I'm getting quotes for a replacement of the Boiler / DHW system as well as the central air in my home in southern / central Massachusetts (02093). house was built in 1989, 2 x 4 construction, 2400 sq ft colonial, insulated most likely only to code. Heat is boiler with baseboard, central air looks to have been a later addition, 1992 is the date on the air handler. Using my February bill with Dana's calculation I came up with 39501 for my heat load. After talking with the first HVAC guy about how grossly oversized the old boiler is he pitched me on a Viessmann vitodens 222 combo unit and a Navien NCB 240 E (with about 4000$ difference between the two)... which I think may actually be more oversized than what is being replaced..... When I asked he told me that oversizing isn't an issue because they both modulate very low 35Kish for the vitodens and 19k for the navien. He recommended the largest sizes because they will better handle my hot water needs. We are a family of four, me, wife, two girls 3 and 5. existing equip is a 1987 burnham NG P205 WNV 130000btu boiler and 40 gal water heater. I know I don't need the extra sizing as the heat goes but is he right about needing the larger unit to handle the DHW load? Thanks Mat Cartier

The problem with low mass combi boilers is that they're well suited only to houses with large heat loads and only moderate domestic hot water loads, which is the opposite of just about any average sized house built since insulation was required by code. What you have is a moderate heat load and moderate hot water needs, which makes it hard to fit.

To run in condensing mode without cycling the minimum modulated output of the boiler has to be no more than about 200 BTU/hr per running foot of baseboard, zone by zone. So for a 35,000 BTU/hr minimum output you would need at least 35,000 /200= 175 feet of baseboard PER ZONE.

It's true that without the thermal buffering of tank a bigger burner is needed to deliver sufficient hot water performance. The max firing rate of the NCB 240E is 199K, but it' min-fire OUTPUT is about 17,000 BTU/hr, and would need 85' per zone or greater, which you might have(?). Most 2 bath homes in MA can do OK with a 180K tankless or combi, but a 150K tankless or smaller kinda depends. The NCB-180E can dial back to about 13,000 BTU/hr out, which would only need 65' per zone to operate in condensing mode, yet can still support two simultaneous 2gpm showers with just a bit to spare.

If the original designer installed enough baseboard to not cycle the P205 into low efficiency you should have something like 200' of baseboard, preferably operated as a single zone (?), so you might have gotten lucky by the gross oversizing of the original boiler, unless it's also oversized for the amount of baseboard installed too (it happens... :-( )

If your radiation is way undersized for even the NCB-180E, a cheap modulating condensing boiler and an indirect water heater makes sense. A popular easy-to-retrofit mod-con boiler that's cheap and available is the HTP-080W stainless fire tube boiler. Unlike the Navien combi units it doesn't usually have to be plumbed primary/secondary, and it comes with a pre-plumbed dedicated port for supporting an indirect tank water heater. (See: http://www.htproducts.com/UFT-Boiler.html ) This boiler is even cheaper than small cast iron boilers of similar max output, and just as easy or easier to install. The min-fire output is 7600 BTU/hr which means it can support a 35-40' baseboard zone at condensing temperatures without cycling. Even if the NCB-180E works with your radiation, the UFT-080W might be the better solution overall, since it's peak domestic hot water flow is not burner-limited. The boiler puts out about twice the heat of a typical 50 gallon tank water heater's burner, and would thus have half the recovery time if set up correctly.

For a deeper dive into the napkin math to use on vetting any proposed condensing combis or boilers see:

http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/sizing-modulating-condensing-boiler

Got the tape out and measured 65' exactly on the second floor, 34' in the front half first floor and 30' plus 2 kick space heaters on the back half first floor.... I could have the plumber set the first floor to just one zone (not sure why its two anyhow). Im beginning to think that the combi move is a bad one... It takes forever for hot water to show up to my low flow shower heads as it is. But i still need enough for my kids to take baths (4GPM?!?!?!). Ive already determined that I'm going to need some point of use electric water heaters to handle sink use. Would the NCB-150E be too small for my DHW demand? It seems more properly sized for the house. Do you have a recommendation for an installer that deals with HTP products? Mass does not want me touching anything in my house... probably for the best. Just saw that the Lochinvar NCK110 mods down to 11... i think its the best I've seen for a combi unit..

