Snowmelt sizing for pumps and expansion tank.

achoyna

Hey guys. Last year your advise helped me build and complete my rear snowmelt solution, which worked wonderfully during last winter here in snow bound Chicago.

My new project is to complete the front snowmelt and i have some questions. When I installed the side paths and stairs down to the mud room, I installed 1/2" pex lines throughout with 4 circuits of approx 180' each terminating in the mechanical room.

I'll be using a water to water heat exchanger to heat the water/anti freeze feeding the snowmelt.

Here are my questions for you guru's:

• How large should the heat exchange unit be? Would 240k BTU be enough?
  
• Do I need an expansion tank/air separator on the snowmelt side of the system? If so, should I use a 2 gallon or 4,4 gallon tank?
  
• I'm planning to use 3 pumps for the system. 1 Grunfos UPS15-58FC to feed from the inside radiant system to the heatexchange unit. And 2 of the same on each side (push/pull) of the snowmelt side of the heat exchanger. Does that sound correct to you for sizing and the need for 2 pumps on the snowmelt side?
  

Thanks in advance for any advise.

Alan

Zman

• The heat exchanger sizing will be based on the required BTU's and the water temps on each side. Most manufactures have calculators to assist with sizing.
  
• The smaller tank will probably work. Figure out your volume and temp range and plug it into the manufactures calculator.
  
• The pump size will depend on the resistance of the system and the required flow rates. Do not do a push/pull arrangement. Use one properly sized circ on the snowmelt side and install it so it is pumping away from the expasion tank.
  

GroundUp

HX ratings are based off of a given temperature and flow rate, so unless your application meets those numbers exactly, the rating is useless. Assuming 6" spacing of the tubing we're talking roughly 360 sq ft of area and a high melting rate for snowmelt is 200 BTU per square foot so roughly 72k would be the load. I have a similar load for my personal snowmelt and do it with a 30 plate 3x8 HX but I have 170 degree boiler water. Lower temp boiler water requires a larger HX to transfer the same quantity of BTUs.

A 2 gallon expansion tank is sufficient and it should be set up like any other radiant system with an air eliminator and single pump (possibly a mixing valve depending on the boiler water temp and HX size) pushing toward the manifold. Another 15-58 would be perfect

achoyna

Thanks @Zman and @GroundUp.

Good to know that one pump should be fine, pumping from the expansion tank (with air eliminator) to the manifold.

The heater side water temps will be around 120f - 130f, so I'll definitely need a larger HX unit. Yes, the square feet of the concrete path and steps will be approximately 320' - 360'.

@GroundUp You mention a mixing valve. Should I be feeding some of the return water coming back from the snowmelt back into the hot water going into the manifold?, as i did with my other snowmelt system that heated both my garage and snowmelt system?

Zman

With the lower water temp, I assume you have a condensing boiler? With a condensing boiler, the mixing valve is probably not needed. Is this a dedicated boiler or are there other loads on it?

GroundUp

The mixing valve would be to reduce fluid temp entering the slab. 120-130 is fine for snowmelt and assuming a condensing boiler, will be perfect without a mixing valve on either side. A mixing valve would mix fluid from the snowmelt return with fluid from the HX supply to achieve a determined supply temp to the slab, but again not necessary with 120-130 SWT from a condensing boiler. HX size to match temps though, is going to be big. Like a 30-40 plate 5x12 unit. Something smaller will work, but the melting rate will be slower as the supply temp to the slab will be lower meaning less BTU transfer

achoyna

It will be used with Navien Condensing boiler. Thanks for the sizing advice @GroundUp.