Using a modulation boiler to control water temp in snow melt system

timschefter

I am designing a driveway snowmelt system for new construction. I used the Wirsbo snowmelt design manual to come up with 12" PEX tube spacing, 160KBTU/hr boiler, 40 % Glycol, 9 zones, R10 under the slab ---etc etc. This will be a stand alone system , the boiler will not be used for anything else. I would be using a modulating/condensing boiler. There would be a flat plate heat exchanger to keep the glycol out of the boiler. My question: Instead of using a mixing system (either injection or a mixing valve) why not use the capabilities of the modulating boiler? They typically can run at 30 to 100 % of output and can be controlled with a 0-10Vdc input signal to modulate the delivery water temperature. You can buy PID temperature controllers from Omega and others for around $150. These controllers can take inputs from a number of types of sensors (in this case a sensor in the slab) and generate a 1-10Vdc signal that corresponds to the difference between the sensor temperature and the set point. These controllers even allow you to control the rate of change of the temperature. I would not do this for an underfloor heat system because the boiler would be running all the time but since a snowmelt system is used intermittently it would seem like this would work. What am I missing?

EBEBRATT-Ed

I have used the Omega controls. Great little controls. You can make them do almost anything.

Try it if you want. If it was me I would still use a three way valve. I think it will give better control. Maybe leave a couple of tees so you can add the three way later if you want

Zman

Most condensing boilers will handle up to 50% glycol with no heat exchanger needed. The Triangle tube and Lochinvar firetube models absolutely will. The lower the return water temp, the higher the efficiency. I would not even consider using a heat exchanger for your application.

In theory, your 0-10vdc control scheme should work. In reality, it may be more trouble than it is worth. I would absolutely run it by your boiler manufactures tech department. Depending on the manufacture, you may need to give the boiler and enable signal and/or a special sequence at startup.

There will be a significant lag between an increase in water temp and a change in slab temp (think hours). You never want supply temps over 130-140 or so to prevent thermal stress. The slab temp sensor will allow you to turn down the boiler when the slab comes up to temp. I don't think it will help much on the way up.

I would either use an off the shelf snowmelt controller like tekmar or HBX or just run the boiler at a fixed temp (110-120). The snowmelt slab is such a huge cold mass, it will take hours to get it up to temp. Even if you request 130 degree supply water, unless you grossly oversize your boiler, the boiler may never get there due to the laws of thermodynamics.

Watching a snowmelt system is a lot like being a spectator at a snail race. Things move very slowly and there is not much you can do about it.

Tinman

I’d scrap the HX idea too. Without a true snowmelt slab sensor, what is sensing moisture?

GroundUp

What size tubing and how thick is the slab? 12" spacing is pretty wide for snowmelt; it tends to stripe and take much more fuel to melt the centers while just wasting heat above the tubing. My standard is 6" spacing with 5/8" tubing and <200ft loops. What are you using to control 9 zones and why are there 9 zones in the first place? 9 loops in a single zone perhaps? I would also scrap the HX idea.

timschefter

4" thick slab. The 12" came from the Wirsbo manual. Seemed like 9" would be better. Sorry I meant 9 loops on one zone. I was not going to attempt to make it automatic as far as on/off goes. If needed to turn on when not there use remote control and monitor with a webcam. The only reason to put a controller on it would be to limit the delivery temperature. Thanks for the feedback.

hot_rod

When melting snow you usually want to throw all the BTUs possible at it, until the job is done. I don’t think you will over-heat the slab if it has a shutdown function when the melt is done. The colder the return, the more efficiently the boiler runs. Keep in mind a 34 degree slab melts snow