Completely Wild Prestige

S Ebels

There's something to be said for system efficiency.
Many of our mod/con boiler installs turn out to be oversized, sometimes by a substantial margin. For example, a 6,000+ sq ft house with an 8-32 in it that NEVER hits high fire even at design. Heat loss called for a load of 103,000 and judging from the reported fuel use on this place I think 75-80K would be more accurate. A 6-24 would have been perfect.

Now consider this. I have another house of similar size and construction with F/A heating system. It has a variable speed/2 stage furnace which fires at 73 &112 output. The heat loss calc on this one was a little less, 94K, but when we get to design temps the furnace is running pretty much flat out. Fuel use in this house runs about 15-20% above the house with the Vitodens.

This leads me to believe that all other things being equal, the hydronically heated home has less system related losses than the F/A house. The "envelopes" are nearly identical in the two homes. You could say that heat loss calcs or correct for F/A applications but need to be taken with a grain of salt on hydronic jobs. They (the calcs) all seem to be oversized for a good hydronic system. Smaller mod/con boilers are definitely something that a savvy installer could take advantage of if they were available.

McKern

Wattage graphs of controlled vs uncontrolled cycling

Situation:

Prestige Solo 110 30K-110K, low fire clocked once at 40K

Heatloss 55K

Pure outdoor reset via P34 (no t-stat control at all), curve attached

Piped direct single monoflo loop (using speed 2 of internal circ)

Stray wattage movement mostly due to voltage fluctuations, normally when firing it's a 98 watt weakling

Both days HDD=46 (based on airport YTZ, which is just slightly warmer than here]

January 30 [P42=01] 10 minute anti-short cycling switched off (20+ hours) - graph with gray background

January 31 [P42=00] 10 minute firing time cycling regulation on (only 19+ hours) - graph with white background

Observations:

Nothing that I would call extreme in the way of short cycling, but still, quite a few more cycles. Only a few were longer than the regular 10 minutes cycles.

Couldn't perceive any comfort differences either way although wild should be better because the boiler can follow the curve religiously.

Internal circulator never shuts off when riding the curve.

Can you say oversized? I knew it going in, but by careful calculations even using a slightly de-rated heatloss, I still figured that under 25-30°, I'd be modulating. If you consider how it spaces the 10 minute bangs, does that qualify as modulating? ;-)

It really should have been modulating according to my calculations! I now have no faith in heatloss calculations or the minimum firing rate specifications of manufacturers. Maybe, I should find a close neighbor with rads and sell them some heating so that this machine can breathe a bit harder than its current 10 minute pulls? LOL

On a side note the house was a couple of degrees colder tonight than normal. I was miffed the curve wasn't as on as I figured it was (it had been excellent for the most part for more than a week now) but luckily I shot the nested outdoor sensor location with an IR gun and it was warmer than the surrounding area.

It was 4 degrees warmer up where the sensor is mounted. Multiply that by the slope and you'll know just how much too cold it was in the house. I'm really digging this life on the curve experiment. Unlike the projections these numbers all tie together. That sensor just has to stop hanging around so close with those sheltered bricks!

The outdoor sensor is on a north facing wall and if I move it, I want it to stand off from the brickwork (I had noticed this lag affect whenever temps dropped quickly but figured it was the effect of the brick's thermal mass on the envelope but it's pretty obvious it is just the effect of the thermal mass of a few very sheltered bricks on the sensor itself). You go to great lengths to shelter it from sun, rain and snow be completely by the book and then it ends up in a warm spot and you come home to a cool house. Go figure!

I'd probably be best off now to go and use the IR gun to find a more exposed spot that gets a thermal gain to match the rest of the house.

(the upper parts of the graph show the additional wattage from the blower motor - 20 watts or so)

Wayco Wayne_2

Man

it sounds like you're having fun playing with all of this. When all is said and done...can you say buffer tank??? WW

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Perry_3

My Vitodens aslo cycles more than I thought

While I have not hooked up instruments to record it; my Vitodens 200 6-24 aslo cycles off a lot. Although I think the run times are at least 20 minutes long.

