Sizing Vitodens boilers

Paul Pollets

You nailed it, Brad!

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rumn8r

Sizing Vitodens boilers

I am considering the Vitodens 200 boiler for multiple hydro air systems. I have been told by some that it is not a good use of mod con boilers because 1) higher temperatures are required to prevent blowing cool air and 2) there is little of no advantage in efficiency for mod cons when run at higher temperatures. Even if the highest temperature curve is selected for the Vitodens, the efficiency is over 90% until the unit goes above 70% load and it is over 96% efficient when less than 40% load (see the graph on page 2 of the Vitodens Technical Data Manual). If I need 2 units because my total heat loss is more than the output capacity of the largest unit, should they be oversized so that they don't approach full load?

Brad White_144

Setting up Hydro-Air for a ModCon

There seems to be a disconnect about Hydro-Air and ModCon boilers lately

Consider that many grasp and appreciate the benefits of a ModCon boiler but remain stuck in the 1950's when it comes to the distribution of the heat created.... you do not need 180 degree water provided your emitters (coils, radiation, floors -heaven forbid at 180!)- are selected for the lowest water temperature that will do the job. More emitter surface, sure. But you buy that once.

Hydro-Air coils can be selected for use at much lower temperatures and as with any high-efficiency air system, it takes more air at a lower temperature to do the job. But you can get coils with three, four or even six rows to extract maximum heat from the water. Enter at 130 to 140 and return 30 or even 40 degrees cooler and you will be below the flue gas dewpoint most of the time.

I would select the emitters and boilers as a system and size the Vitodens singly or in tandem for the heat loss. You really do not want to over-size them, even if paired. Remember, you *did* size the emitters for low temperature to begin with. Let the Vitodens do what it does best.

My $0.02

Brad

rumn8r

Brad,

Thanks for the reply and for your posts to my other thread. Pardon my confusion but there are many conflicting opinions here. Even the Vitodens Technical Manual suggests high temperatures for hydro air systems in the graph with the different response curves. From my other thread I understand that I can likely run at condensing temperatures if I size the hydro coils correctly and still get warm air. However, this is a large investment and I really hate making bad decisions. Therefore, I am exploring the senario that I might have to use higher temperatures and noticed in the Vitodens Technical Manual that there is not a dramatic reduction in efficiency if that is needed as long as the unit isn't running at full capacity. Am I missing something? What is the downside of oversizing mod con boilers?

Brad White_144

The downsides of oversizing

any boiler is that at low loads you will cycle more (less of an issue than with high-mass boilers but it is still a drag on efficiency and equipment).

Each time a boiler fires, it takes at least a minute to stabilize the combustion products. ModCons take less time and the Vitodens is the best of them IMHO, but the cycling can still occur. You really want the boiler to be in cruise control, which means small enough so that it is trying to keep up with the load.

If you oversize it so that the boiler thinks the current load is 15% of the capability will not enhance efficiency, but will short cycle or pulse the Vitodens. Modulation is not an excuse to oversize.

The dictum to run high temperatures (even as stated in the Vitodens manual) is more conventional thinking and it is well that they make the distinction. Their alternative is to explain that every system can be made "low temperature" and run better, but one can never write every exception nor system. And what of retrofit markets where the distribution is already existing? So, yes, they have to dictate what is most common.

Newer systems? Blank slate- make the system to your desires.

rumn8r

Thanks, I'm beginning to understand. So short cycling is not considered in the efficiency numbers in the manual? If not, how much can short cycling reduce efficiency for a low-mass boiler? If it is, then I still don't see the downside.

Brad White_9

Short Cycling

is to be avoided. Don't buy an 18 wheeler to make local deliveries. Get a vehicle proportional to the duty.

Mike T., Swampeast MO

So short cycling is not considered in the efficiency numbers in the manual?

Unknown. However from everything I've studied about that boiler and everything I've observed from watching mine, those output level and temperature efficiency numbers are based on true modulation where the boiler is supplying heat at almost exactly the rate it is being consumed (lost).

