Vitodens short cycling

Ron Gillen

J-P

J-P, it doesn't chase the target in the way I'm familiar with on any other system. It's not like if it's X F outside it will be Y F boiler temp. At any given outdoor temp there does'nt seem to be a specific temp for the boiler. It's strictly heat in = heat out and it checks that constantly through the sensor in the low loss header. As I mentioned above, I have no indoor feedback other than that sensor and it never fails to keep indoor temp spot on. How does it sense the volume of the system? It doesn't care. It only follows the results of what it last tried. I have never tried it but I read here once that you could leave the windows open, with no indoor feedback, and watch the boiler respond.

Mike T., Swampeast MO

Thank you Ron! I suspect you're just as curious as me and have been watching "boiler temperature".

The reason you don't seem to see a correspondence between outdoor temperature and "boiler temperature" is because you have the low-loss header. It's doing its job providing the hydraulic separation and the boiler is doing its job providing energy in=energy out via these uncoupled systems.

In my case with a directly connected system of high volume I find PERFECT correlation between target and boiler temperature as long as the load is above minimum modulation. "Target temperature" and "boiler temperature" will stay equal for days on end (or at least every time I check and I've also watched it quite closely for hours).

A change of 0.2F supply temp (as I measure on the surface 3' away from the boiler) is a nearly perfect indicator of some GROSS change in operation of the boiler or change in desire of the occupants.

ALH_4

sizing

Mark,

I think 40k is the perfect size. It would be nice to have a turndown ratio higher than 4:1, but 10k is a decent starting point. It has been stated here that this pulsing may still be very efficient. I would have to see that to believe it. We use modulating boilers to match the load. If pulsing was ok, then we would not need modulating boilers at all.

I have become a believer in matching the boiler to the heat load and matching the volume of the dhw tank volume to the domestic hot water load. There are obviously limits to the practical size of the water heater, but for most homes, letting a 40k boiler work on that water heater for a few extra minutes is the best way to go.

At least that's how I see it.

Andrew

jp_2

Time or weather

it wouldn't know the difference between the two?

cold and sunny, or a warm night? lots of mass, little mass.

I do not think the boiler needs to know this info.

scott markle_2

Load dependant differential

While I have not observed this issue as bad in the other direct piped higher reset curve 6-24 systems that I have installed, I'm seeing it in my own aprox. 20k, all low temp. radiant house. I really love the vitodens 200. I think the sun moon dial timmer/shift adjustment is an excellent interface for simple one zone or TRV systems. I'm amazed by how consistent indoor temperatures can be with no indoor feedback at all.
This said, I worked recently for very respected local radiant design/instaler who told me he didn't like using vitodens boilers because they didn't provide an interface option for Tekmar controls which he favors. This got me looking at Tekmar especialy the tn4 controls. I could go on for a while about how smart these systems are, but related to the topic what I really appreciate is the automatic load dependent differential. This feature raises the differential when loads are low thus eliminating short cycles and maintaining accurate indoor temps. all in a nearly continuous circulation arrangement (for the zone with the highest heat demand)
What I just noticed on the Tekmar web site is that tN4 controls can in-fact be interfaced with this boiler. see attached pdf. service bulletin. Does anybody have any experience or opinions with this pairing. Is this for multi -boiler setups only?
I recently did a gb-142 with a Tekmar 420 and although the loads are very low, short cycling is a non issue. For viessman to excuse this operation as "normal" denies that there are simple control strategies that can address it. I believe that this "no compromise company" should develop a work around for this issue. This would probably involve new software for the control board that would provide a load based differential option.
Sensible and sustainable future housing means structures with much lower heating loads, we need the 65k on the upper end for DHW and those rare below design weather events. This does necessitate punishing our expensive equipment with these needless 5 second combustion intervals for the majority of the heating season.Yes It's a good guess that because of the cold surfaces of the heat exchanger we are getting a high level of the 11% or so "condensation bonus" in "pulse mode". I wonder though just how quickly the "flame" is able to become stable and reach it's full combustion efficiency. I imagine that the germans have worked this out pretty well and that these "pulses" are somewhat "space shuttle like". Just because we can ignite a burner with such precision doesn't mean that we should depend on this. Why tax the hardware for lack of better software.
Please, you guys with the data loggers and voices in the industry call viessman to task on this. Yes we love the matrix-x burner the variable speed pump logic, modular construction and uncompromising quality of this boiler. But we dont want to spend extra on Tekmar if the job doesn't require it. Please give us automatic diferential control. Incidentally there ARE differential adjustments within the comfortrol, including a coding 1 address setting for burner switch dif.- fixed or load dependent. Problem is it doesn't work on the Vitodens Why Not?
P.S I Included a photo of my last job(for credibility) This system uses a tekmar 270 boiler control. The 270's DHW function is used for setpiont control of unit heaters. TRV equipped panel rads in home run, on full setback. DHW handled by gb thru it's included 10k sensor and relay.

