New Boiler / Indirect System Advice (Viessmann vs. Weil-McClain)

EJ81

Hi All,
Apologies for the long post. I'm newly registered here, but have appreciated your collective wisdom for years. Robert O'Brien's articles on boiler sizing have been especially helpful. I would greatly appreciate your advice as I specify my new system (and yes, I understand the install is more important than the equipment). There was a great June 2014 post titled "Modulating-condensing boilers," but I would love to hear six years more experience can tell us.

I am specifying a system for a new 3,500 s.f. home. Manual J indicates 67k BTU demand. Primary heat will be served by Trane air source heat pumps, with cutover to hydronic coils in the high 20’s. There will be 3-4 small hydronic radiant flooring zones (<400 sf total) in the bathrooms. Domestic hot water will be provided by an indirect tank. Fuel will be propane and the system will be direct-vented.

I am leaning toward the Viessmann Vitocell 300-V (EVIA-79) for its anticipated longevity. I would like to be able to handle 5 GPM at 110 degrees faucet temp for 45min+, with surges up to 11 GMP for 10-15 min, hence the large size. In my calculations, I assume DHW priority and storage at 150 degrees, mixed to 120. The system will have recirculation on the two primary baths.

I am trying to decide between two very different boilers: the Weil-McClain GV90+ (84k or 113k output) and the Viessmann Vitodens 200-W (102k or 127k output). I am drawn to the virtues of cast iron / traditional boiler longevity and (I believe) simplicity of the GV90+ vs. the modulating efficiency and intelligent capabilities of the Vitodens that will allow me to size larger for DHW while maintaining efficiency for heat. My goal is to buy a system that is quiet and, apart from routine maintenance, won’t have to be touched for 15+ years.

Thanks in advance for any advice you can offer, and happy holidays!

Getting to a few pointed questions:
a) Am I completely off track in any aspect?
b) Are there any strong recommendations between the two models re: longevity / maintenance / repairs, etc.?
c) Aside from the long recovery period (which I can tolerate) are there any concerns that this DHW tank is too large for the Weil-McClain 84k or Viessmann 102k boilers I listed? I am under the impression a thermostatic mixing valve will compensate for the declining temps as the reserve draws down.

Summary
New System; propane fuel; direct vent
67,000 BTU heat demand; served first by air source heat pumps, hydronic coil backup
2 x hydronic coils
3-4 x hydronic flooring zones (small, bathrooms and a foyer)
1 x Indirect DHW (family of 5; 3.5 baths; would like to support sustained flow of 5 GPM @ 110 faucet temp for 45min)

Robert O'Brien

I assume you did the math regarding indirect pipe and pump sizing in conjunction with the boiler size to determine that it can supply your needs? No knock on the GV90 but it isn't your old school low tech CI boiler, have you looked at Viessmann CU3A? The only mod/con that is going to approach CI lifespan. Or alternatively, a Weil McLain CGI? It's a direct vent version of their standard boiler and as low tech a DV boiler you can get.

EJ81

Hi Robert, thanks for responding. Regarding the CGI, I'm trying to stick with something in the "high efficiency" family, which rules that out. I'll need to get pricing on the CU3A. Of course it looks like a top notch choice, but it may stretch my budget too far as I think it's a pretty big premium over the same size 200-W.

For the others, have you experienced any meaningful difference in reliability? I previously was considering the TT Prestige, which many pros listed among their favorites in the 2014 article, but I've seen several reports of the plastic fans failing and other repairs.

As for pipe sizing and flow, I'm admittedly at the limit of my understanding here. The larger 200-W appears to align, but it's unclear to me if that's an absolute compatibility issue, or simply a recovery time issue for the smaller ones. Simply asked - will an undersized boiler not work properly at all, or just take a long time to recover?

EJ81

After way too many hours of research and planning, you have me sold on the CU3A. I spoke with a Viessmann rep and talked through the high-level system plan. He recommended using the dual coil indirect (Vitocell 300-B, EVBA-79) and piping it in series.

This should pull the return temps down and approach condensing levels even on DHW. With flow in the 6GPM range and the high mass, the boiler should be able to keep up.

I'd love to get everyone's feedback on where I landed. I have attached a diagram of what I'm planning to do, as well as one outlining the connections.