With a Vitodens driving hydro-air, here's how I'd prevent short-cycling of the boiler:

Say it's the 8-32 model with minimum output of about 33 mbh. I would establish a minimum boiler temperature (this setting overrides lower heating curve targets) such that 33 mbh is instantly absorbed by the hydro-air coil. Thus, if the house is loosing 33 mbh, the boiler will be operating continually at minimum output. If the house is loosing 11 mbh, the boiler will be operating 1/3 of the time BUT the burner will be operating continually at minimum output whenever the thermostat calls for heat. In other words, the boiler will not cycle at any time during a heat call.

Multiple hydro-air units could well complicate this. Multiple hydro-air units that are further divided into zones complicate it even further. With multiple or sub-zoned hydro-air, the minimum temperature required to fully absorb the minimum boiler output when only a single unit calls may be unacceptably high. While not utterly required, buffering would likely be beneficial.

Oversizing the boiler will only increase the minimum heat output, thus forcing the boiler to cycle much more frequently and likely cycle DURING a heat call. With a Vitodens there is absolutely no excuse for burner cycling during a call for heat (unless the heat being lost by the structure is less than minimum output).

MikeL_2

con more important than mod

All the advantages of a mod-con unit can be accomodated in other ways except for the condensing part, which still gives them a 10-15% advantage in absolute AFUE numbers, and probably half that in steady state efficiency.

I run a Buderus GA124/17 which is a induced draft, cold start boiler. It supplies a 4 coil AH with water through a superinsulated Amtrol buffer tank at about 112F on design day temps of 14F. The return temp is 88 F or so at this supply temp. The boiler will run 20 minutes every 60 minutes on design day temps and the circuit runs constant circulation with the AH at 350cfm unless there is a DHW call in which case the circulator shuts down.

No standy losses in this design as the boiler heat ends up in the buffer tank, and the inducer keeps the flue loss near zero in the off cycle. No super high temps either. Just low controlled heat. Since the boiler is heating a 40 gal buffer tank and dealing with low return temps, it never cycles during the heat cycle (Tekmar 262 and RTU, so no actual call for heat as it''s a PID controller).

But this does nothing to get the heat lost up the flue during boiler run time, which a condensing boiler would capture. So I think that the mod part is not nearly as important as the con part. SOmeone should make a flue gas economizer for residential boilers. It would be a retrofit that buys you 10% easy in absolute AFUE numbers and is much cheaper than a new boiler.

Mike T., Swampeast MO

With regards to efficiency it's VERY hard to separate the "mod" from the "con" in a boiler capable of both modulating and condensing.

If you force the burner to cycle to maintain setpoint, both the supply temperature and output levels will be higher and efficiency lower on both accounts than if the boiler had modulated continually.

It's just these sort of seemingly minor differences that add up to make mod-cons typically MUCH more efficient than implied by AFUE.

Even with the primary circulator running, standby losses from the Vitodens seem exceptionally low. The gaps between the coils of the HX are so narrow and relatively long and so precisely maintained that natural convection never seems to get going. When firing, draft through the Vitodens HX seems to be truly forced--not merely induced. This is probably how the Vitodens detects excessive wind--it knows that very little (if any) air should be moving without the combustion fan running. Another indication I see that the draft is truly forced comes from the condensate. Right before ignition, the fan runs at full speed to clear the combustion chamber. When doing this it builds up enough positive pressure in the combustion chamber to deliver a little slug of condensate from the trap.

Brad White_9

I take the opposite view

Not to dismiss condensing, but given a choice of only one, I would take modulation. (Yes, of course I want both! They do go hand in hand so well and cross-benefit each other.)

This is my rationale:

Condensing is a function of firing rate and return water temperature; the efficiency difference from this compared to conventional equipment can be measured by combustion testing and AFUE standards. Typically the efficiency, steady state, will jump from 87-88% to at least 92% and possibly 98%. Fantastic and I mean that.

But if you take away modulation, instead use bang-bang on-off control, you have no control over your firing rate. You only have your return water temperature to work with. The return water temperature is a function of heat emitted to the space coupled with as low a starting HWS temperature as will do the job. This varies with outdoor temperature of course (try to control that!)

So, your on-off boiler (assuming outdoor reset and a healthy differential to lessen cycling) will still cycle on-off on any day warmer than design. And let's face it, all boilers have some over-sizing built-in.