In my case, I estimate that my house heat loss is lower than the "heat loss" calc showed, and the fact is that most often the house heat loss is less than the minumum firing rate.

I have no reason to question the Vitodens minimum firing rate - it seems to match pretty well my estimated heat loss based on system temperature drop and estimated flow.

What I really need is a Vitodens 4-16, or even perhaps a 3-12 (which might be cutting it close in my case).

I am not sure why Mfr's are not really stepping up to the plate on smaller mod/cons (with quality equipment).

Perry

Uni R_3

As they say in Jamaica, \"Soooooon mahn\"

Wayne,

I'm thinking that my curiosity will be paid off. I might as well work with the machine, so when I finally get off my butt and get the top of my indirect piped - then I can liberate the electric 40 gallon heater.

The buffer will be charging with return water at ideally a rate of about 4gpm. That when during each 10 minute firing cycle the water in the buffer tank will get cycled through once.

It's just will not be to extend firing cyles as much as extend the time in between. As long as it doesn't ramp up I'm fine. I could probably set the maximum fan speed to the same thing as the minimum fan speed on the CH side of life! LOL

I don't think that letting it cycle as it sees fit would be much better with a buffer tank because I think it would still be pretty random.

Watch where you install those outdoor sensors! ;-)

McKern

Hrmmm...

Perry too true unfortunately. There should be far more selection in the 60K and under range.

Wayne, yes the buffer situation will be interesting. On the one hand diverting 4gpm through the buffer will mean one cycling of buffer tanks contents per cycle. The 10 minutes cycles make that all too easy.

I'm just worried that if it was trying to store 10° warmer water, then I might be squeezing a balloon. 40 gallons is 330 pounds so a greekD-T of 10° means 3.3K in 10 minutes or an extra 20K per hour from the boiler. So would I be working the boiler harder and then shutting it off longer? Yes, I'm confused... ;-)

Wayne, what are your plans for cleaning the HXs on the Prestiges you've installed?

Ross_7

prestige installs

Wayne,
How many of these boilers have you installed so far?
Any problems? Thanks!
Ross

Wayco Wayne_2

I think I

have 6 out there. I have one 175,000 that serves a radiant floor and a snowmelt too, but when it isnt snowing it makes me over sized, so I installed an 80 gallon buffer tank in series with the supply. The flow must go through the buffer before it hits any zones. Boy, it works great. It cycles longer and once it shuts off it coasts on the stored heat for a long time. I have no plans to clean the HX's at the present. I was told it was self rinsing because of the vertical heat exchanger. We'll see what time brings though. WW

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Mad Dog_2

Industry will usually change only when.......................

The demand or outcry is great enough. Studies like yours coax them along. I was talking to Mark Hunt Yesterday and we were talking about the same thing. The Wall-hung Mod/con manufacturers REALLY need to put out a unit that can top out at maybe 30 K. We come across situations where that would be the max needed. For ex. a three-story extension on a home. Everything else is overkill. Mad Dog

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Mad Dog_2

Guys like you, Mike SwampEast, Gerry, Steve Steamhead and HR

Are the real world labs. Mad Dog

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Mike T., Swampeast MO

This is the very reason I keep stressing that heat loss calculations already have all the necessary "fudge factor" you need--particularly with a mod-con. Granted there's not much you can do at present if you can't find a mod-con small enough, but there's certainly no need to oversize!

My own house has 100 mbh loss via HVAC-Calc conducted in a normal manner. (93 mbh when I'm extremely picky doing things like considering closets as unheated "cold rooms" and compensating for "cold walls" in the conditioned space.)

I'd venture to say that many would be relucant to use a Vitodens 6-24 (with max input of 91 mbh) with a 100 mbh calculated loss, but it didn't bother me too much--after all it's my own house and I have only me to blame if something goes wrong...

ZERO problems with available heat energy and I've conducted many tests including raising temp significantly in most of the house in weather at/near design (8F).

Have clocked the gas meter many times and will say that the Vitodens low-end modulation input rate of 25 mbh is dead on.

