on-demand wh vs wall-hung boiler pricing

Royboy

wondering if anyone can help me understand why wall-hung mod-con boilers seem to be consistently & significantly more expensive than on-demand water heaters of comparable efficiency and output. it seems to me like the on-demand units would require more rapid & precise modulating control which I would expect would make them more expensive. I know some boilers have built in pumps which adds cost, but that doesn't account for the price differences I see. and I get that they are designed for different purposes, just trying to get a handle on why boilers seem to be so much more pricey for relatively similar devices ...

Henry

Modcon are much more expensive ,mainly due to the type heat exchanger used and burner. Just look at the weight difference with a Asian on demand and a North American Modcon.

Royboy

thanks for the response, Henry. so ... can you or anyone else elaborate on what makes boiler HX and burners need to be heavier/pricier than on-demands? I can understand why an on-demand HX would want to be low mass in order to get the heat into water with as little in the way as possible, but why can't you build boilers that way if it results in a less expensive product?

kcopp

Boiler need an "H" stamp

Section IV — Heating Boilers
Provides requirements for design, fabrication, installation and inspection of steam heating, hot water heating, hot water supply boilers, and potable water heaters intended for low pressure service that are directly fired by oil, gas, electricity, coal or other solid or liquid fuels. Rules pertaining to use of the single ASME certification mark with the H, HV, and HLW designators are also included.

Royboy

and there's no equivalent certification for on-demands?

SWEI

Look at Navien's first couple generations of boiler, which were basically modified tankless water heaters. They had a lot of issues. Time will tell for the current generation...

Jamie Hall

It's simple. Really simple. Consider the duty cycle of the two. The on-demand water heater? Perhaps an hour a day, max. The wall hung furnace? If properly sized, they can run up to 24/7. The furnace had better be heavier built, no?

Jamie Hall

Oh dear. I meant boiler. Sorry!

Harvey Ramer

In my opinion they are overpriced and underperform. Not a lot of stuff there when it comes down to it.

Royboy

so Jamie, when I look at the duty cycle, I figure the short on-off cycles & the (to my mind) more demanding job of 70-80° ΔT with precise output at varying flow rates would be a harder thing to build for. but then again I know relatively little about what's in these things, hence my confusion & question.

and the question, really, comes from clients asking me to provide cost effective heat sources for low-temp radiant slab systems, which don't need more than water heater temps. they often ask about doing it "with a water heater." I can and have installed conventional water heaters to run slabs but I'd rather burn fossil fuels more efficiently than a "conventional" water heater does. I explain that there are other appliances out there that get a lot more out of the fuel via better heat exchangers and condensing the combustion gasses. but I get hung up in explaining why they need to pay for a boiler that costs roughly twice what a on-demand unit when both are advertised as providing comparable efficiency. that's the question and I'd like to have a more comprehensive understanding for myself so I can offer a plausible explanation to clients who pose the question to me.

and I truly appreciate that this is a place I can put this question out and get such high-level informed responses from industry experts!

Royboy

@ Harvey: genuinely not sure if you're being straight or sarcastic

Harvey Ramer

I'm being straight. But Modcon boilers are the right tool for the job out of our current selection. It is what it is.

Don't even go down the road of using on demand water heaters, of any flavor, for space heating. They aren't designed for it and it's been proved countless times over, they don't work well or at all, in that application. You are by no means the first and won't be the last to be tempted by the lower price.

If it worked well, we'd all be doing it!

kcopp

The real life analogy would be if you need to go pick up a load of 2x4's at the lumber yard.... Yes you can strap a whole load onto a car but you really need a truck to do the job...

Royboy

@ Harvey - well, I have done it, used on-demands for space heating. 3 that I can think of at the moment. all for low-temp radiant slab delivery. 2 Takagi Jr's that have run combi systems (DHW via flat plate HXs & storage tanks) for more than 10 years with 1-2 cleanings of their heat exchangers over that time. both seem to be doing the job and a couple HX cleanings seem no more than what one would do to maintain a boiler. 3rd system a condensing unit from big box bought by client (contrary to my reservations) running fine with no service for 5 years. have I just been lucky? (I'm not being flip ... I admit I don't understand the intricacies of this hardware - hence the question)

Royboy

@ Kevin ... yes, and I am really trying to learn about what makes on-demands not well suited to putting their output into space heating. I get that its potentially complex but some specifics about why they are not good for this task would leave me feeling a bit more appropriately informed.

Jamie Hall

@kcopp 's comment sort of sums it up: you are looking at a heavy duty load. Some things aren't that different, but what is different is the construction of the heat exchanger itself. The material it is made out of and how it is put together. Better, more corrosion resistant alloys. Heavier gauge. Better assembly quality control. The controls are very very inexpensive for both gadgets. One might think that the apparently fancier controls for the on-demand water heater would be more expensive, and they are -- by a matter of a few dollars only. In fact, the actual hardware is probably a few cents different -- no more.

No one would ever say that a water heater won't work for a low temperature heating application, like radiant. It will. The question is, for how long? A well-built boiler, designed for the job, should last a lot longer than the water heater. Again, the analogy of the pickup truck is apt -- but I will compare two trucks: a half ton mini and a three quarter ton heavy duty. Sure, I can put a half ton of dirt into the mini and get away with it. I may even be able to do it for 50 or 60,000 miles. My three quarter ton heavy duty? Probably good for 200,000 plus (it's only got 150,000 on it now, so I don't know yet!). The mini would have been cheaper -- but not that much cheaper.