I'm targeting efficiency here, so have tried to size pipes, etc., to keep head loss low enough to run the ECM pumps in the 20 watt range. As noted above, air-source heat pumps will provide heat down to about 40 degrees (I revised my analysis up from high 20s).

I actually pulled historical hourly temperature data, so based upon Manual J (adjusted for my actual setpoints and hourly-specific differentials), should see average efficiencies of about 97.2% for heating. DHW will of course pull it down to <90% levels during tank recovery.

Any feedback or suggestions would be much appreciated.

Jamie Hall

It's a very good boiler. You may even see efficiencies approaching the numbers you quote -- though, like gas mileage for cars, it's unlikely.

Whether you even come close, though, will depend -- as it does with all heating jobs -- on the skill and ability of your installer. Expect him or her to take extra time getting it properly set up; it's anything but plug-n-play. Since efficiency seems to be your goal, though, it will be worth the extra billing.

EJ81

Thanks, Jamie. I appreciate it and am 100% with you on the importance of the install. That's why I've invested so much time in trying to understand the "right" way to do it myself. A small example is the pex runs to the air handlers - had they been 3/4", the head loss would have crushed any hopes of circulator efficiency, so I've specified 1". I don't want to micromanage, but do want to be well-informed.

The Viessmann rep offered to have a tech come out to tune it once everything is set - I definitely intend to take him up on that offer. Thanks again for replying.

EJ81

Apologies for the repost, but someone when editing, I deleted the prior comment.

Back to the original thread, after a lot of research, I'm sold on the Viessmann CU3A. A discussion with the Viessmann rep convinced me to go with the dual coil indirect tank (Vitocell 300-B, EVBA-79) piped in series. I'd love to get everyone's feedback to where I have tentatively planned. System diagram and wiring/connection plan attached.

As others have noted, I am 100% aligned on the importance of install (hence my being here to be sure I understand it myself).

I'm targeting efficiency and have worked to keep head loss manageable so we can run ECM pumps in the 20W range. Based upon historical hourly temperature data, manual J, and my setpoints, we should be able to take significant advantage of the outdoor reset and modulating capabilities to run in the 97%+ efficiency range for most heating calls.

Thanks for looking and for your thoughts.

PerryHolzman

I believe you are likely on the right track with the new Viessmann CU3A.

The heat exchanger is much different than what is in the Vitodens. I have a 14 year old Vitodens 200. You are not going to be able to have a 15 year maintenance free operation with a Vitodens. The Vitodens HX needs to be cleaned every year; and other parts do wear out (Fan, control boards, etc).

The CU3A heat exchanger design appears to solve the some of the plugging issue that the Vitoden has. So, it should be less maintenance from that end (that's a personal opinion - I've never worked on a CU3A)

I'm also glad to see them bring back the Dome MatriX burner. I have that on my Vitodens 200; however later Vitodens models went away from it due to its cost (even though it had overall better technical performance).

As for other things (Fan life, control board life, etc. : I got 12.5 years before I needed to replace parts - which were not cheap parts either). I'd like to hope that they have improved the life of the fan and circuit boards.

I do recommend a the following transformer based surge protector to protect your boiler for decades as low cost cheap CMOS chip surge protectors usually die within 5-7 years and there is no way for you to know that most of them are dead and no longer protecting a likely $$$$ circuit board (it does seem that mod/con circuit boards are very expensive - regardless of brand). Transformers often last many decades and even over 100 years.

https://zerosurge.com/

I have the commercial 20 amp model wired into my boiler power supply circuit.

I later added a double conversion pure sin wave output UPS to protect the boiler from power outages (with accessory battery packs for multi-day operation).

I wish you well with your new installation.

Perry

EJ81

Thanks, Perry. I am arguably overinvested in surge / UPS / etc., for my computers and home theater, but never would have thought to do so for my boiler. Great advice, thank you.

Shahrdad

EJ81 said:

After way too many hours of research and planning, you have me sold on the CU3A. I spoke with a Viessmann rep and talked through the high-level system plan. He recommended using the dual coil indirect (Vitocell 300-B, EVBA-79) and piping it in series.