I do not see any "short-cycling", but I do see a lot of what I call "pulsing". Just a few seconds of burner operation--at full input--every few minutes. As best as I can tell, this "pulsing" mode of operation only occurs when actual heat loss in the structure is about 6mbh or below.

Once load increases much above 6 mbh, the burner will fire at minimum input for at least an hour. Target temperature is generally achieved in just a few minutes but boiler temp exceeds target at a much slower rate. The burner continues to fire--at minimum input--until boiler temp exceeds target by about 8-12F when it will shut down. Either that or the boiler will have to increase output to maintain target and thus begin genuine modulation (constant but variable fire to meet the load).

In general, the outside temp must in the 20s or below for the boiler to continuously modulate to load. On very sunny afternoons it's not at all uncommon for the boiler to kick out of modulation with the outside temp still in the upper 20s.

---------------------------------------------------

With a calculated heat loss of 55 mbh and presumed actual minimum modulation rate of 40 mbh, I suspect that your house never requires more than 40 mbh to maintain indoor temperature--thus your observations of relatively short firing periods. If your boiler is firing strictly by some fixed amount above and below target, I suspect that your firing times will remain quite consistent with the time between them varying with the weather. This is the exact sort of firing behavior I observed with my old boiler (using mechanical reset--plus/minus 10F around target--constant circulation, TRVs and the thermostat connection "jumped" to produce a constant heat call.

Regarding your outside sensor:

I'd suggesting devising some sort of stand-off mounting in the current location. Be wary of putting the sensor where it receives direct sun unless you can find a "sweet spot" that typically only receives winter sun around high noon. If it gets significant sun early or late in the day you're quite likely to underheat the house.

The outside temp sensor supplied with my Vitodens has a cast zinc baseplate. When I was talking to Viessmann about perhaps moving my outside sensor (to a place that received some sun) or perhaps using two sensors and switching between based on sun level/time of day the technician asked me if I had the sensor with the zinc base. I said, "yes". He asked me if there were any problems with "false" readings. I said, "no." He told me that they have found that such sensor, in some instances, winds up measuring the temp of the wall itself--not the air. They now use an all plastic sensor body and have eliminated the problem.

Mike T., Swampeast MO

Yes, have noticed. Used F/A electric resistance (forgive me please) in the attic renovation of our rental house next door. Pressed for time and couldn't find the proper electric resistance coil. Settled for one about 160% of HVAC-Calc value but where half could be disabled for about 80% of HVAC-Calc value. Disabled half thinking, if anything like with my Vitodens, there won't be a problem.

While I didn't do the insulation/weatherization work myself, I did verify that it was at least reasonable and matched what I used for the calculation.

First deep cold spell, occupant complained that it wouldn't stay warm. I asked if he was using significant daily setback. "No." Checked his electronic setback thermostat to verify.

Had to enable the other half of the heater.

Mike T., Swampeast MO

Uni R

Is the control system of your Prestige designed with wild control in mind?

Mark Hunt

Mike


Questions. I ask because I have my own suspicions about this.

Can you describe in a bit of detail how the duct system was installed? Supplies vs. returns in rooms? How were the returns brought back to the AHU? Were the ducts sealed?

My building science training kicks in here. After watching what happens in a FHA home when the duct system is, "less than optimal", I have a sneaking suspicion as to why this happens. Several actually, but I have noticed the same thing. Not in every FHA home, but quite a few. I am also NOT blaming the FHA system as the sole cause. Construction quality plays a HUGE roll here.

Panned joist bays, using sheetrock wall cavities used as ducts, in-sufficient supply or return. I do not want to put up a marathon post, but all of these things have some part to play in this.

Again, this is NOT a FHA bash.

Mark H

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McKern

Oxymorons aside... ;-)

Mike, the default for the Prestige (and most likely the Ultra as well) is that when demand is less than the minimum firing rate, it fires 10 minutes at a time and adjusts the gap in between to maintain temps. It works very well like that. I had no idea it was doing that until I saw the first Wattage graphs.

I hadn't read about that anywhere or heard that mentioned so I posted that here. Effectively, that control means that there is no way that the boiler can short-cycle.