Yes, ANY low-mass combi is going to be a bad fit for your loads.

The NCB-150 is too small for your hot water needs, and is still going to short-cycle on a 34' baseboard zone. The Lochinvar NHK 110 won't cut it for domestic hot water either, with less output than the NCB-150- it'll barely cover a 2.5 gpm low-flow shower at your wintertime incoming water temps. Any low-mass combi that can robustly support a 2- bath house will usually risk short-cycling on a 65' zone.

Call HTP for recommendations on local installers: (508) 763-8071 They are in a position to know which installers are doing dozens per year without a problem and which installers are tying up their tech support lines with questions that are clearly spelled out in the documentation.

In the Lochinvar lineup , the KH-55 or WH-056 and an indirect water heater would work.

http://www.lochinvar.com/_linefiles/KHN-PS-01.pdf

http://www.lochinvar.com/_linefiles/WHN-PS-03.pdf

They will drop back to ~7800 BTU/hr out at low fire, yet can still deliver more than 52K at high fire, which is more than a 1.2x oversize factor for your fuel-use calculated load, which is enough to cover the cold snaps when it's below your 99% outside design temp.

So with calls in to 12 different plumbers / HVAC guys (actual speaking calls... not just the ones that I left messages to that never called back...) and all of them telling me that my numbers don't actually show the size boiler that I need... I downloaded the Slant Fin heat load calc and did some more months of winter heating fuel use using Dana's method... All months came between 39000 and 44000, Slant fin gave me 43000. Slant fin also showed me that I have between 1.8 and 2.5 times more baseboard (just the finned sections) than what it recommends per room. For reasons that my numbers are wrong.... I'm having them tell me that they use home size using just the square footage and nothing else... I've gotten a couple that size it to the amount of baseboard, some that "let their supply house" do the heat load calc... using magic apparently... and some that "have done tons of these houses" and know that "its around 80000 btus..." I believe my numbers are correct and even if they aren't exact I'm pretty sure they're wrong toward being oversized but Im getting price quotes using the UFT-100. I'd prefer the 80 but aside from what seems to be a small difference in equipment cost will I be losing much by giving in to the UFT-100? (which I would absolutely play around with the high flame setback...) Is it worth standing my ground while all this equipment in my basement is still in working order? 82% (optimal) working order?

Posted By Matcartier on 24 May 2018 02:29 PM
So with calls in to 12 different plumbers / HVAC guys (actual speaking calls... not just the ones that I left messages to that never called back...) and all of them telling me that my numbers don't actually show the size boiler that I need... I downloaded the Slant Fin heat load calc and did some more months of winter heating fuel use using Dana's method... All months came between 39000 and 44000, Slant fin gave me 43000. Slant fin also showed me that I have between 1.8 and 2.5 times more baseboard (just the finned sections) than what it recommends per room. For reasons that my numbers are wrong.... I'm having them tell me that they use home size using just the square footage and nothing else... I've gotten a couple that size it to the amount of baseboard, some that "let their supply house" do the heat load calc... using magic apparently... and some that "have done tons of these houses" and know that "its around 80000 btus..." I believe my numbers are correct and even if they aren't exact I'm pretty sure they're wrong toward being oversized but Im getting price quotes using the UFT-100. I'd prefer the 80 but aside from what seems to be a small difference in equipment cost will I be losing much by giving in to the UFT-100? (which I would absolutely play around with the high flame setback...) Is it worth standing my ground while all this equipment in my basement is still in working order? 82% (optimal) working order?

Slantfin's baseboard recommendations are set to minimize the total amount of baseboard needed to heat the place using high water temperatures.

For a condensing boiler to be operated at condensing efficiency takes about twice that much baseboard, so you actually NEED 1.8-2.5x as much baseboard than the high-temp system recommendations to be able take much advantage of the condensing efficiency. At an average water temp (AWT) of 170F (180F out, 160F return) baseboard delivers about 500 BTU/hr per foot, and that's probably the rough load/foot ratio in the Slantfin recommendations. At and AWT of 130F (135F out, 125F return- the beginning of condensing, ~90% combustion efficiency) baseboard puts out about 250 BTU/hr per foot. To hit 95% efficiency it has to be an even smaller ratio.