Tinman

The singular reason that prevents me from going that route is the potential liability of using an appliance that does not meet the standard for that application. Like others have said - no H stamp. In the unlikely event that something goes horribly wrong, I wouldn't want to be on the wrong side of the inevitable questions that would be asked. It's hard enough staying on the profitable side of things without someone trying to take everything I own.

Royboy

@ Jamie - thanks. from what you wrote I'd conclude that boilers are better built and would last longer as a result. and on-demands can be used, they just aren't going to last as long in general (or maybe in space heating in particular). then it seems more like a life-cycle cost comparison.

and I've been driving Tacomas for the past 20+ years, typically to 250,000 miles or so

Royboy

@Stephen - and thank you for that clarity! I live on the edge in some regards, and have been quite fortunate to date in terms of bad outcomes. gets a little more tricky when you're doing it professionally. and I have no desire to give either myself or hydronic heating a bad name - but I do try and save $$ for my clients when I can.

I'm not posing this question re:systems that need 180° water. just trying to better understand why an appliance that's designed to output hot water at the 120° temp & volume I'm looking for and can handle relatively high input temps seems to get such bad reviews for the application. I get that you have to pump harder through most of them than you do through boilers, but that's more of a downside than a deal breaker to me.

hot_rod

Royboy said:

@ Steven - and thank you for that clarity! I live on the edge in some regards, and have been quite fortunate to date in terms of bad outcomes. gets a little more tricky when you're doing it professionally. and I have no desire to give either myself or hydronic heating a bad name - but I do try and save $$ for my clients when I can.

I'm not posing this question re:systems that need 180° water. just trying to better understand why an appliance that's designed to output hot water at the 120° temp & volume I'm looking for and can handle relatively high input temps seems to get such bad reviews for the application. I get that you have to pump harder through most of them than you do through boilers, but that's more of a downside than a deal breaker to me.

Tankless are designed for low flow, high ∆T operation, boilers are designed for low ∆, high flow. So the heat exchangers are engineered for opposite conditions.

Trying to move higher GPM in tankless heaters often involves high head "mining pump" circs. Even then you may not see full output.

SWEI

There are a few that do have H stamps. We have employed them successfully as a backup heat source on a couple of solar space heating projects. They both shared some important characteristics: high ΔT, very low duty cycle (<100 hours per year typically), and no requirement for dynamic control of supply temp setpoint.

Royboy

@HR: was hoping you'd weigh in. so if heating design is workable with lower flow rates and you employ a pump with sufficient head to get your flow rate through the more restrictive HX ... would that address your primary concerns?

@SWEI: would you mind sharing what ones are H stamped if you recall? I know Takagi is OK with use of at least some of their units for space heat and used to adjust warranty (down) when for space heat. last I knew Rinnai also approves of space heating. but I've never been tuned into H stamps on equipment.

hot_rod

Royboy said:

@ HR: was hoping you'd weigh in. so if heating design is workable with lower flow rates and you employ a pump with sufficient head to get your flow rate through the more restrictive HX ... would that address your primary concerns?

Nice to hear from you, will you be at MREA this year?

Really it comes down to what you and the customer are comfortable with and that it doesn't open you to a liability issue, regarding certification of the equipment. That may not be an issue in your neck of the woods

I have installed a handful of small electric tank water heaters and tankless electrics over the years for micro load radiant, bathroom floors and walls, kitchen floors, etc. I felt they were safe, efficient and affordable, and a better option than electric cable embedded in the tile set in some opinions.

Just got back from Winnipeg and Saskatoon, 50% of the radiant boiler sold up there are electric, with crazy low KWH rates, soo... if the shoe fits.

@SWEI: would you mind sharing what ones are H stamped if you recall? I know Takagi is OK with use of at least some of their units for space heat and used to adjust warranty (down) when for space heat. last I knew Rinnai also approves of space heating. but I've never been tuned into H stamps on equipment.

Royboy

hot rod said:

Nice to hear from you, will you be at MREA this year?

Really it comes down to what you and the customer are comfortable with and that it doesn't open you to a liability issue, regarding certification of the equipment. That may not be an issue in your neck of the woods

not sure about MREA yet, but will check in for sure if I do.

I know I'm at a different place on the comfort/liability scale than most professionals here and have been fortunate in that realm so far. and the local norms here are somewhat in synch, but I'm not wanting to use that as an excuse for poor design or work or equipment choices.

so this thread is an attempt to increase my comfort level with using on-demands for radiant slabs when they seem desirable. plus I'm open to learning they should not be used and in that case I'm just looking for some detail on why, for my own understanding and to better respond to clients who suggest this option.

SWEI

Takagi had an H Stamp on their noncondensing units about 5-10 years back. I haven't followed up to see what's current.

The secret to making them happy is to give them conditions that mimic what they see in DHW service: 40+ °F rise at flow rates that work for the HX and a low duty cycle. This can work well with a buffer tank when we dispense the heat slowly using an ODR-controlled mixing valve. As I mentioned above, it makes a nice backup for solar space heat, though you are dumping heat into the tank rather than adding it only as needed to the space.

Some have seen success using the thermal mass of a slab to create similar conditions, but I've never been comfortable with my ability to control that.

Royboy

thanks for responses to date. I'm now a little more clear about what are seen as the primary issues with using on-demands in lieu of boilers for low-temp heating loads. and I'd certainly welcome more perspectives on the topic ...