All I wanted to do that day was see how the boiler would behave without that limitation in place. It could fire whenever it wanted to. I had no minimum time between firings set or anything like that. It could fire at will to maintain its target temperature.

It probably ended up firing an average of about 5-8 minutes at a time but far more variable. I was simply curious to see what the alternative to 10 minute firings would be.

McKern

Matt, that's such a fine group!

Thanks Matt, that means a lot.

I get a lot out of this place and try and give back what I can.

Mike T., Swampeast MO

I can give every detail on the ducting system as I did install that part...

Two smallish bedrooms, a fairly large living/kitchen area, good sized bath with laundry and a storage room.

Upstairs of a 1 1/2 story home with a 12/12 pitched roof and numerous dormers of different styles.

Quite compact system. Horizontal air handler installed in the "attic of the attic" but such space still within the true insulated shell. Supply and return plenums as well as most branches in the same space. I agonized over the design...

For calculation purposes did consider the ductwork in "unconditioned space".

Most supplies from ceiling or high on walls--know that's not ideal for heating but good for our awful summers and most practical with the given design of the space.

FULLY DUCTED "HI-LOW" RETURNS in the two bedrooms and the major living space. Magnetic sheets provided to cover the appropriate return grills with instruction to "cover the top returns in the winter and the bottom in the summer".

Returns sized 120% - 140% of supply area depending on shape/configuration of each.

Mainly custom sheet metal for the ductwork. "Noodle" supply to the storage room and one "noodle" that I had to use to connect to a return. Sheet metal insulated on the outside with bubble foil (cut perfectly, attached with contact cement and sealed with aluminum tape). All drives/esses/takeoffs sealed with duct mastic. All segmented fittings sealed with aluminum tape before the bubble foil insulation. All seams on metal ductwork sealed with aluminum tape before bubble foil insulation.

All new windows. Simple vinyl-frame, double glazed. Inexpensive but not truly "cheap". Insulation level dismal (R-11 walls, R-13 roof) but figured that way for loss/gain calculations.

Mike T., Swampeast MO

Oxymoron: "Pointedly foolish"

Wild: "Living in a state of nature"

An on-off thermostat is an oxymoron to a mod-con because a mod-con wants to be wild.

Is there any mod-con other than the Vitodens that does not have thermostat connections? That "wild", in other words the ability to "live in the state of nature" is PRIME reason that I chose the Vitodens.

Mike T., Swampeast MO

How else other than wild and living in the state of nature can you describe the ability to maintain space temperature without sensing space temperature?

McKern

I'm not sure what to say...

The only sensor driving my boiler is in the attached pic. Let me assure you that the t-stat is actually switched off and has been for well over a week.

Mind you, I had to manually set the curve - no dials here. In fact manually dialing in the curve was what I wanted to do. That way I knew exactly what it needed for temps.

Yes, my boiler does have t-stat terminals. They could also be used to control parallel shift of the ODR curve as simply as an on/off switch. The flexibility is what I really liked about the Prestige. I'm not sure whether or not I want to use a t-stat. I like having that flexibility.

It seems a bit odd to see Vitodens 10 times in a thread about a Prestige (which only got mentioned half as many times so far). This post was just meant to inform people about how the Prestige behaves with the default 10 minute anti-short-cycling control on verses letting it cycle off and on itself. This possibly applies to the Ultra as well, although I do think that there are some small differences in how the MCBA gets hard-coded. I'm very much interested in what people with Ultras have to say about their boilers - the Vitodens choir tends to just try and make people uncomfortable with their purchases or what they sell if they don't happen to be made of the same metal.

I'm quite fond of the Prestige. I chose one over a Vitodens for many reasons and as unbelievable as that might be to you and all of the Vitodens' owners out there, money wasn't a factor.

You're obviously very pleased with your boiler - that's good because you obviously care a great detail about hydronics and your home. But please respect the fact that the Vitodens might not be the best choice for everyone. I enjoy your posts but using this thread to to expel the virtues of the Vitodens seems a tad bit unnecessary.