What size pump for radiant floor?

[Greenfield Dave]

Hi Guys! Long time lurker, first time poster. Here’s my situation.

I built a 36X48 garage with loft, and used knowledge from this site and others to put PEX in the floor. 1500’ to be exact, in 5 zones. Put 2"pink board down, used the twist in PEX clamps, thought is was Miller time! But I didn’t put clamps at the point of the loops, and they floated up in the concrete mix. Woops! So we decided not to make saw cuts, it didn’t crack too bad, it does have fiber in it. I have insulated all the 2X6 walls and put OSB on them, but the ceiling will have to wait till money is in season! I just bought a stainless manifold, and am going to heat the system primarily with my OWB, but will use an electric water heater for backup/supplemental heat. I have a sidearm heat exchanger, expansion tank, and just enough skills to be dangerous!

My questions for you experts are:
1. How big of a pump do I use?
2. Should I control the incoming temperature to the water heater/sidearm with a tempering valve? Or run that loop wide open and control the temperature to the floor?

I have a layout of how I think it should be plumbed, but it’s too big to attach here. (pointers needed..)

Wood Doctor 8000
Stihl 041 Farm Boss; 009
POS Swisher log splitter
House heated with an older boiler, old cast iron radiators and under floor PEX

Any input is appreciated
Dave

 

[8nrider]

hey have you pressure tested the system after the pour?

 

[atlarge54]

Yes you'll want a mixing valve to temper the floor loops. What size pex? Hope it's not 1/2". I've got 800' of 1/2" in 5 loops and a taco 009 only flows about 5 gpm. I think you'll need more flow than that.

 

[jebatty]

I believe 1/2" is rated at 1-1/2gpm for maximum heat transfer, and 3/4" is rated at 4 gpm. I think you should size your loops and calculate your pump head, then size your pump, to achieve these flow rates.

 

[jebatty]

I’ve got 800’ of 1/2” in 5 loops and a taco 009 only flows about 5 gpm.

Based on my comment above, you are getting less than 1 gpm per loop, which is probably less than where you should be.

 

[Greenfield Dave]

No, I have not pressure tested the loops yet, but will before proceeding much further.

It is 1/2" PEX, the loops are 300' each.

Thanks!!

 

[jebatty]

Good resource for calculating line head:
http://www.freecalc.com/fricdia.htm

 

[in hot water]

300 foot loops of 1/2 pex are fine.

The RPA "RadPad" suggests a max. of .6 gpm per loop for 1/2 pex.

.6 X 5 loops= 3 gpm at just under 4 feet of head, using water.

A small Grundfos 15-58 on speed 1 should handle that, depending on the boiler and other piping pressure drop.

You should be able to provide 25 or so BTU/ square foot, keeping the slab temperature below 85F in a 70 room air temperature. What does the heat load calc for the building require?

 

[atlarge54]

Not to hijack this thread but I do have some related questions. My manifold is plumbed with 1" copper and a mixing valve also 1". How much does a mixing valve reduce flow? My pump has 3/4" flanges, would a switch to 1" flanges make much of an improvement? If .6 gpm is ok maybe I'll go back to a 007 and see how it works. My loops are under lots of flooring plus carpet does the .6gpm only apply to concrete? What temps. are generally run in concrete loops?

 

[Greenfield Dave]

300 foot loops of 1/2 pex are fine.

The RPA "RadPad" suggests a max. of .6 gpm per loop for 1/2 pex.

.6 X 5 loops= 3 gpm at just under 4 feet of head, using water.

A small Grundfos 15-58 on speed 1 should handle that, depending on the boiler and other piping pressure drop.

You should be able to provide 25 or so BTU/ square foot, keeping the slab temperature below 85F in a 70 room air temperature. What does the heat load calc for the building require?

The garage is well insulated, with 3 OH doors and 3 service doors. I have no idea how to calculate heat load or loss. It has 9' ceiling height.

What is RadPad? Earlier in this thread, someone suggested 1 1/2 GPM in 1/2" pex... Now I'm really confused!

Thanks for all the input!

 

[in hot water]

Not to hijack this thread but I do have some related questions. My manifold is plumbed with 1" copper and a mixing valve also 1". How much does a mixing valve reduce flow? My pump has 3/4" flanges, would a switch to 1" flanges make much of an improvement? If .6 gpm is ok maybe I'll go back to a 007 and see how it works. My loops are under lots of flooring plus carpet does the .6gpm only apply to concrete? What temps. are generally run in concrete loops?

Most valves and mixing devices have a rating called Cv. It relates to the number of GPM you can flow through that valve, or device with a 1psi pressure drop.

Generally a 1" mixing valve, unless it is a high flow Caleffi with have a Cv of around 3. Or a 3 gpm flow. 3 gpm at a 20 degree delta T will move 30,000 BTU/hr. Upsizing the flanges will not change much, look at the hole diameter in the pump volute.

Proably an 007 will do the job, it depends what else you have in the "circuit" beyond the tube loops. Such as valves, piping, boiler or heat exchanger. You need to know the pressure drop through all of the components in the "loop" Over-pumping just consumes more electrical energy and has a small effect on output. Grossly over-pumping will lead to pump problems, noise, and velocity induced wear in all of the component. Worse case cavitation and no flow.

.6 GPM is a recommended safe flow through 1/2" PEX as per industry standards, Radiant Panel Association, etc. Certainly a 1/2" tube can carry more flow than that but velocity, wear, and noise are considerations. For off and on use like home plumbing you can exceed the .6 flow rate, but not safely for continous flow hot water circuits.

The temperature you need to supply has to do with the heatloss of the room. The higher the heatload the higher the required supply temperature. Generally for residential floors you want to keep the SURFACE temperature below 85F. Above that they become uncomfortable to bare feet as average skin temperature runs in that temperature range.

Radiant design software will spell all of this out, by the way. Loop diameter and length, tube spacing, flow rates, supply temperature, and total load. It should always be the first step for a trouble free radiant system.

Not all spaces can be comfortably heated with just radiant floors, depends on the load. 25- 30 BTU/ square foot is generally the max for a residential radiant floor output. Some say 25-27BTU/ square foot output. Commercial slabs, where people aren't generally barefooted can run a bit warmer temperatures and have higher outputs. The ambient air temperature also plays into that output. cooler ambient= more output via larger delta T.

Now to get 85F surface may require 95- 115 or higher temperatures depending on the load and how much is lost downward through the under slab and edge insulation.

I've seen un-insulated concrete slabs with 130F supply unable to warm a room. Edge insulation is also very, very important as it see outdoor ambient temperatures. A radiant slab edge trying to maintain 85F against a 0F or lower ambient presents a large delta T and load. at least 2" around the slab edge, down to frost level.

hr

 

[Greenfield Dave]

Man, "in hot water" is on it! You really seem to know your stuff! I will use the pump you are recommending, and would love to send you the layout for editing...But can't figure out how to insert it, because its a little to big to attach. Send me a PM with your email, and I will send it to you...


P.S. I did do as you suggested and put 2" pink board both under the concrete and around the perimeter.. At least I knew to do that..

Thanks very much!!

Dave