No Primary/Bypass loop ok for some boilers?

Mike T., Swampeast MO

Primary/secondary is not always required.

No "consumer reports" type of thing for boilers that I'm aware of. As a general class, condensing/modulating boilers are so new that nobody really knows how long they'll last...

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You have a converted gravity system with standing iron radiators, right?

Any significant changes like radiators replaced with some other sort of emitter or a "zone" of baseboard added?

If the answers are "yes" and "no" respectively, then:

1) I'd be especially leery of using <I>any</I> boiler with an aluminum heat exchanger in a gravity conversion system. Really believe you'd be better off with stainless steel.

2) Such a system is <I>usually</I> perfect for use with a single circulator and without primary/secondary, BUT--and this is VERY important--you will need thermostatic radiator valves (TRVs) on the radiators. Why? Because of the way the piping was originally sized. While huge and with essentially zero restriction to flow, it's "backwards" compared to sizing for forced circulation. This means you need to move more water with forced flow than with gravity to achieve reasonable flow balance in the system. The TRVs will not only make each room act like their own zone, but they will keep the flow rate down to a level acceptable for "direct drive" through most mod-cons.

3) Think long and hard about a Vitodens by Viessmann. While the boiler cost itself is higher, quality is exceptional and such a TRVd gravity conversion is exceptionally well-suited to its control method and general operating philosophy. Since the two smaller models have a built-in circulator, there is little else required beyond the boiler, venting material and the TRVs with especially simple installation.

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Some "Do's":

1) A heat loss calculation is MANDATORY! A free program is available here. You can also download a one-structure use version of HVAC-Calc at www.hvaccalc.com for a nominal price. If you're willing to spend a few hours, I highly suggest you conduct one or both of these yourself--then "play" a bit with different insulation levels.

2) Do your utmost to find a contractor who is already familiar with mod-cons. If you can't find one, at least find a contractor who is enthusiastic about using and learning.

3) Do insulate your basement piping at least reasonably unless you want a heated basement. Those huge gravity pipes can give off an awful lot of heat under constant circulation of heated water...

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Some "Don'ts":

1) There's little sense in putting a highly efficient boiler into a poorly efficient shell. Give utmost consideration to any reasonable insulation/weatherization improvements <I>before</I> you install a new boiler--such could well be the difference between sizes.

2) Do not oversize the mod-con! If your system fit the two criterion above and if you have at least reasonable insulation/weatherization measures a mod-con will go a LONG WAY!!! If faced with a choice between one somewhat "small" and one somewhat oversized, choose the "small" one!!!! My own house has a calculated heat loss of 100 mbh. I use a 6-24 Vitodens with 91 mbh rated input, 81 mbh rated (DOE) output and 70 mbh rated (I=B=R) output. No problems whatsoever. Capacity to spare! Unless your house is huge and/or extremely leaky and/or in an extremely cold environment, I'd say there's a good chance that a boiler the size of a Vitodens 6-24 will be sufficient.

3) Do not fall into the trap of oversizing the boiler for domestic hot water. A boiler the size of a 6-24 can provide a lot of hot water provided you have a proper indirect. High-quality indirects with high heat transfer ability can be very expensive. If such proves a budget buster, I'd honestly use a stand-alone water heater before I chose a lesser-quality indirect.

4) Do not neglect a system flushing, cleaning, and likely application of a corrosion inhibitor/rust "binder" and a filter. Old systems can be full of rust, corrosion and scale that can conceivably shorten the life of even the finest heat exchangers.

Al Roethlisberger

Is there a \"consumer reports\" for various new boilers anywhere?


Beyond reading every thread on Boiler brand "X", is there a resource anywhere that follows customer satisfaction on some of the current brands/models of condensing boilers?

I will be doing some upgrades, maybe a complete new boiler, for my hot water heat and DHW next year.

So, I am looking for any general objective recommendations(or concerns/warnings) as I start to consider brands/models. So all comments are welcome.

Some brands I have been looking into so far:

Buderus GB142
Knight
Laars Mascot
Munchkin
NTI Trinity
Viessman

Others I should consider?

I don't have anyone local to really do the job of educating me or selling me on a particular brand, so this is likely really up to me to figure out and install. So any info is helpful.

Thanks
Al

ALH_4

P/S

My picks are the Viessmann Vitodens and the Triangle Tube Prestige depending on budget.

Some models of the Vitodens can be piped without P/S in certain situations. I have done it many times. The Prestige has very low head loss through the heat exchanger, though they do recommend P/S in all cases according to their manual. I wouldn't use P/S on a Prestige, but that might void the warranty unless the factory would endorse the design.

Maintaining minimum flow through the heat exchanger is the issue. As long as you can do that with a differential pressure bypass and have enough residual head to run the system, no P/S is required. Really, it's really just a modified P/S that uses only one pump.

Patrick Linhardt, \"Linhardt's Field Guide to Ste

Prestige P/S not needed

Hi Andrew,

I was in Blackwoods NJ in October for a class on the Prestige boiler. They are recommending that P/S not be used unless desired. Size the system circulator(s) for the pressure drop thru the boiler (chart in back of install manual) and the zone(s).

Best regards, Pat

ALH_4

good

Thank you for the information, Pat. When I had read the manual, all the piping instructions indicated P/S and the text stated that it was required. The pressure drop chart in the manual seemed to indicate otherwise. I am glad to see they are allowing for installations without P/S. The low head loss heat exchanger is a major selling point for the Prestige in my opinion.

Mike T., Swampeast MO

When I read the manual it seemed that the main reason they wanted P/S was for the "freeze protection mode" to work properly. Odd...