This should pull the return temps down and approach condensing levels even on DHW. With flow in the 6GPM range and the high mass, the boiler should be able to keep up.

Was the rep able to tell you how much actual water volume is left in the tank with two coils? It has to be a good bit less than the 79 gallon tank with the single coil.

Thanks!

S.

EJ81

Hi Shahrdad,
Truthfully, it was never a topic of conversation. In reviewing the technical specifications, the dual-coil model (300-B, EVBA), and the single-coil model (300-V, EVIA) are physically the same size; the dual-coil weighs 18lb more.

The specifications list a coil capacity of 2.9 gallons in both cases. While the guide doesn't reference this as a "per coil" value, intuitively, it must be, since the AHRI certified specs, using the same flow and water temp, reflect a considerably higher heat demand and associated DHW production (249 MBH vs 169 MBH). The first draw spec on the dual coil tank also is lower (67 vs 76), which would hint at less usable storage.

Shahrdad

I keep going over the physics of this in my head. I can see how having more coils and more heat transfer surfaces can lead to higher extraction of heat and cooler return temperatures. But I also keep thinking about how the location of these coils and the heat differential between the coils and the surrounding water could negate much of this gain.

With the single coil tank, the coil is near the bottom where the water is coolest and where the temperature differential between the coil and the surrounding water is the greatest. This large temperature differential will lead to faster heat transfer from the coil to the water. With two coils, the first coil--where the hot water from the boiler enters--is located closer to the top of the tank, where the hottest water will also be. The temperature differential between the coil up there and the surrounding water will be much lower, and consequently, the heat transfer between the coil and the water will also be much lower. And when the water from the top coil finally reaches the bottom coil, it will already be a bit cooler, and the heat transfer will be lower due to the lower temperature differential. So in the end, the water returning to the boiler from a double coil tank and a single coil tank might end up having the same return temperatures.

I wonder how long it will take to recoup the cost of the more expensive double-coil tank in gas savings, assuming it really does lead to more condensation and bigger savings in gas. And there is also the possibility that whatever is saved in gas money will have to be spent on electricity for a stronger circulating pump that has to overcome the higher resistance of twice as many coils. With the high cost of energy in Germany, I am sure those clever German engineers have put in a great deal of thought into these DHW tanks already.

I am myself seriously considering replacing my 50 year old CI Weil McLain boiler with a CU3A and an indirect tank for DHW. My house is a large 124 year old masonry house with an original gravity system. This Viessmann boiler looks better than any other I've seen so far, but I think I'll be going with the single coil tank. It will be fun to see which ones we choose to install and how happy we will be with our decisions!

Best,

Shahrdad

pecmsg

I may have missed it but where is this home located?

Are any of the Hydro Coils installed in unconditioned space?

heathead

One Coil or Two? The pumping loss through two coils vs one on a Viessmann is not noticeable. The two coils will work to heat the water more cost effective, by creating lower return temp to boiler so it can condense more ie be more efficient. Most Viesmann's are set to have the boiler temp 27 deg higher than tank temp when recharging the tank, so if tank drops to say 115 then it gets the boiler water up to 142. That 142 degree water is then is heating the top coil and then giving up that heat faster, then the return water after the lower coil being much lower back to the boiler. Now is that enough to payback the dual coil tank I don't know. If you put that extra cost of the two coil tank vs one into a heat pump water heater it would have better payback in the summer, depending upon energy rates on electric vs Gas. Use the Heat pump water heater in the summer and indirect in the winter. With the Cu3 you have to heat up 13 gallons of water in the boiler every time it needs to fire the indirect water heater in the summer. In the Winter the boiler will be hot and no heat loss or lost energy heating that 13 gallons of water before you start heating the water in that tank. That said the more water heaters you add the more to go wrong, but then you also have a backup of hot water if something goes wrong in the boiler and you need to wait for parts. Also some free coiling and dehumidification in the summer with heat pump water heater. Don't know what your hot water needs are and if a heat pump water heater would be able to supply enough to meet your needs in the summer. It is not a straight forward answer, just thoughts. I think that your choice in boilers is a great one and will be saving money every month heating that old house.

