I built a pole barn for a garage/shop last fall & now it's time for heating.
I have 1/2" PEX in the slab & need a boiler to heat it. I'd also like to have hot water for 2 sinks -bathroom & kitchenette + I put in a H/C mixing hydrant to use for whatever. Not taking shower, washing clothes, etc.

It's not insulated yet, but I put 1.5" foam between the girts, so it has somewhere ~R6 in the walls, plan to blow in R13 in the walls and R13+ in the ceiling (2x6 ceiling joists, so it will be buried, but not much more)

From my research, rule of thumb is 50k BTU/hr for 1200sq ft, my barn is 30x40, so that's nice & easy - and probably wrong
A/C sizing suggests 24k BTU/hr
Found a program that let me put in the walls/windows/doors & calculated heat loss (radiantworks pro)
For my location, it used a design temp of 4*F & calls for 23,700 BTU/hr of heat load.
At 45*F, I only need 8400 BTU/hr

My plan was to go with a Combi unit that produces hot water.
The smaller ones (55-80kBTU/hr) are more expensive than larger (~100k BTU/hr)

My main question is, how big is too big?
The Noritz CB199 is 109k BTU/hr & would simplify the install a bit over the smaller Rinnai & Weil-McLain units (plus save $500+)

Not sure if this link will work, but here are the comparisons
ecomfort.com list
Gut says to go with the Weil-McLain AB80-C, but the Noritz simplifies install a lot & saves $$ - My cheap-ass can't get over that.

A separate WH plus a boiler might be an option. The boiler only has to be big enough to supply the amount of heat your linear feet of PEX can radiate. Hopefully you have rather short PEX loops.

quote:

Originally posted by Woodman:
A separate WH plus a boiler might be an option. The boiler only has to be big enough to supply the amount of heat your linear feet of PEX can radiate. Hopefully you have rather short PEX loops.

4 loops at 275ft, should be pretty easy to balance.
The combi boilers I'm looking at all can modulate output down, some moreso than others. I can't really find a definitive definition of 'short cycling' other than extreme examples. 3 minute runtimes are bad, but is 15 ok? 10?
The AB-80c can cut down to 10% output - so even at 45* outside, it could run almost non-stop, but I can't find any info on if that's an efficient way to run it. The CB199 can only modulate down to ~17k, but if 10 minutes isn't considered 'short cycling', then it could be fine.

Beware of the boiler capacity at the max rated temperature for the PEX tubing.

You may want to consider a boiler specific for radiant floor heat.

Ten minutes is fine. Your cycle will probably be far longer. 8,000 BTUs is barely more than a single residential gas range burner on high. Aren't radiant floor applications low temperature? 120˚ or so?

I'd have to read back up on it, but I think you want your return water temp to be within 20˚ of boiler set temp five minutes of cold startup. High efficiency boilers should have a bypass built in to temper return water temp. You'd want to read up on adjusting the water tempering loop if one exists. We install them hard-piped between the supply and return in some applications to reduce condensation upon the heat exchanger.

Your loops are 275/4 or 275 each? Concrete floor?

This message has been edited. Last edited by: Woodman,

4 PEX loops of 275ft each in concrete.
The design calls for 95.5*F supply water, return of 75.5*F in the secondary (floor) loop. Mixing valve will add return water to keep the slab temp down. Boiler supply primary loop will be ~140*, depending on load and hot water usage (boiler has an internal circulator to push through primary loop when heating domestic water w/o the floor calling for heat.)

If 10 minutes of boiler run time is fine, then I should be golden with the 80k boiler (which i thought i was) and almost comfortable with the 109k - but I think I'll stay a little smaller.
Thanks for the sanity check - I wanted someone with experience to confirm what I thought I was seeing.
I'm going to get a quote on somebody doing it & talk to a friend-of-a-friend to make sure I'm not screw up somewhere else, but I think I've got most of the plan covered.

On typical mounted fin radiator systems, I was taught not to go longer than 90' because the water will cool too much. A quick online query reveals 300' for ½" PEX is 'max'.

Going with 25BTU per foot as your expected output, your system can radiate a maximum of 27,500 BTUs. No sense allocating more boiler output for heating as the pipe will not radiate much more than 33,000 BTUs.

https://www.pexuniverse.com/pe...g-sizes-floor-output

You do not want cold return water hitting the boiler longer than a few minutes during cold startup. There is probably a bypass to temper the return water to prevent condensation upon the heat exchanger. This piping (it may be internal on your boiler) adds supply water to the return water to raise return water temperature, minimizing condensation upon the heat exchanger during cold start.

Getting ready to spend a couple hundred$$ at PexUniverse & already got the manifold from them.
I hadn't seen that chart, but your numbers are very close to what the design software said for the system radiation capability (I want to say it was like 27300)

thanks for the tip on cold startup. I haven't read the startup procedures in the manual(s) yet - still in the design phase (I've read the installation to the point of wiring/settings on 3 or 4 different brands).
I would think that the primary loop through the closely spaced tees would be the 'bypass' you talk about (without the main circulator running)- but I bet it's in the manual too.

quote:

Weil-McLain AB80-C

Wow, that is a really nice boiler! The condensing issue on cold startup is not in the manual. It's probably more of a chimney-vent boiler issue; in my area, big old inefficient houses with big gravity-boiler headers in the basement have the issue and manufacturers require extra piping to honor warranties. But condensation would likely not be an issue in ... a high efficiency condensing boiler.

Thanks Woodman.
1 more item I'm struggling with. I can do the math, but something doesn't make sense.

Pump selection.
This should be a fairly easy thing, but I"m getting different info from 'design software' than I expect.
napkin-calcluation:
275ft of PEX per loop should be about 8.25ft of head.
Say 1ft for the manifold, round up to 10ft for the 1" copper Supply/Return.

One of the design programs says 4.6ft head
The other says 13ft of head.
Not exactly consistent for me to figure out what is up. Hoping for some experienced insight.

We use the single speed Taco 007 for nearly every application and it always works; sometimes the heating guy at the supply house up-sells us on a 3-speed because of an old oversized gravity header now being used in a forced-water system. A Taco three speed circulator pump should work fine for your application, if you want a little extra control.

The rugged 0015 3-Speed circulator is specifically designed to provide dial-in precision to meet the requirements of any hydronic heating system, including radiant and higher head systems. In radiant applications, its’ 3-Speed switching capability gives you the fine tuning control to match a wide combination of tube diameters and lengths of run.

https://www.tacocomfort.com/pr...irculator/index.html

Thank you. I couldn't figure out why I was so far off from the most common google result (007)

Please tell me you didn't use regular Pex for your loops! If you did, buy the cheapest boiler you can, because they won't last long. Regular Pex has no oxygen barrier like hepex, and intruding oxygen will eat up a boiler in about 10 years. Pumps and other system components won't like it either.

quote:

Originally posted by wreckdiver:
Please tell me you didn't use regular Pex for your loops! If you did, buy the cheapest boiler you can, because they won't last long. Regular Pex has no oxygen barrier like hepex, and intruding oxygen will eat up a boiler in about 10 years. Pumps and other system components won't like it either.

It's oxygen barrier.

quote:

Weil-McLain AB80-C

Haven't even seen one of these, nice choice!!

Thanks for the advice. I was looking at a Navien unit for cost & it matches load a little better, but I just found out the WM is made in Indiana (or maybe NC) and the Navien is Korean. So I'll spend a little more & keep someone working. I'd say buy a car to keep me working, but we can't build them fast enough to keep up & I need a break.