The chances of available boiler capacity increments equalling your particular design are about as good as me being elected Archbishop of Canterbury.

I have a Monitor MZ, a non-modulating condensing boiler. Yes, it is oversized for heating but does drive an indirect. My gas consumption, even with their OD reset and wide differential, is about the same on a per degree-day basis as my Burnham Series 2 with Centra 4-way mixing valve. It is true. I am no better off had I kept the 1986 Burnham/Centra combination active.

If I had modulation, even two-stage, but the conventional 4:1 or 5:1 turndown, my boiler could match my heat loss and condense beautifully the entire year long. With bang-bang control, it starts condensing and eventually rises above the dewpoint in colder weather.

Let's stipulate that any house with a heat loss in the 40 or 50,000 BTU range is limited to a Munchkin T-50 or M-50 with boost for domestic. The rest seem to fall in the 80-90's MBH input range.

Can I quantify what "Mod" would do on top of my condensing? Not numerically, but given that my only improvement is "condensing", my gas consumption has not dropped compared to my old Burnham 204, I have to wonder.

My $0.02 and hard experience.

Brad

ALH_4

mod vs. con

I will agree with that completely. In my mind modulating condensing boilers are designed primarily to modulate, but also to handle the unavoidable condensate and attempt to recover some of the latent heat available in that vapor. By modulating, the boiler can match its input to the load in far more situations than the fixed fire boiler that matches the load a few times a year if ever. Nature is rarely if ever "on/off", and the more closely we can match our heating systems to natures continually changing conditions, the more comfortable we will be.

Maybe that's worth $0.01 ;-)

-Andrew

Brad White_9

Worth a lot more, Andrew.

"Nature is rarely if ever on/off"- how true- Well said.

And worth every penny!

rumn8r

Mike T.,

So the boiler cycles DURING a heat call because it reaches the supply set point according to the curve selected before the heat call is satisfied?

rumn8r

odcman,
Why is the boiler running only 1/3 of the time on design day temp of 14F? How warm is the air coming out of your registers?

rumn8r

If I go with the Vitodens I will definately need two units. Here is the heat loss for different areas at design temp (no safety factor):

air handler #1 ~90 k btu/h
air handler #2 ~40 k btu/h
Desert Aire (pool room) ~90 k btu/h
pool heat exchanger: 135 k btu/h for 2° /h rise
indirect water heater

I can't just add everything to get the total btu required because heat to the pool will also heat the air in the pool room and the Desert Aire provides 36 k btu/h when running. I also have to make sure that the pool room air gets priority over the pool water because air temp must be maintained 2° above water temp to control evaporation (which will cool and add humidity that must be removed). This is another reason why I am interested in a modulating boiler.

Total heat loss (without safety factor) is 220 k btu/h. Two 8-32 boilers will provide from 33 to 224 k btu/h so I would probably have to go with 11-44 boilers which will provide from 49 to 306 k btu/h. I would still have to figure out how to control the different heat consumers. Suggestions are welcome.

Mike T., Swampeast MO

Exactly!

Perry_2

Not exactly exact...

Mike is correct if you have no house temperature feedback to the boiler.

In my case - with Cast Iron radiators - and with the RS feedback the boiler will not cycle based on the outdoor reset curve - unless the house temperature is in accordance with the call for heat.

In a perfectly matched system the outdoor reset curve by itself would work. Most houses are not perfect. Thus, things like TRV's and RS sensor exist - for where they can be installed at a good match for the system.

The overall key is to match the boiler and controls to the house system. Get them matched - and wonders happen. If they are mismatched - then wonders don't happen (and you get frustrated).

How to adapt a Vitodens to your hydro air coil... I'm not sure myself. However the same principals apply. You need to match the system and controls to your house for it to work really great.

Perry

MikeL_2

On design day, the house loses 21k Btu per hour, about 1/3 the capacity of the Buderus GA124/17. With the differential I have set, this works out to 120 minute burn per hour, which will get my 40 gallon buffer tank up to 125 F. The boiler then stops and the circ keeps going for about 40 mins which is what it takes to drop down to the design temp minus half the differential.

I have actually never instrumented my air ducts, but based on the AH coil sucking almost all the heat out of the inlet water, the air must be no warmer than about 120 F at the top end on design day.