Mike T., Swampeast MO

Yes, I've adjusted the "Differential" setting that appears in the setup menu of the Comfortrol but it [appears] to have little or no effect on the operation of the Vitodens. Viessmann did tell me that it won't hurt to adjust it and I might put it to a rather extreme value next year just to see what happens. I have not found that setting mentioned anywhere in the Vitodens documentation.

With the Viessmann communications accessories mentioned in the Tekmar article, any 0-10V controller can operate the Vitodens. The reason they say a Tekmar controller with the "Tekmar Stager" must be used with the Vitodens may well be because of the Vitodens' inherent ability to control a two-temperature system via a mixing valve. To the Tekmar controller such [might] be treated as a "staged" system even if there's really only a single boiler. If used as a single-temperature boiler the other Tekmar controller mentioned might work. I'd contact Tekmar for clarification.

If you have a lot of on-off zones driven by a Vitodens I can see where the Tekmar TN4 controller with their advanced thermostats could be beneficial--as long as you can bear the price. TRVs/FHVs would probably cost a lot less.

I kind of doubt that numerous people with Viessmann would tell me that the "pulsing" I'm observing in this directly connected, fully TRVd system is "normal" if such weren't the case. I was privately informed by the highest level of Viessmann management that there was no need for a LLH on my system.

I do however sometimes wish I could speak fluent German and call German technical support. They probably see FAR more similar systems over there than here in North America.

scott markle_2

Mike rest assured that the deferential settings accessed thru factory settings or coding 1 are of no effect on this boiler. I have experimented with extreme adjustments, no help.

As far as the low loss header, It's my feeling that if we want to maximize condensing efficiency, these decoupled systems may be some hindrance to achieving lowest possible return temps. Literature from the international Viessmann site supports this conclusion. Support at Viessmann has taken my concerns seriously, unfortunately a comfortrol programing solution has not been found. I'm about to remove my low loss header as an experiment suggested by canadian support. Hopefully I can do this while it still somewhat cold.

It seems to me that the low mass boiler industry has adopted a p/s recommendation policy regardless of installation specifics. The GB142 is packaged with this, no choice. In my opinion a direct piped vito is a thing of beauty. I was pushed into the LLH regardless of the fact that my flow rates were all easily calculated. And now I'm disconnecting it to see if I can improve system behavior.

With your experience as a gauge I'm fairly sure the short cycle issue will remain. However I look forward to taking that extra constantly running 15-58 off my electric bill.

Again I really think that critically thinking installers should press Viessmann to address this, I'm encouraged by the ongoing discussion of this topic at this site.

Mike T., Swampeast MO

I'm not surprised at all that the differential setting has no effect on the Vitodens. While I thought I [might] have seen a slight difference at one point, over time I saw none.

It's utterly simple to prove that secondary flow rates exceeding primary flow rates increase the return temperature and thus reduce efficiency. It's just as simple to prove that without variable primary flow it's impossible for a mod con to maintain target in a low-loss header in nearly any real-world system. Such were the points of a very long post here last year regarding "Primary/Secondary with Mod-Cons".

I too have contacted KWE Technologies (Viessmann's programming arm in Canada) with my concerns and recommendations. While I certainly understand Viessmann's general philosophy of modular design where the same control unit (Comfortrol for instance) can function with wildly different boilers, I have a sneaking suspicion that a Vitodens-specific control unit may be in the works.

The Viessmann white paper titled "Condensing Technology" (produced for the English speaking European market by the way) is VERY specific that primary flow should not exceed secondary by more than 30% without a significant impact on efficiency.

Unfortunately such seems to be lost in the North American market and training. At times I believe the Europeans have given up on North American heating engineers and their near refusal to use proportional flow control and differential pressure bypass instead preferring banks of circulators and on-off wall thermostats. Thus they just say, "Use a low-loss header." Sadly, I often think they're right...

