# Boiler vs. On-demand water heater



## Corys812

I have a question for all you excellent and knowledgable plumbers and heating gurus out there. Forgive me if this thread exists, I did a search and it returned no such topic.


What is the difference between an On-Demand water heater (a mod/con, ie. Noritz) and a wall hung mod/con boiler?


People keep asking me about the difference. I just have to shrug my shoulders. Sometimes all I got is "you wouldn't use a dishwasher to clean your cloths would you?"

I know on a lot of the units it voids the warranty of the heat exchanger but I see no other problems. 

I know water heaters have smaller passageways in the heat exchanger but that seems to be overcome able with a higher head pump. 

What do you guys think? This question has been bothering me for a while and I just now thought to pose it to you lot


----------



## Scott K

Here is a somewhat relevant thread off of heating help: http://www.heatinghelp.com/forum-thread/134159/Boiler-Swap


----------



## ZL700

Easiest way to say you can't in most cases is to reference part of a letter from Rinnai

Rinnai Tankless Water Heaters are certified to the American National Standard/CSA Standard for Gas Water Heaters ANSI Z21.10.3/CSA 4.3. The Scope of this standard includes water heaters for use with combination potable water/space heating applications, commonly referred to as open loop systems. The Scope of this standard does not apply to products that will be used in space heating only applications, commonly referred to as closed loop systems. Products intended for use in closed loop systems are typically certified to the American National Standard/CSA Standard for Gas-Fired Low Pressure Steam and Hot Water Boilers ANSI Z21.13/CSA 4.9. The Scope of this standard applies to products that will be used in space heating only (closed loop) systems.
For this reason, Rinnai America Corporation does not recommend or promote the use of its Tankless Water Heaters in any space heating only (closed loop) applications (see Figure 1). Rinnai will not honor warranties for units installed after May 1, 2008 in closed loop applications. Rinnai Tankless Water Heaters may be used in combination potable water/space heating (open loop) applications (see Figure 2). Rinnai Tankless Water Heaters may also be used in combination potable water/space heating (open loop) applications where the space heating system is a closed heating circuit that is separated from the Rinnai heated potable water supply via a heat exchanger (see Figure 3).


----------



## Greenguy

Not totally correct about the Rinnai's if you look at the RC98i they have design specs for space heating as well as for their fan coil heating applications. Keep in mind a wall mounted hot water system has its out put based on GPM a wall mount boiler has its output based on water temp. 

The reason this is important if you use a Rinnai for heating you need atleast 6GPM to hit high fire and max water temp. Other wise of you are trying to heat an indirect tank to 120 with 120 water you may find you have issues. Also using a Rinnai for any type of heating application cuts the warranty down to 3 years from 12 for DHW supply. 

I had a rush job a ways back had to use a Rinnai for 2 zones infloor radiant, 2 zones baseboards up stairs and a 75 gal indirect tank. Was not a job I enjoyed doing. A wall mount on demand domestic hot water system is not a boiler... Scott K said something similar in another thread and it's true.


----------



## ZL700

You missed the point for the OP, and didnt fully comprehend what I posted. 

Also you are incorrect, the RC98 units show only combi use, no closed system heating use only, so the job you did is not condoned by Rinnai, so you have no warranty.

It can't be used as heating only, and to skirt the boiler safety laws, CSA & ANZI, it must be in a combi app only, with a DHW fixture draw if used for heating.

But rinnai and others recommend AH unit apps only, radiant or other types can't evacuate the stagnant water enough in the larger mass heating systems, thus there are safety issues. 

In the end tankless aren't tested to CSA 4.9 safety standards as a boiler, so they will never have heating application ASME so why use them?

Read the whole letter here as proof to understand:

http://www.blueridgecompany.com/documents/Combination_Systems.pdf

Good luck, tankless are not boilers as you stated yourself


----------



## Greenguy

The system I installed had two flat plate heat exchangers 1 to separate the Rinnai and the baseboard heating the loops the second was to separate the in floor zones, which allowed me to have two separate temps without the need for any mixing valve. I never said it was a closed loop, let me explain.

http://www.rinnai.us/media/Hot_Water_Design_Manual_Rev_D.pdf

If you check the pdf file below page 49, in my system the heating loops are separately fed and each has its own sprirovent and expansion tank, like I said before the customer wanted this unit, I would have preferred a prestige solo or an IBC. To make sure it wasn't a true closed loop system the supply for the shower is taken directly off the primary loop. the whole system was designed that it could be flushed with ease. There was a lot of extra junk put in to make it work with the Rinnai. if you look in the manual for the RC98i the only way to get the high water temps needed is to use the Commercial Control MCC-91, in doing so drops the warranty down to 3 years. 

As the warranty that’s for the Sales department to deal with, we all know what it’s like when the customer is stuck on an inferior product we either install it or walk from the job. Btw the plumbing/gas inspector approved the install.


----------