EJ81

My home is quite the opposite of Shahrdad's. It is located on Long Island (downstate New York) and is a new construction project with spray foam encapsulation (including the attic). The mechanical room technically is unconditioned, but it has concrete on three walls and benefits from the air-sealed first floor deck, so heat loss is pretty minimal. Everything else sits within conditioned space.

To Shahrdad's point, I am skeptical I will ever recover the added investment. I'll need to save nearly 300 gallons of propane in today's dollars. At some level, I ultimately disregarded the ROI math in favor of an exercise in technical capability and a desire to push the system to peak efficiency.

The efficiency curve of the CU3A, like most modulating boilers, drops to the mid/high 80s when run at the higher temperatures typically required for DHW. The 79-gallon tank really is sized for surge capacity, so I am going to experiment with temperature curves. If I can meet the demands with a 1.4 curve, I should achieve condensing return temps with this setup. If not, I'll have a real-world trial to share with all of you.

Given the above scenario, if I can achieve 10% higher efficiency (which would be a lot), I would theoretically return my investment after using 3,000 gallons of propane just for heating water. In my case, that will be many years.

Shahrdad

I've been wondering how well a large water-content boiler like CU3A deals with two systems that require vastly different temperatures. Most of the winter, my radiators are just lukewarm, and when it's zero or below, they only get to 130 degrees. The house stays at 70 degrees.

In their application guide, Viessmann doesn't show any primary/secondary pumping when only one CU3A boiler is used. With up to 19 gallons of water inside the boiler, I wonder how well it goes from providing very hot water for the tank to just lukewarm water for the radiators on a moderate winter day when the rads need only 105 degree water.

This is the typical setup Viessmann shows in their application guide for a one boiler system.

EJ81

Not sure if you saw my system diagram (attached). I am using the Viessmann 3-way mixing valve with boiler-controlled actuator to address this exact circumstance. Having said that, be prepared to open your wallet - it rivals the cost of a low-end wall-hung unit.

In my diagram, ZC1 and ZC3 are temperature compensated zones; ZC2 is a fixed-temperature radiant floor zone (secondary heat for comfort only) and uses a basic mixing valve to achieve the desired temperature. DHW is controlled by the boiler controller.

heathead

EJ81 is your ah1 and ah2 air handlers? If so I would not want a mixing valve. The higher temp is not bad for the air handler coils especially when first blowing air.

Shahrdad

Oh that's really cool. Did the Viessmann rep design that for you?

I think my 125 year old system's water content is several times what your much newer system contains. Just in the basement alone, there are about 200 feet of 3 inch steel pipe (two supply and two return), and there are more big pipes going to the large cast iron radiators on the three floors above. I'm sure there are several hundred gallons of water in the system, which would quickly swallow the 19 gallons of hot boiler water up. I'm not sure with so much water, a three way valve would be necessary for my system, especially since it just has to deal with two temperatures: The house system and the DHW system.

Which one of the boilers are you going to get?

S.

EJ81

HH - more to the story. My primary heat sources are two air-source heat pumps. It's a high-efficiency, variable speed, variable capacity Trane system. They can meet the heating demand down to about 17 degrees, but given the very high cost of electricity in my region, it will be more cost-effective to use the propane beginning in the mid-30s, so they'll be programmed to switch over. Because of the variable speed design, they ideally will run near constantly at slow speeds, with very very low setbacks. At 180 degrees, the heating coils can provide ~60k BTUH each for zone.

Based upon temperature data in my region and my home's heating demand, average demand across the roughly 2500 hours of time when temps are below 40 degrees will be about 17k BTUH per unit, and in a typical year, will peak at ~33k BTUH for a few hours ( at 10 degrees).

I plan to experiment with the temps to find the sweet spot between comfort and efficiency, recognizing that perceived temperature with the flowing air will be cooler than actual, but running at the full 170+ degrees would leave me back in the same situation where my boiler never condenses.

EJ81

Shahrdad, I actually spent way too many hours studying the application guide, speaking with the Viessmann rep, and reading this forum, so created it myself. I did have the Viessmann rep look at it to make sure I didn't miss anything. It was a very tedious copy and past exercise!

To your point, my entire system, inclusive of the radiant floor loops and DHW coils, has at most 50 gallons of water, and more likely closer to 40. This includes the radiant flooring loops and boiler.

