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Different take on tankless vs buffered (15 Posts)
Different take on tankless vs bufferedAny opinions on how contractors bidding on my small-house remodel can reach such wildly different conclusions on tankless vs. buffered mod cons for combined heat-and-hot-water install? FYI, depending on insulation TBD, Manual J indicates 23k-33k MBTU/hr on coldest night and someone will very likely want a hot bath that night so I figure another 30k MBTU for DHW, based on experience with existing DHW tank. I can give more details if you wish.
Traditional radiant contractors: Smallest available boiler+buffer+indirect will avoid cold water sandwich and avoid short cycling in shoulder season. I get that.
Full-service heating-plumbing contractors: With high enough BTUs (150k MBTU/hr), tankless avoids cold water sandwich, and vendor will honor warranty for any reliability problems due to shoulder season short cycling. That's supposed to give me confidence that tankless reliability won't be a problem. I'm open to what they say but...
I don't get it...if shoulder season short cycling isn't a problem with tankless, then why would a boiler need a buffer?Viessmann installers in SF Bay area, feel free to contact me via my profile page re whole house remodel
BoilerIt sounds like you should buy the smallest condensing boiler you can find and attach an appropriately sized indirect.
A 50-60K Triangle tube or Lochinvar with a 40 gallon indirect would be a good call.
The less domestic storage you have, the more boiler output you will need.
The tank in the combi units is domestic water storage, it is not a boiler buffer tank.
If you get the boiler sized correctly, you should not have cycling issues.
OK back up, no buffer needed for small efficient house?Zman wrote:
>It sounds like you should buy the smallest condensing boiler you can find and attach an appropriately sized indirect.
A reputable contractor and also a rater recommended the Navien. Also, for some reason some of the larger rebates $1000+ favor tankless (big rebates nice but not the determining factor for me). If I say, try again, I think I should give some reason - else, who am I to argue with both a rater and a reputable contractor?
> A 50-60K Triangle tube or Lochinvar with a 40 gallon indirect would be a good call.
> The less domestic storage you have, the more boiler output you will need.
Do you mean, the more I will need to get DHW taps to respond in a reasonable amount of time and to support low-flow draws and intermittent draws? Or some other reason? FWIW there are a lot of low-flow and intermiitent draws plus soaking tub is in use (even if it wasn't, that capability would be needed for home resale value).
> If you get the boiler sized correctly, you should not have cycling issues.
30k BTU heat load on coldest day I think means a lot of hours of operation averaging 10-15k during shoulder season, no? Some contractors and lots of forums seem to make strong statements about small, efficient homes needing a buffer. For example,
If I read that correctly, user SWEI says of a 30k load house "There is literally nothing available in a gas mod/con that will properly handle that small a load. You're going to be short cycling most of the season." I assume he meant, it will short cycle without a buffer.
On related note, Lochinvar seems to have the lowest modulation (10.5 k) and even that sems borderline...IIRC, TT, Burnham, Viessmann, and Navien are all closer to 15k at lowest modulation.Viessmann installers in SF Bay area, feel free to contact me via my profile page re whole house remodel
First of AllYou never add btu/hr for domestic hot water unless the DHW load itself is larger then the heat loss of the home. For some condensing boilers out their such as Viessmann, Burnham Alpine and I believe the Loch. You can cap off the high end of the modulation rate for heating independent of DHW.
The main cause of condensing boiler short cycling is not the max btu/hr the boiler can put out. The boiler will never give it until its needed. The problem is the low end and the size of the boiler pump that creates short cycling. There is no manufacture on earth that will warranty for a short cycling boiler. If a part failed it wasn't because it was defective but rather beat the heck out of. ie, improper installation.
I'd take a look at this. Wouldn't need to be piped pri/sec and blows away a tankless and in my opinion a boiler/indirect option especially for such a small heating load and domestic load.
http://www.viessmann-us.com/en/Residential/Products/gas/Vitodens_222.html"The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
Yes I like the Viessmann 222-F but...Chris, first of all I had already posted about the Viesmann 222-F in another thread (that you also posted in) regarding poor availability in my area. So far I've found only one contractor in my area who installs them - and his availability is also very poor.
The 222-F is like some member of the opposite sex I've only heard about but think I'm in love with...which is always dangerous but fortune sometimes favors the bold. Other times, it clobbers you.
> The problem is the low end and the size of the boiler pump that creates short cycling.
Of course. Now consider this: Even 222-F has min fire at 16.4 kBTU. My post-remodel target is 25k BTU on coldest night. Considering shoulder seasons and cool-mild climate zone, even the 222-F, even if I go with only one zone, may need a buffer, no?
> You never add btu/hr for domestic hot water unless the DHW load itself is larger then the heat loss of the home.
Existing inefficient water heater at 30k BTU and 30 gallons, to be scrapped, but is just about right as far as DHW demands around here. So, yes I expect DHW load itself will be larger than heat loss of home.Viessmann installers in SF Bay area, feel free to contact me via my profile page re whole house remodel
Not the 222F-19Has a min of 12 Input..You can cap off the high end of the modulation rate for heating independent of DHW. You also won't have to pri/sec pipe it either. Set the pump so the boiler works on off a 40 Rise and your boiler flow rate is only 3.1gpm."The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
Yes, 12 kBTU min input for US version of 222-FYou are correct sir.
Apparently it's the UK version that mods only down to 4.8 kW (16.3 k MBTU/hr)
While, as you say, the US version mods all the way down to 3.5 kW (11.9 kBTU/hr)
Memo to Viessmann - may I suggest it would be wise for www.viessmann.com to point to the above doc.
Short cycleA short cycle is more than 6 cycles per hour.
