vito 200 is the most efficient mod-con available in NA.

What are the features of this house, and the expectations of the owners, in terms of zoning and delivery? If the owners really want this boiler there are ways to make it work without short cycle, but for that mater it works fine with short cycle. Just don't put it next to the bedroom of someone like me, I couldn't stop listening for that solenoid click.

One possibility would be to operate the boiler in setpiont mode from it's external call for heat contact,(with the sun moon dials set to zero.) This is how I run my boiler, additionally I have set the burner potentiometer to it lowest setting, so it never goes above minimum modulation for heating (except during ignition). The control I use to determine firing times is designed for steam boilers. It works very well with my high mass radiant delivery which has some similarities to cast iron steam systems (load and soak). It took some time to properly adjust the control, but have eliminated the pulse mode. Because the burner potentiometer is set to min. the boiler never reaches it's external call for heat setting of 115 deg. The control has an indoor sensor which can downwardly adjusts the "curve" in response to internal gains (works well with the wood stove). I have a 35 minute cycle length setting and a 1 minute minimum runtime, I have never seen continuous operation. In "cold" weather run times are around 10 minutes per 35min. cycle. Supply temps generally level out at around 105, with a healthy delta T.

Obviously this hack is not for everyone. I wouldn't envy the service tech who's never seen a vitodens and has to figure out what's going on with a system like mine. Not for every one, but this worked for me. I could imagine similar strategies utilizing a P/S buffer tank and secondary mixing.

This is what mike was getting at, but without throwing away ODR and operating the boiler at DHW production temps for mild heating loads.

AlHeating

Efficiencies

Hi Scott !

Interesting what you say about condensing in the flue pipes. i've never seen that this way. Many boilers don't require a drain on the exhaust pipe because the HEX can take it (with its own drain). This way, it looks like the boiler is more efficient that it really is.

However, when we look at the flue gas temperature, it's usually 10 degrees coolder than the water outlet temperature. At a maximum of 185 degrees outlet (in my case with the Knight ... sorry i'm "sold" ahah) , the flue is around 173 degrees. I know it's not a perfect way to measure this, but the combustion analyser gives me a good idea of what the combustion efficiency can be. I'm surprise to see how efficient can be a condensing boiler even in "non-condensing situations". With the cold outdoor temperatures we have here, an I/O reset has to be set in a LARGE range to satisfy some peaks. It condensate a most of the winter, and the rest of the time it's about 89-90% efficient which is really not that bad. And it's about the same thing for any boiler.

I agree that we can say the Vitodens 200 is (one of)the most efficient. The problem (for us) is the limited temperature range, the venting limitations and the cost of all that. I always ask myself where is the line that make an investment like that "a good move". For a small residential boiler, it will cost MUCH more to install a Vitodens, to get what ... a few pourcentage of efficiency ?? What's the real impact on a small gas bill ?? I have absolutely no doubt that it will last a few years more than any other mod-con boiler. But is it enought so it worth it ?

Joe Furfaro

Buffer Tank

Instead of a buffer tank could the Vitoden be programmed so that at a certain outdoor temp it would disable itself for a short period?
My system incorporates a LLH so I was considering putting an external timer(controlled by a outdoor thermostat) on the two external pumps. In this senario the pumps would be disabled for a period of time.
Let's say one hour off 20 minutes on. When the pumps are off the Vitoden will not see very little drop in temp(because of the closed primary loop) and stay off for a longer time.

Ron Gillen

Is it colder in Germany?

We always seem to want to "Americanize" the Euro systems. Funny how the Vito operates properly in Germany and not over here. We want to add buffer tanks, mess with the constant circulation, add third party controls, all to avoid the operation it was designed for. My system operates stictly on ODR. Runs almost constantly at or near design (-25F) clocked many times @28,500, 95F supply 85 return. Pulses during the swing season, and only fires a few times at 5 AM early fall and late spring. Just as the designers intended it to do. It may be some what oversized but not by much when you consider how well it handles the domestic load when coupled with the Vitocel indirect.

Unknown

From what I'v heard (here) in Germany the burner lights at low fire and modulates up, this has to do with LP vs. Natural. The NA model was designed to light stably with propane which is almost non existent in germany.

It gets tricky to weigh the gains that can be achieved at the upper end of efficiency. I appreciate quality and while I'm not a metallurgist, or combustion engineer it's clear to me that Viessmann makes very nice machines.

Al, What you see as a limitation I see as a design prerequisite. Why go to the lengths required to build a reliable long lived condensing design and not make it condense as much as possible. Mod cons are gaining popularity because they are easy to handle flexible to vent and provide full out door reset without mixing valves, some even do fine (thrive) in direct connected arrangements. These are plenty of compelling reasons to use a condensing boiler that have nothing to do with latent heat. Viessmann puts condensing as a primary design objective, not a secondary benefit.

I'v seen vitos on normally sized air handlers with 4 way mixing valves for radiant. A lot of people didn't understand this stuff at first. Good condensing design starts with low temperature distribution. I think 165 is a reasonable limit it also protects glycol which suffers at higher temperatures. Hydro air is still viable it just requires bigger coils. I think chasing the upper limits of efficiency has merits that go beyond the percentage points gained on a small gas bill. Conservation should not be driven solely by economics, that's how we got in this mess.

Roland_15

Buffer Tank

Hi Scott, Do you have any pictures of hydronic heating installations in Deutschland? Can you post some? Thanks,Roland Schmitt

AlHeating

I'm really happy to see people thinking like you do.

"Conservation should not be driven solely by economics"

It's ALL TRUE !! But it seems that here, in North America, the energy is not enough expensive !! This is one of the reasons why I'm full of admiration for the "European vision" of efficiency. You guys don't look driven only by the cost ... Here, as long as the energy will be that cheap, we will not see as much efforts (like on the other side of the ocean).

We have some units here, produced either in Germany or the States, that can light on low fire even with Propane. But people don't look like knowing them. Here in Quebec, our electricity cost 0.06$/kw, Natural Gas 0.60$ a cubic meter. People see only the cost. It gets on my nerves, but that's the way it is !! We do many optimized system. But only for people not afraid of change... or European people living here !! Ahah

Unknown

(here)

Roland, By (here) I meant the wall, not germany.