The manual was also specific about maintaining a minimum flow level based on maximum boiler output. Might be difficult in a system zoned with valves or circulators. I suppose differential pressure bypass could be used, but since that's really system (e.g. emitter) bypass it wouldn't [seem] to be in the spirit of maintaining a "guaranteed safe" flow rate at maximum boiler output...

I'm not sure that low head loss in the HX has much to do with any requirement for primary-secondary. After all, the head loss through the HX itself is known and can be accounted for. System-side head loss in a gravity conversion (even fully TRVd) is extremely low regardless of what boiler it's connected to.

Curiously however, I could not find any mention of a maximum flow rate for the Prestige. Unusual. Other mfgrs are generally quite specific when it comes to maximum acceptable flow. Perhaps this is mainly control related and the Presitige uses a different scheme.

If you look carefully in the manuals for other mod-cons you'll usually find something along the lines of, "Primary/secondary is recommended but not required. All diagrams of suggested systems in this manual use primary/secondary."

ALH_4

decoupling

But the pressure drop in a boiler like the Vitodens gets pretty high when trying to drive through enough flow for a 15-60. Devices like the Low Loss Header were developed to "hydraulically decouple" the system where the system flow rate must exceed the flow rate achievable through the heat exchanger. With cast iron boilers, the bypass is for thermal decoupling, with condensing boilers it is for hydraulic decoupling. A heat exchanger capable of handling the flow rate needed to maintain a reasonable temperature rise at high fire eliminates the need for a low loss header or P/S.

Al Roethlisberger

Thanks! What is special about the Vitodens?


Thanks Mike, that is all great advice.

I will be considering all of this and other feedback over the next few months.

I have searched long and hard for a local pro to work on my current system with no success, beyond some simple maintenance, so along with limited budget... I'll probably be doing much of the install myself... hence all the questions.

I have read up on the Vitodens, but unfortunately it seems to be only available to contractors, not DIY'ers. Is there something particularly sensitive/difficult with the Vitodens that requires a pro to install versus others.

As for DHW, can't I just use a standby tank instead of an instant/indirect water heater? I know that would be less efficient, but it might be substantially less expensive.

I assume the Vitodens had DHW priority built in like most modern units, so I would think this would be an option.


Thanks again!
Al

Mike T., Swampeast MO

Are boiler controlled variable speed circulators available for the larger (11-44 & 15-60) Vitodens boilers?

Have you read this paper? I know it's produced by Viessmann, but much of the information seems quite generic.

Here are some statements from that paper that I've always found quite curious:

Previously, low-loss headers were used to ensure the presence of a minimum circulation volume inside the boiler. Modern condensing boilers no longer demand this facility. However, it may be the case that the max. permissible boiler throughput is less than the circulating volume inside the heating circuit, e.g. in underfloor heating systems. Here the higher heating circuit volume flow should be balanced with the boiler circuit volume flow using a low-loss header. This avoids raising the return temperature. The capacity of boiler circuit or heating circuit pumps must be matched, so that the higher volume flow is circulated in the heating circuit, to reliably prevent a mixing of warm flow water with the return...Careful design and control are required if the use of a low-loss header cannot be avoided to achieve the best possible condensing effect...When balancing the low-loss header, the volume flow on the boiler side should be adjusted approx. 10 to 30% lower than the volume flow on the system side (lower return temperature)." [emphasis added]

What that's telling me is that earlier [I'll bet non-modulating] condensing boilers had to have a flow rate through the boiler high enough to virtually eliminate the possibility of hitting high safety limit unless there was a fault. (Anyone who has observed mod-cons in action knows how fast they transfer heat...) Such was especially a concern with TRVd systems--even with differential pressure bypass.

I suspect that modulation was the solution to this problem. Less output means the ability of less flow without the danger of hitting the safety limit. Determining just how to do this however isn't particularly easy and it makes me appreciate why Viessmann really wants a fully integrated system where every part knows just what the other is doing...

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Back to the "problem" with the 15-60. At max input and max flow the emitters must be capable of 30°F delta-t. Viessmann recommends a number of different circulators which have varying degrees of "extra" head available above that of the boiler. [Again, are any of these variable speed?]

If you're using some form of radiators with such a boiler there shouldn't be any problem with "direct drive" unless the radiators are significantly undersized or you've micro-zoned with zone valves. With TRVs there shouldn't be any problem and a fixed speed circulator is no problem other than increased electric consumption.

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Now back to the statements from that paper.

It's underfloor that can cause a problem. Why? Because 30°F delta-t may be either unachievable (constipated emitters) or undesirable operation (too high a temp variance across a floor).

Note however that for most efficient operation and presuming that the larger Vitodens models use a fixed speed circulator, you need to keep secondary flow higher than primary 10% - 30% higher than primary at all times. Not particularly easy with a multi-zoned system unless you're using FHVs...

Mike T., Swampeast MO

From one hard-core DIYer (and one presumed old house lover) to another...

Search here at Heating Help for the material, construction, design and control elements that make the Vitodens unique. As was said here at least three years ago, "The burner alone is worth the price of entry." While many mod-cons have entered the market since, the Vitodens burner remains unique in form and function.

Again, is your current gravity system substantially intact?

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Three things I find invaluble in my sensitive renovation of a 1903 home:

1) TRVs -- FOR COMFORT

2) Vitodens -- FOR EFFICIENCY

3) Central Vacuum -- FOR CONVENIENCE

Al Roethlisberger

Yes, my system, other than the current 15yr old boiler is intact

One radiator upstairs was removed about the same time the Carrier boiler was installed and the system was converted to a closed circulator system.

The boiler was installed and the circulator is currently pumping *towards* the expansion tank through the boiler, and the circulator now runs the water reveresed through the system from the direction the water originally flowed.

Other than that, the system is completely intact.

Thanks for the feedback all!

Al