If you have a boiler with a descent amount of volume (firetube) with a turn down of 10.5 k., you would short cycle when the load is some where in the 4-8k range.It all depends on the volume of the system.If your load is going to get that low, you should determine the volume and do the math as to whether you need a buffer.
If you believe in navien and their marketing claims, you should run right out and buy one. They sound great.
Do the math is rightOK, you convinced me that for economy reasons do the math is right - there is no other way to be sure I need to spend for buffer + parts + labor.
That said I would be surprised if any of the 5-6 contractors I'm in touch will will calculate volumes or flows or anything else in the next 2 weeks. I will not be surprised if I end up hiring an engineer.
How Manyzones of heat and what is the smallest zone on design day ? Really must have this to further discussion . You can get more DHW using a storage type tank than with a tankless . 3 way diverting valves in tankless combi are problematic . I have changed no less than 7 of them in 18 months (Navien) . That what you want ?You didn't get what you didn't pay for and it will never be what you thought it would
Just trying to be (too?) polite> I have changed no less than 7 of them in 18 months (Navien) . That what you want ?
As a matter of fact, no, tankless makes zero sense to me - not to say it's a bad idea, just that I don't get it - I can easily imagine having to wait for hot water, turning tap high to prime, failure bec. it gets used too much, guests getting confused, members of the opposite sex rolling their eyes...I can also imagine 15k low fire still being too high in the shoulder season...
To repeat: The rater, a reputable contractor, and for that matter the governor of California all want me to get tankless (rebate issue). So, OK you can forget the governor and the rebate. I still have to politely explain to the rater and the contractor that their beloved 1st choice is not my first choice...now I hope you don't mind my saying that in my experience 100% of all contractors treat me like a child - a stupid homeowner who doesn't know anything. Maybe I am and maybe I'm not. They already drag their feet and blame me for being indecisive. Again, sorry, but par for the course as far as I'm concerned. I imagine they have to deal with bonehead homeowners 99% of the time.
What do I tell them?!
As for "how many zones", the reason I have no answer is, I don't know that I will even get to have /any zones at all/, when I seem to need the smallest boiler you can buy (seems to be the Loch - and it's hard to find anyone to sell me that one either).
Politicians and salemenI just spoke with my local "rater" this morning. He knows almost nothing about boilers and admits it.
Your contractor is interested in the sale.
The Governor? Really?
Like I said before, you will not regret a small condensing boiler with an indirect.
If you post info on the heat loss, radiators and zones, the math on short cycling is pretty easy.
An engineer is always an option. I recently consulted a LEED accredited engineer that automatically doubles the heat loss. I quote "it is a felony to undersize and a misdemeanor to oversize". I would ask a few questions before going that route.
I guess that leaves taking advise from folks on the internet.
No good choices....
Cold weather is a motivatorZman thanks for your support.
> The Governor? Really?
Both Shwarzenegger and Brown have made big deals about promoting the underlying statute, Title 24. I don't know who in the state decided that rebate forms for prescriptive upgrades would have checkboxes for tankless or condensing furnace, but none for mod-con boiler. I submitted a question about that local rebate authorities.
I could easily get $1000 for tankless plus caulking. In theory there are rebates for performance upgrades that go up to around $4000 and a boiler is eligible, but it's an incredible bureaucracy. And it seems to prefer mini-splits anyway bec. I'd get air conditioning credits that I don't need. Minisplit makes no sense here becuase electricity costs 4x gas here, COP is only 3, and pointless expense bec. people in my region don't need any cooling. Doubt neighbors would like the noise either.
> If you post info on the heat loss, radiators and zones, the math on short cycling is pretty easy.
Zones depend infinitely more on what an installer is willing to warranty than anything I would say on a forum. I don't believe there is any one correct answer. Seems to me one zone has lower installation cost and reliability advantage but zones maybe cheaper to operate, plus maybe easier to change things around as remodel progresses.
> An engineer is always an option. I recently consulted a LEED accredited engineer that automatically doubles the heat loss. I quote "it is a felony to undersize and a misdemeanor to oversize". I would ask a few questions before going that route.
FWIW I didn't hire a rater to get points. I had to hire a rater bec. in practice it's needed to permit additions larger than 100 square feet in Calif. (Technically homeowner can prepare the energy calcs but steep learning curve.)
I'm thinking maybe more contractors would bid seriously if engineering plans were on the table.
Almost nothing available that is truly suitedfor many of the small, efficient houses we are building these days. I stand by that statement in a large number of of cases, but the critical ratio is that of the design day heat loss (maximum heat demand) to the the minimum output capacity of the heat source. The higher that ratio, the longer the boiler or heat pump will run, the better the comfort level, and the more efficiently (in a modern appliance) it will heat and/or cool the space.
Most currently available mod/con boilers have a minimum firing rate of 16-17k for their smallest model. Here are some of the lowest minimum modulation rates [in square brackets) of which I am currently aware (followed by their maximum output capacity in parenthesis):
Viessmann 200-W B2HA 19 [11,580] (64,655)
Lochinvar WHN055 [10,450] (53,250)
Lochinvar Cadet CDN040 [8,545] (37,600)
Daikin Altherma [5,000] (28,760)
As far as domestic hot water, keep in mind that a typical gas tank heater puts out 24,000 - 28,000 BTU/hr and the typical electric only 15,400 BTU/hr. An indirect water heater fired by any of the above heat sources will offer faster recovery.
rules of thumb for bufferedThanks, SWEI, I imagine I do get that about having a high ratio of design load BTUs to low fire BTUs, as a major determining factor in reliability, comfort, and efficiency all around.
Now do you have a similar rule of thumb for add buffer vs. don't need to add?
Maybe it's something like...when there are many hours a year when expected load is less than 50% of turndown? If so I don't know how to estimate how many hours it will be just chilly enough to need heat, or how many hours of hiccuping an appliance can tolerate.