Will seek advice in near future, put PEX in addition slab, learning curve...

sobitthen

Was talked into PEX while pouring 16x16 slab.
Ran 2 150 foot runs splitting room in 2 as 1 side is bedroom the other an office.
Did not calculate anything yet, addition is on a 100 year old 16x26 cabin, both wrapped in styrofoam and well insulated overhead.. lots of trees, Pines to south, hardwoods rest, so not a lot of sunlight in peak times, early morning, late afternoon in heating season.
With the cold coming I got into panic mode, bought pile of components in 1/2", backflow, pressure relief, pump, 2 one manifold, that has 1", and a H3N Takagi LP heater...
I had planned on using PEX with aluminum transfer under old floor in cabin, have the PEX, then other projects coming fast and tossing a pair of 1000w electric baseboards in there for this season.
My mind used to work well, Lyme, co-infections, Lone Star, and few other NW WI nasties sort of took away my short term and ability to read, some days better than others, will try and read and learn, working on tight budget, hoping I bought the correct components from trying to research this the past 3 months...
Was looking up more information and stumbled upon this site, hopefully I can straighten this out before I make some big mistakes..
Thanks all.. I will do my best to try and comprehend the advice...

Ironman

You may be a little light on tubing and have to run a little hotter SWT.

The first step that should have been done was a Manual J heat loss calc to determine how much heat you need. That's the foundation for designing everything.

You can download a free app from SlantFin.com and do one yourself.

What type of heat source are you considering?

kcopp

He said he bought a Tankless water heater to heat the room... wrong choice in my opinion. (LP Takagi)
Perhaps you should get some help from someone qualified who does this type of install before you spend more money on equipment that will not work well.

Ironman

kcopp said:

He said he bought a Tankless water heater to heat the room... wrong choice in my opinion. (LP Takagi)
Perhaps you should get some help from someone qualified who does this type of install before you spend more money on equipment that will not work well.

I missed that one, but you're absolutely correct. He should send it back and get the right tool for the job: a mod/con boiler.

hot_rod

So total building size of 676 sq. ft.? Rule of thumb 25 btu/ sq ft = 16, 900 BTU/ hr load at design (coldest day)

You bought a 199,000 BTU/ hr water heater so first you have a big size mismatch. Also tankless water heaters are designed for low flow rates at high temperature difference, say for example heating 4 rpm of 55° water to 120°. This is kinda the opposite of what a radiant boiler is designed to do, high flow at low temperature difference.

If you can trade it for a small mod con boiler, maybe a 50,000 that could turn down to 8- 10,000 btu/hr.

It would take some work and $$ to force that tankless heater to work for your application, a large buffer tank, probably a high head circulator pump.

If you want hot water and radiant heat, consider a 110 or 120 Combi boiler, it will have a 10 to 11:1 turndown and better match your heatload. Again, that is a rough estimate on your heat load.

jsmith6752

do not waste your money on the shinny stuff under the pex. Even the goverment does not suggest it. Just use foam insulation,

Ironman

jsmith6752 said:

do not waste your money on the shinny stuff under the pex. Even the goverment does not suggest it. Just use foam insulation,

Are you referring to buble wrap or heat transfer plates?