Ok to plumb a loop of two 1/2" kickspace heaters with 3/4" PEX?

Jells

I'm thinking of heating a 450 ft apartment with 2 kickspace heaters in surface cases that my calcs show should work fine. Nicer than cluttering the small rooms with baseboard heaters like I've done in the past. Last time I installed these I used monoflo tees off the existing 3/4 copper loop to 3/4 PEX to the copper of the units. If it's just these 2 units could I just run a loop of 3/4" PEX through the 1/2" heaters in a simple loop rather than using tees? FWIW the heaters will run off a 40k but water heater, their total btu emittance at 140 deg is 16.6k at the low fan setting.

Gilmorrie

Yes, I've done that successfully.

Gilmorrie

But only with an hydronic (boiler). I'm not real keen on using a domestic water heater for space heating.

Jells

Gilmorrie said:

But only with an hydronic (boiler). I'm not real keen on using a domestic water heater for space heating.

Thanks. See this post for the big picture https://forum.heatinghelp.com/discussion/comment/1641265
I'm open to other ideas, but haven't seen a good one.

Jells

Studying this issue, I'm wondering if I can do this with 1/2" PEX the whole 60' loop rather than 3/4. It's a bit confusing because the PEX is a smaller dia than copper, and the fittings smaller still. But especially if I use 1/2 and bend it where I need to go 90 deg there will be very few fittings. What are the parameters of pressure drop that I need to stay inside of? I've read some say it's OK to replace 1/2" copper with 1/2 pex because the pipe is smoother and there's less fittings. I tried figuring it out from these https://mrpexsystems.com/wp-content/uploads/2016/08/17-Pressure-Drop-Charts.pdf https://s3.amazonaws.com/s3.supplyhouse.com/product_files/T050-300-B-Bluefin-flow rate chart.pdf https://www.etnasupply.com/ASSETS/DOCUMENTS/ITEMS/EN/Uponor_LF2935050_Manual.pdf but it makes my head explode. Best I can figure is that for a 60' loop with few fittings it doesn't matter that much. But I know I'd be safe with 3/4 even if I had to use lots of elbows because it's too stiff to bend 90 in a joist bay. Any guidance?

Jells

Youngplumber said:

If you are are using a delta t of 10 with the other numbers you've given you need 3.25 gpm. If you do that you need .634 inch pex which means 1/2" won't due. 5/8" would be pushing it but likley OK. Or you could jump up to 3/4"

But I'm just piecing together from the bits of information I've been scanning through.

Thanks! Could you possibly 'show the work' so I can understand? Is it just using 'pipe length equivalent' numbers per fitting? How do you arrive at a delta T of 10? I've been wondering what the real temp drop of such a loop would be.

FWIW I used the 1gpm 140 deg BTU numbers for the '120' units on this sheet just to be on the safe side. Https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Beacon-Morris-K120-Submittal-Sheet.pdf.
But I'm thinking of adding a smaller unit under the cabs in the 100 sq ft kitchen for a total of 3 1/2" heaters in the loop.

Jells

While I think I could wrestle 1/2" pex through a series of 16" center joist holes and then 90 deg down the bay, I don't think 3/4 is possible, just too stiff. I'd have to use ells.

Jells

Thanks @Youngplumber
Hmm. There's just so many 'off the beaten path' variables in what I'm doing, like putting 140 water through an exchanger block then round this loop, that it seems like I should just keep oversizing everything. Like even the exchanger block. I assume one rated for say 125k BTUs rather than 60k will transfer more heat and have a narrower in-out gap even at far lower BTUs output. For $40 bucks more it seems a smart overkill.

I actually measured the BTU's used by the current ventless heater in one room of the apartment at 6200/hr. So even doubling that, not really necessary since with door open a lot of that heat was going to the other 2 rooms, I have a load of 12,400. So my goal of around 20k with 3 heaters is plenty of overkill even if it get cooler water than 140. It's worth overbuilding it just so I never hear that the tenant is cold.

I tried to do a heat loss calc for this building when I wanted to add radiant to my own kitchen and bath. It's attached partially on one side, bricks with no insulation, 20 year old replacement vinyl window, unheated hallways, and it was nearly impossible. A radiant guru told me it wouldn't work, I wouldn't get enough BTUs through. I did it anyway, a suspended joist bay install under the existing terrazzo floor, and it's been sweet for 15 years.

Jells

I got the 1st heater and the 1/2" feed splits into two 3/8" OD tubes running through the vanes. Best case with a .031 wall that's 0.153 sq in, about a .44 dia tube. I think it makes me nervous doing a simple loop through 3 of these with the flow constricted so much. I'm going to use Monoflo loops of 1/2" pex from a 3/4 pex main loop to to these units.

Predictably, the wall case came damaged and has to go back. Grrrr.