I am presently deciding on which mod/con to install in a new house that I am building.

The heat loss has been calculated to be 37k.

The heating will be radiant on all floors, slab in basement, Warmboard on main floor and Thermofin U Overfloor in loft.

I have decided on a TT Smart 60 indirect for hot water.

Based on the postings from this forum and others, I have decided on a Viessman W200 boiler. 

The models that I have looked at are the WB2B-19 31,000-67,000 MBH input, the WB2B-26 31,000-93,000 MBH input and the WB2B-35 31,000-126,000 MBH input.

I understand that the boiler wants to be able to satisfy small heating loads in addition to supplying the DHW needs.  Based on the fact that all 3 of these boilers appear
to have the same minimum modulated output, it would make sense to me to select the WB2B-35 to be able to better provide the DHW load. 

The boiler installation WILL NOT be a DIY job.  I just need a sanity check on my logic.  I like to fully understand all aspects of the project and what the options are and what the tradeoffs are before I speak with a contractor

Thanks

hmm. that's interesting. I wonder why they would bother offerring three units with the same minmod? Are there price differences?

There are a few hundered dollars price difference between the different units.

My guess is that they are actually the same physical unit. They probably have different control software that limits the output of the smaller models.

Their marketing department has been working overtime to determine what price/performance points they want to have in the model line.

Back to the original question, is it better to use the higher output unit for the better DHW performance or use the smaller unit and have it take longer to service the DHW?

normally I size the boiler to the heat load and then size the domestic tank for the boiler output and DHW load. because normally I would rather have a smaller boiler and bigger tank.

In this case though, I don't know. What's your domestic load? How many showerHEADS at once, how many back to back sequential showers, and regular/low flow heads?

the only way I'd bother going bigger on the boiler is if it allowed a smaller tank AND there is no other penalty here. and I'm not confident of that, but maybe your suspicion is right.

There are 3 bathrooms. Master bath has a showerhead and an 80 gallon whirlpool, bathroom in second bedroom has one
showerhead, downstairs bathroom has 2 showerheads (for use with a sauna both showers probably used at the same time).

There could regularly be a whirlpool and shower and laundry going at the same time.

All showerheads are 2.5 GPM

I'd call that something like a 7.5 GPM demand for about 20 minutes.

with the small boiler you'd need something like a 120 gallon tank (assuming 140 storage temp).
with the larger, about 80 gallons should do it.

slightly conservative but pretty close. 46 degree water/115 at the tap.

Thanks Rob

I really appreciate all the time you and the other professionals take to answer questions on this forum.

Now I know what I should be looking for when I get quotes for the job.

BTW, I designed the house with Warmboard based on your recommendations in these forums. The loft was originally going to be attic
space only so I didn't put Warmboard there but when we decided to finish it, I selected the Thermofin also based on your recommendation. I even
ordered it from your web store.

Thanks

we appreciate your patronage. by the way I spoke to a viessman rep and he agrees that the boiler is basically the same boiler for all 3. no real good answer yet as to why there are 3 though. seems silly to me, but viessmann is a sharp company. seems like there must be a reason.

After doing a heat load using radiant floor appropriate software, I look to the Maximum AND the Minimum output ratings. Since the heat load represents approximately 5 days of the heating season, the minimum fire is the number I am concerned with. The lower the minimum output, the longer the cycles i.e. more comfort, combustion and system efficiency.

The Vitodens is very sophisticated and controls cycle time among other things, but the best part (beyond efficiency) of any modulating/condensing boiler is the outdoor reset control, which is compromised when not programmed correctly or when trying to compensate for an over-sized boiler.

Dr. Viessmann has concluded that a minimum fire is required for efficient combustion in this particular HX. The same heat exchanger is often used by various manufacturers with a change in burner output settings, controls, etc.

I am factory certified for Viessmann Vitodens condensing boilers, it is a very nice unit, but all models are too big for your application.

In your case (if the load were accurate) I would look to the max output first - there are a couple of 50s out there and the Prestige 60 you mention - and be happy with the much lower minimum.

There is no better way to heat domestic hot water when a condensing boiler is required than with a simple and reliable indirect water heater. Condensing boilers should be sized to the heating load and coupled with an indirect-fired water heater, which in turn is sized to the DHW load, which is determined by peak demand. In your case this peak demand would be filling your tub.

It is almost never a good idea to oversize a residential boiler to for domestic hot water duty. It is not common practice.

Your designer should have made this decision for you.