To me there's nothing at all difficult about estimating flow rates and designing nearly any system to operate within the acceptable flow range of a mod-con and a single circulator. The only real problem with such is with underfloor systems which can have a quite high head loss. That said, I'm almost positive I can design a directly connected radiant floor system using Warmboard, FHVs and a 6-24 Vitodens in an efficient shell. (Offer for free assistance with such still stands!) Even if head loss is the problem it's NOT a flow volume problem and as long as you don't have a constipated system like bare-tube staple-up you can STILL keep the secondary flow rate quite low.

For my purposes and concerns I prefer the Canadian Viessmann support to the U.S. They seem to understand directly connected systems better but there is the ego-stroke I get when I identify myself as "Swampeast Mike" and hear, "You're a celebrity."

scott markle_2

Euro LGM

I also want the "Euro LGM" if it would start my boiler at low fire.
I'm certain this would eliminate the short cycle!
What I'm not understanding is the part about propane and gas valve adjustments What is it about propane that requires a different ignition sequence than natural gas?
Is this hypothetical gas valve adjustment that mike speaks of for a natural gas unit?
Am I hearing that propane must be ignited at a higher firing rate? why?
Also excuse my ignorance but what does FHV stand for?

Mike, my house when I remove the LLH will basical be direct a connect radiant system. I have included a hand drawn schematic of my system for your review, The thermostatic mix valve is set at 105 but never gets that hot. (this was in the interest of extending the life of older embedded rubber tubing)
The second floor which is half the area of the uncovered ground floor slab is quick track. The novel thing about this install is the use of return water from the higher temp upstairs wood floors as supply for the plate heat exchanger. The exchanger was included mainly for O2 protection but has the added benefit of dropping water temps just enough to keep things balanced. No zoning to speak of although upstairs manifold can be bypassed for extended unoccupied periods. The system heats beautifully. Looks like about 230 gallons since 10/31/06 lots of long showers too, although my GFX recovery unit is helping with that.

The underlying idea behind this design was cascading temperatures to maximize dela T thus achieve the highest level of condensation possible. Unfortunately I feel my LLH is probably interfering with this to some extent .I wish I'd read the euro viessman essays sooner

Here is the pdf. from the other side of the pond that got me thinking ,good description of net vs. caloric value etc. As well as the sketch of my place. Please let me know what you think. Mike I'd love to see some conceptual schematics on your ultimate low temp. direct connect vito system.

scott markle_2

Recent posts removed?

This topic got really interesting when suggestion of a north american propane vs. euro natural gas ignition sequence was brought up. I just came back to review these posts and found them removed. What is the issue here? is it political. The more information the better, even if it's unverified speculation. I think this is a very important topic. Yes it points to a potential flaw in an otherwise spectacular product. But By asking questions we are assisting in the development of these important technologies.

DanHolohan

Removed

by request of hb.

jp_2

not sure here?

if what you say is true, then there is no reason for choosing a reset curve, or adjusting the curve?

also, how would the system measure 'heat' or 'energy'?

in your statements there would be no use for the target to begin with either?

Mike T., Swampeast MO

How many GPM is your secondary circulator (Grundfoss 15-58) currently moving?

As long as the head loss through the flat-plate HX isn't too high I [think] it will work without the LLH. The key is going to be that "balancing valve" before the "primary" (unisolated via HX) supply manifold. Use a true metering valve so you can make precise adjustments!

As you move more and more of the total system water through that "primary" manifold, its head loss will increase significantly via those small diameter loops and you might not have enough residual head in the built-in circulator for really high-load situations. If you can't move enough water, the boiler will have to modulate WAY down--perhaps even cycle--and you might not have enough heat when you need/want it most.

Of course your total space heating load is very low and presuming you don't use much or any daily setback you won't need much flow (or boiler output) anyway...

A truly interesting situation! At least it looks like it will be quite easy to experiment and if it doesn't work you can always re-install the LLH.

scott markle_2

flow rates

To be honest I'm not exactly sure. I used longest loop plus HX head and total required g.p.m. to calculate pump size. But I'm not really sure how figure out actual flow rate based on known unbalanced multiple tubing lengths and pump performance curves. Perhaps someone could explain to me to me how such an problem would be set-up mathematically.
As for manifold balancing, without the use of individual flow meters this seems a fairly tricky business. From what I have gathered, in the real world most balancing manifolds are left wide open unless loop lengths are really different. This is probably because most circulators are oversized and even if shorter runs have higher than required flows there is generally plenty of residual head for runs which require it. This is part of the beauty of the Tn4 alternating zone control strategy as it minimizes the amount of time that multiple zone valves are open simultaneously. Thus if there are flow imbalances which might starve longer loops the amount of time that this condition exists is minimized.