So, sizing the boiler to the radiation by the high-temp output of the baseboard is really for non-modulating boilers, and even then it's a MAXIMUM boiler size, not an optimum or even appropriate size. It only means that installing more boiler than the radiation can emit even at high temperature doesn't heat your house any faster or better than a boiler sized exactly to the radiation, so there is no point to installing a bigger boiler than that- it's a maximum.

But when you have 2x as much radiation than the house needs to be heated comfortably there is no point to installing the maximum amount of boiler that the radiation could possibly use, even if it's a cast-iron boiler.

With modulating condensing boilers you need to look at the MINIMUM fire output, and be sure that you have enough radiation to emit that much at condensing water temperatures. Oherwise you won't be able to get condensing efficiency without abusing the boiler with excessive cycling. For baseboard to hit a true mid-90s AFUE out of the boiler the min-fire output should be no more than about 200 BTY/hr per foot of baseboard. At that ratio it balance at an AWT of about 120F or a bit less with no-cycling. Ideally every zone should be able to emit at least 75% of the minimum fire output at an AWT of 120F. Below that it edges into short-cycling territory.

The min-fire output of the UFT-100W is about 9500 BTU/hr in condensing mode. That means ideally every zone would have at least 9500/200= 48 feet of baseboard. With that much baseboard it can deliver ~125F water out of the boiler at combustion efficiencies in the mid-90s and will not cycle on/off AT ALL during calls for heat, even if it's the only zone calling for heat. If it has only 36' it will cycle some, but it won't short cycle. Any shorter than that becomes a potential problem.

The min-fire output of the UFT-080W is 7600 BTU/hr. So it could balance perfectly with zones as short as 7600/200= 38', and would still tolerate zones as short as 28'.

So zone by zone, break it all down. If you have any zones shorter than 48' it will put a lot less wear & tear on the UFT-80W than it would on the UFT-100W.

For reference, here is the chart showing the output per foot of a popular Slantfin baseboard at different AWTs & flow rates:

https://www.slantfin.com/images/stories/Technical-Literature/ratings_fineline30_r.pdf

If you're having problems getting line breaks to show up, try using a different web browser. Both Firefox and Chrome work reasonably well.

The kinds of feedback you are getting from installers is pretty typical. Some old dogs never got past the 5th grade arithmetic, they can do BTUs per square foot, and BTUs per running foot of baseboard, but not the real math behind hydronic heating design. That is why I prefer to present it in 5th grade arithmetic on forums & blogs, since most peoples' eyes would glaze over immediately if showing anything more complicated that that.

A napkin-math rules of thumb like "minimum fire output no more than 200 BTU per foot of baseboard" is something that actually works without requiring anybody to go to hydronic design school or study up on control algorithms, doesn't require running the calculus on thermal mass & minimum burn times- it's just a solid stake in the ground for a min fire output that works-always. It's fairly idiot-proof (even for creative idiots. :-) )

Getting the average "Plummin' an' Heatin'" pro to even run a fuel use load calculation seems like a big ask, which is why you need to do it yourself. That puts another solid stake in the ground to know what the max-fire output of the boiler needs to meet or exceed. You clearly don't need 2x more boiler output than than the amount of fuel it took to actually heat the place, no matter what sort of BTUs per square foot or BTUs per foot of baseboard math the installers want to run.

I say stand your ground. With both a load tool and fuel use calculations indicating loads of less than 45,000 BTU/hr, there is no point to installing a boiler (like the UFT-100W) that delivers 90,000 BTU/hr more than twice the actual heat load.

After three months of messing around with plumbers and hvac guys I've finally found one that has no problem installing the UFT-80. The surprise was that they are a larger HVAC firm and the designer didn't even try to persuade me to go bigger. The price was reasonable and I have faith in the company to install it correctly. Thanks for your help


Mat Cartier

They're out there- YAY!

Care to share which company is doing the work? (If you'd rather not post it on the forum, use the "Private Messaging" function on this site. I rarely look at the PM- stuff- just looked and saw somebody messaged me with a question over a month ago... :-( )

Amazingly it was Gem Plumbing. They came through with a comparable price for the a/c as well. I don't have it in me to battle them over sizing for that though... I've been reading Allison Bailes and it seems like a legitimate Manual J is the only way to get that one right. And between measuring the duct work and getting someone out for a blower door test... Im spent.