Thanks for your response Badger

The system hasn't been designed yet. I have been educating myself so that when I get the system is designed I will have the system that I want in the house not
the system that the designer/installer would make the most profit with. I fully support companies making profit on their work, my concern is having the best performing
most reliable system I can achieve. I would like to decide what is the best boiler to use then try to find a good installation person that installs that brand.

I initially considered 3 brands of boiler. The Buderus, Triangle Tube and Viessman. There are many positive things said in this forum about all 3 units. I will admit that
I am a little biased/concerned about an aluminum heat exchanger so I have eliminated the Buderus unit. I consider the Triangle Tube and Viessman to both be good
solid reliable units. I wanted the Viessman because it seemed that when anyone wanted "the best boiler" they always seemed to mention the Viessman. I also liked the
way they continuously control the combustion.

When I look at the Viessman line, they made the 3 models with the same minimum modulation limit, but differed in their maximum output. So if I was to use a Viessman, my
original question was that given the same minimum modulation capability, wouldn't it make sense to have the largest maximum output capability to service the DHW better.

Rob graciously answered that the 67,000 BTU Viessman would need a 120 gallon indirect and the 126,000 BTU unit would need an 80 gallon indirect.

I really appreciate your answer that the correct thing to do is to use a smaller boiler with the larger indirect tank even if that means not using the Viessman unit. This is exactly the type of
things I like to understand so that when I am having the system designed, it is done properly and I understand WHY solution A is better than solution B. I had not previously read that the
outdoor reset can be compromised by trying to compensate for a larger boiler.

I believe the heat load calculation to be correct, I have used an online head load calculator, the Taco software and a couple of others. The calculations were 35.4K to 37K. The house is
a walk out basement level approx 2/3 below grade, the main floor with cathedral ceiling and a loft over the bedroom area on the main floor. The entire house is insulated with closed cell
spray foam, R21 in all walls and R38 in the ceilings. I have caulked all joints between the framing members to prevent air infiltration. There will be a 200 CFM Aldes HRV system for the whole house.

Given the manufacturers under consideration, you think it would be better to use the Prestige 60 and the Smart 120 indirect rather than the Viessman.

Thanks for your input

I would prefer the solo 60 in this case. If you use the viessmann i'd consider a small buffer tank.

There is never a good excuse for a buffer tank coupled to a condensing boiler that modulates.

I couldn't even begin to agree with you on that one. Note I'm talking buffer, not maintained storage. full benefit of reset at all times. load goes over min mod and it's basically invisible again.

no matter how good your modulation is, bathroom zones are an issue. especially when there are multiples. and in this fellow's case, the viessmann's min mod is equal to his max load. any zoning would just make it worse.

I routinely see situations that would benefit from additional water mass. I wish all modcons came with 10+ gallons internal, frankly.

I am willing to offer general advice on hydronic design, but since I design systems for a living specification is by contract only.

A Radiant Panel Association certified designer/installer (like Rob and I) could make your search much shorter and more productive.

I appreciate your efforts and wish that more contractors/builders shared your concern and dedication.

As for the best boiler, I have installed nearly all the condensing boilers available in the US and find the size, near piping and control programming infinitely more important than the make and model. Naturally, regular competent maintenance is critical to performance and reliability.

The best high efficiency boiler is the one that best fits the heat load and radiation of the building that it will heat.

A expensive crutch to compensate for poor structural and/or heating system design e.g. a ModCon with a minimum fire too high for the application. 

If you are forced to over-zone a home or you are trying to turn an all-glass three season into a four season porch (it will probably need all of the low-fire anyway) then a buffer tank may be in order. Since nearly all ModCons control short-cycling a properly sized boiler will rarely short-cycle (built-in program).

When designing with solar, or wood-fired boilers that have to dump cyclical loads I use a buffer. When I sold non-modulating condensing boilers a buffer was occasionally needed. But with a ModCon? Almost never.

Frankly, with loads this low (I would confirm with Wrightsoft/Uponor module) a condensing water heater might be the best investment. 96 percent thermal efficiency coupled with a sub-system with outdoor reset could satisfy any load with standing capacity for the big tub and any draw for smaller loads down to a towel warmer.

And you know I LOVE ModCons!

agreed. in many cases though "over zoning" is necessary. there are an awful lot of people out there with restricted bathroom floor areas and no desire for visible supplemental emitters and they are usually kept at a higher temperature than any other room in the house. those tiny zones can call constantly.

So I would contest your statement, as I would spec a buffer tank on many of the mod/con systems I see and also design, but I can't fault you for trying to avoid them.