Welcome! Here are the website rules, as well as some tips for using this forum.

If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.

# Need help calculating tankless water heater size for Pex radiant heat

Member Posts: 4
Hi everyone,

I'm trying design and install a DIY pex radiant heating system in my 1989 ranch house. Here are the numbers:

-1500 Sqft
-Built in 1989, moderate insulation, have a lot of windows.
-joists are 16 inches apart, plan on doing pex in between joist installation.
-Floor is made up for 5/8 subfloor with 3/8 engineered hardwood floor.
-Live in Michigan, close to Detroit

I was planning on doing a closed system with a tankless water heater. I want to get a an idea of what size tankless heater i need and whether there are some recommendations in this department. I'm not finding a clear guide when it comes to this.

Thank you

• Member Posts: 1,189
You need to do a lot more reading. You will need to figure out how to do a heat loss on the space which will tell you how much capacity you need. You will then need to learn how to design the loops to get the heat you need from the heat loss out of them. Finally you will need to learn how to do the near boiler piping.

Why a tankless water heater instead of a modulating condensing boiler?
• Member Posts: 4
Can you point me to a guide where i can do those things?

i was thinking tankless water heater since it's pretty low cost. I do have central air, but i'm trying to get to a point where i no longer need it.
• Member Posts: 12,710
Well now. The heat loss calculation is really not at all hard. There are a number of on-line applications which will really walk you through it -- I happen to prefer Slant/Fin's, https://www.slantfin.com/slantfin-heat-loss-calculator/, but there are others. That's step 1 -- and you will want to note the heat loss, room by room, to be able to determine how much radiant heat each room needs.

Do not use a tankless water heater. Yes, they are inexpensive (often flat out cheap), but they are not designed for, intended for, warranted for etc. space or radiant heating applications. Use a mod/con boiler intended for the job and set it up properly (outdoor reset enabled, floor sensors, etc.). You'll be much happier.

As for design of the radiant heating floor system, while it's not quite rocket science, it's also assuredly not simple. There are guides out there for it, but take it slow and learn all you can before you start planning, never mind installing. It's very easy to put one in which works exceedingly poorly.
Br. Jamie, osb

Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
• Member Posts: 4
Thanks!

Is there a good article covering why you should chose a broiler over tankless water heater?
• Member Posts: 12,710
I don't know of a good article... but there's better than 20 years of experience here on the Wall, and we're all pretty much in agreement. Search for "tankless water heater"
Br. Jamie, osb

Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
• Member Posts: 2,540
techygear said:

Thanks!

Is there a good article covering why you should chose a broiler over tankless water heater?

Tankless water heaters have pretty restrictive water channels , these are designed for Street pressure moving water through them . Terrible street pressure can be as low as 30PSI , not a problem getting through the heat exchanger . Taking that into consideration , you'd need a hell of a circ to push heating fluid through there even at a lower 20 PSI , 1 PSI = 2.31 feet of head which equates to 46.2 feet of head just for the heat exchanger , not taking into account the added mechanical energy for the tubing and all other devices in the system . You'd need one hell of a circulator to move that much energy through the system . that's why you don't want a tankless water heater .
You didn't get what you didn't pay for and it will never be what you thought it would .
Langans Plumbing & Heating LLC
732-751-1560
Serving most of New Jersey, Eastern Pa .
Consultation, Design & Installation anywhere
Rich McGrath 732-581-3833
• Member Posts: 1,169
I have a tankless doing my heated driveway. Put a primary secondary on a 199 ,000 btu unit with a 1-1/4 manifold and good equipment ( pumps , air-separators ) and you will be good.
• Member Posts: 12,710
Whether you decide to go with a tankless or a proper mod/con -- and I will grant you that @Snowmelt has been using his tankless for quite some time, so they will work for the purpose -- the heat loss calculation is the very first thing to do. Then go look for something with a reasonably matching heat output.
Br. Jamie, osb

Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
• Member Posts: 1,189
A mod/con will also have the proper controls for heating and usually pump relays and such built in. The pricing can be similar.
• Member Posts: 5,737
Snowmelt said:

I have a tankless doing my heated driveway. Put a primary secondary on a 199 ,000 btu unit with a 1-1/4 manifold and good equipment ( pumps , air-separators ) and you will be good.

It should be noted that snowmelt is a fairly infrequent load. I would strongly recommend a modulating/condensing boiler for heating your house
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 5,566
edited October 2019
Let's reason for a moment: if a tankless could take the place of a boiler, there would be no need to make boilers.

You can't properly design from economics; the laws of physics will out-weigh the laws of economics every time.

As already pointed out, a tankless is not designed, controlled or approved for space heating.

Almost weekly this issue comes up here from someone who's been convinced this can be done and they're asking for help to try and fix a botched job. You're on the front end and you can avoid this scenario before it happens. Please heed the warnings of the pros who have commented. There's probably 200+ years of combined experience just from these few.
Bob Boan

You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 4
Rich_49 said:

techygear said:

Thanks!

Is there a good article covering why you should chose a broiler over tankless water heater?

Tankless water heaters have pretty restrictive water channels , these are designed for Street pressure moving water through them . Terrible street pressure can be as low as 30PSI , not a problem getting through the heat exchanger . Taking that into consideration , you'd need a hell of a circ to push heating fluid through there even at a lower 20 PSI , 1 PSI = 2.31 feet of head which equates to 46.2 feet of head just for the heat exchanger , not taking into account the added mechanical energy for the tubing and all other devices in the system . You'd need one hell of a circulator to move that much energy through the system . that's why you don't want a tankless water heater .
I'm planning on making a closed loop system, can't most of this be solved with a decent pump or maybe even multiple pumps?

Again, just trying to learn
• Member Posts: 462
Design engineers are experts and they design boilers as well as DHW's you want to cobble a system out of parts that can be shoe horned into a service they were not designed for. The experts here talked me out of using a tankless DHW for space heating and I have thanked them many times over.
There is a massive body of knowledge here available for you and I to use, am humbled by it.
Respectfully suggest you spend spare time listening to this site.