I got the CU3A 35 (125 MBH). Bigger would have been better for DHW, but given my heating demand peaks in the ~65k BTUH range, I'll be stepped all the way down most of the time, even with the 35 model. That's what sold me on the 79 gallon DHW tank vs. the 53 gallon model.

Shahrdad

One of my best friends owns a huge house of about 12,000 Sq feet on the street behind me (www.magicchefmansion.com). She had a geothermal system installed for AC as well as for heating during milder weather. She also has four boilers which were previously the only source of heat to her house. She says she's saving some money on heating while running the geothermal heat pump, but really misses her warm radiators and doesn't like the blowing breeze. When it gets really cold, she still fires up her boilers, which she says offer a much more pleasant heat. I was telling her that geothermal heat could also be used to heat the hydronic system, and she was going to inquire about doing that.

Shahrdad

This is a good video that talks about how the Viessmann manages a system such as yours or what I will get.

https://www.youtube.com/watch?v=LSiiK2GXoiU

heathead

Ej81,

I agree that the coils don't need to high temps. But having a mixing valve just because the system is still hot from an indirect hot water call may not be a wise investment and may cost you on efficiency the whole time with the mixing valve. The mixing valve isn't making the boiler more efficient it makes it less because it mixes the water down and then the coldest water isn't being returned to the boiler. The boiler comes with a outdoor reset. That outdoor reset will be set to the temp to the highest load needed which would be air handler coils. I am not understanding why you would want a mixing valve just to lower the boiler feed water after a call from the indirect, thinking that the boiler is to hot for the coils after a domestic hot water call. I think a Viessmann controlled mixing valve would be better for your radiant floors and not your air hot water coils. I am sure someone with more information can help if needed with a way to deal with the hot water in the boiler from the domestic hot water call, if you are afraid that will over heat the air from fan coil air after an indirect hot water tank loading. I can't see the cost and complexity to add a mixing valve to a air handler coils. I might just be under thinking the excess heat from the indirect call would do to the fan coils and comfort level.

EJ81

Great video, thanks for sharing. I wish I had seen that much sooner! Good luck with your project.
That house is quite a place!

EJ81

heathead, I don't think you're underthinking it - it's undoubtedly an expensive, complicated solution to a relatively small problem. The garage heat will be hottest of the three heat zones (but runs the least), and DHW of course will be the hottest of everything. At this point, I can't remember all of the decision points, but I believe the MV was a recommendation from Viessmann. In any event, the equipment is purchased and about to be installed, so I guess time will tell.

Shahrdad

I would LOVE to see photos of the installation. You system is a lot more advanced and complicated than mine. I have wondered whether installing the larger 119 gallon tank would result in fewer cycles. The larger tank will have a smaller surface to volume ratio and retain heat better, and it will take longer to exhaust the hot water supply and require the boiler to kick in.
It would be cool to see what your return temps will be with two coils versus someone who has the same tank set at the same temp but with one coil.
I'm waiting for warmer weather before switching over. Hydronics isn't big in my area. A lot of HVAC people dabble in it, but I have yet to see a system that's piped as it should be or a boiler that is not grossly oversized.

S.

EJ81

My 1,000 gallon propane tank gets dropped tomorrow; plumbing everything should commence in the next couple weeks.  I’ll post pics when done and share results, but that realistically won’t be until the end of heating season.

Shahrdad

I just was just watching a Viessmann video about commercial water heating, and at around 57 minutes into the video, they showed how they use a plate exchanger to make the water going back to the boiler from the DHW tank give up more of its heat to the incoming domestic cold water going into the tank, thus driving return temperatures to the boiler even lower. Looked like a really cool idea, though again, not sure how long it will take to recoup the cost of it in gas savings in a domestic setting. The engineering of it is really cool though.

Shahrdad

Also, here Jodi Samuell addresses the question of whether or not to use two coils in the indirect tank connected in series. He says this setup will lead to cooler return temps, but whether or not that is enough to offset the price difference, he's not so sure. I looked at the price sheet, and two coils doesn't seem to be that much more expensive than one.

https://youtu.be/nOp0vNV_qjY?t=2081