Review my math?

RJMCTAFO

4" slab on grade with 2" underslab and edge insulation

Came up with 53k btu load

8 loops 1/2 " pex at roughly 225' loop lengths @ 120*

Will be heated using a flat plate heat exchanger off of an outdoor wood boiler. Planning on using a 1" thermostatic valve.

225 x .03 = 6.75 ft head

Pressure drop on exchanger is 5.9 psi
1 psi =2.3 ft of head
5.9 x 2.3= 13.57

Head of 20.3?

Usually have my supply house do all the math but wanted to start planning more myself. Any input is greatly appreciated.

Ironman

What's the square footage of the slab? The spacing of your tubes?

A thermostatic mixing valve is the wrong component for a slab. It will cause it to over-heat. You should use a smart valve or variable speed injection mixing controlled by outdoor reset.

120* is also on the high side for a slab. You don't want the surface of the concrete to exceed 86*.

A 53k btu load doesn't jibe with the amount of tubing you stated. What type of use is the structure?

If you would answer the above, we'll be able to help determine what you need.

Also, what size plate HX?

RJMCTAFO

Slab is 30 x 50
Was contemplating using an I series valve. So I will go that route. Building is a post and beam barn that will be used as a workshop. I am still going to have the design guy do it for me but I want to learn more of how to do it myself.

30 x50 with 16 ' ceiling height R19
2 12x14 insulated overhead doors and one insulated steel entry door. Checking my heat loss to see if I screwed something up.

http://www.supplyhouse.com/FlatPlate-FP5X12L-10-10-plate1-MPT-Low-Pressure-Drop-Heat-Exchanger-5-x-12
Found this which has a lower pressure drop.

This is the first one I have done with a wood boiler / exchanger set up.

Thanks

Zman

You caught me in the mood for a project.

The relationship that you are not seeing is Flow and resistance. The heat exchanger, for instance is rated at a certain resistance at a certain flow rate. Every system has a system curve that shows it's resistance at different flow rates. Every circulator has a performance curve that shows it's flow rate at different resistances. Where these 2 curves meet is the gpm's you will see once you hook it all up.
To really figure out which heat exchanger, you need to go to flatplateselect.com. The one you just posted is remarkably well suited for your application. It has about 1 PSI resistance at 6 gpm and more importantly will put out 54,000 btu's with a water temp of 120 (input 180)
Your calc for the tubing is pretty close as well. The head loss for the entire system will end up around 10-11 ft at 6gpm (probably the ideal flow rate)

I agree with Bob that the heat loss looks high. From a tubing point of view it is good to oversize. The same is not true of the boiler.
Do not use the t-static mixing valve, the head loss is off the charts and the I valve will give you outdoor reset and boiler protection.

Did you have a brand preference on circs?

Carl

RJMCTAFO

Thanks for the help. I am unable to check out the website right now but I understand what you are saying about flow vs resistance.

I am partial to Taco and right now the Viridian is cheaper than a 007 with the rebate.

With your info I am thinking a Viridian VT2218 with a 1" I series valve in setpoint mode. Piped p/s off the exchanger with a 007 primary pump.

Zman

The 2218 looks perfect for the radiant side. The 007 may work depending on that circuit.
Post details?
Carl

Ironman

RJMCTAFO said:

http://www.supplyhouse.com/FlatPlate-FP5X12L-10-10-plate1-MPT-Low-Pressure-Drop-Heat-Exchanger-5-x-12
Found this which has a lower pressure drop.

This is the first one I have done with a wood boiler / exchanger set up.

Thanks

Personally, I never use anything smaller than a twenty plate and I would probably look at using a 30 plate on your job. I deal with plate HXs and ODWBs all the time.

Within reason, you can't over-size a HX. The larger the plate HX, the less resistance to flow. You're also dealing with an ODWB which is probably open to the atmosphere making one side of the HX an open loop. An open loop is subject to fouling, clogging and other undesirables. The spacing between the plates is extremely narrow and clogs easily. This needs to be considered in choosing a HX. Don't design on best case scenario; design based upon worst case.

Ironman

RJMCTAFO said:

With your info I am thinking a Viridian VT2218 with a 1" I series valve in setpoint mode. Piped p/s off the exchanger with a 007 primary pump.

I don't think you grasped what I was trying to communicate to you about not being able to use a fixed water temp with a slab. Because the slab is high mass (essentially it's a big rock) it takes it a long time to heat up and a long time to cool down. This is what we call the flywheel effect. If the water temp to the slab is too high at any given outdoor temp, the slab will continue to give off that excess heat for long after the thermostat is satisfied and has shut off the circ. This will cause the space to over-heat. The only way to compensate for this is to use outdoor reset which will vary the supply water temp based upon OD temp. The colder it gets outside, the higher the supply water temp and vise versa. Once the reset curve is set properly, it will maintain the correct slab temp at any OD temp. In fact, when tweaked right, the thermostat will become almost unnecessary. It will just act like a high limit for the space.

I've got a building with a slab next door that's setup this way and I haven't put the thermostat on it yet. It ran all of last winter and the indoor temp stayed a constant 68*.

You need to get the I Valve that has outdoor reset and set it up properly. You don't want to use a set point valve.

RJMCTAFO

Gotcha. Makes total sense. I am used to using the boiler control for outdoor reset. The light bulb came on when you said that for me. That is how our shop is set up with constant circulation and a 4 way valve. No tstats.

Zman

Good catch Bob,
It definitely need outdoor reset and the boiler protection sensor.
A 3 way valve will work just fine since you don't need to worry about circ through the boiler.
I would install a t-stat to act as a high limit, with the design intent to be constant circ.
Carl

RJMCTAFO

https://us.v-cdn.net/5021738/uploads/editor/gk/c9ccrudbuwxj.jpg
So this is my plan. Thanks for the help guys. I will check into maybe upsizing the exchanger also. Its a friends barn so its kind if a side project. He is religious about taking care if the boiler water so I know he would change the filter on the boiler side and I would treat the radiant with Fernox.

Zman

You don't need the loop with the 007 at all.
The heat exchanger takes the place of the closely spaced tees

Ironman

I see no need for doing p/s on the radiant side. If the Viridian can move enough gpm for the floor, that's all you need with the I Valve.

If you're gonna pipe it like that, then I'd do variable speed injection mixing and use a Tekmar 356 to control the mixing pump. You're already showing the mixing bridge. If you go this way, it's a better setup than a smart valve, so the I Valve would be eliminated.

RJMCTAFO

Taco shows p\s for all installations of the valve. Was going off of that. Trying to keep it simple so I will just run the viridian on the outlet of the mix and let it rip then.

Zman

They only do that so the flows will not compete. With the HE it cannot be an issue.
The Virdian still needs to be with the mix loop or all flow will stop when the valve closes. Just move the expansion tank,air eliminator, fill valve T&P ect and get rid of the 007

SWEI

Do you actually need a 008 on the boiler loop? It's OK to size for a 35-40˚F ΔT at maximum fire. You might even consider another VT2218 there.

RJMCTAFO

On the boiler loop I have about a 50' run to the wood boiler. He has the pump and filter setup already so was planning on using what he has. Thanks for the help guys I really appreciate it.