Adding a heat exchanger zone to a pex in-floor system

Mfremstad5

Hello all. I am currently looking to build/design my heating system for the farm shop. I have 7 runs of 300' pex in the floor. Was thinking of adding a forced air heat exchanger as well. My thinking is I would keep the shop 50-55 degrees constantly. However, when I'm working in the shop I would like the option of jumping the air temp up. Was thinking I could add a separate zone to my system with a separate thermostat so I can increase the temp when I'm working in there. Would I have to size my boiler up to handle this load? Would I need a mixer for the two different zones or would I be able to set the delta on my exchanger pump instead? I'm heating 2100 sq. ft with 14' walls. I was thinking of getting a 199k btu boiler to handle the additional load but not sure if that woukd be too much. Thanks.

hot_rod

A heat load calculation for the shop would spell out the boiler size.

If it is well insulated you are probably in the 20 or so BTU/ sq. ft.

2100 X 22 BTU/ ft = 46,000 BTU/ hr boiler output. I'd look at a 60- 75,000 size. Certainly no more than a 100,000. Spend the $$ for a real boiler, not a tankless water heater.

You may need two temperatures, 160° or so for the hanging heater, maybe 90- 100° for the floor. I find 66°F plenty warm for my shop. I set it back to 62 in the evening.

Once you experience radiant floors, you will hate the noise an air movement of the forced air unit heater. And the ceilings will be 80°. You may end up just idling the shop at 60, boost when you go into it. Within an hour the temperature should increase 4- 6°

DCContrarian

What we don't know is the heat capacity of your shop building — how many BTU's it takes to raise the interior temperature. Without knowing that there's no way of knowing how many BTU's are needed other than trial and error. And it's not something that's normally calculated for buildings.

The trial and error method would be to estimate how much heat your under-floor coils are capable of putting out, and see how long it takes them at maximum output to raise the temperature the way you want to. That's assuming they have enough output to be capable of doing it. If you, you could do the same experiment with a rented portable propane, kerosene or electric space heater.

GroundUp

I do systems like this pretty regularly. Unless the shop is a complete sieve, even 100k is probably overkill for the loss but if you've got a single zone, oversizing doesn't hurt much of anything if it can be used at some point for a faster heat-up as described. Most heat-only boilers will have the ability to run an indirect water heater tank as priority with a higher water temp than the space heating, which is often what I use for a unit heater system like this. That way the floor can be set to 100* or whatever, and the UH can be set (via the DHW control) to be run at 160* or whatever with a second thermostat. If you want them to be able to run simultaneously, you'd need to run the whole system as a high temp system and mix down the radiant which will reduce the efficiency of the boiler. A couple weeks back I did an 80k heat-only boiler in a 2400SF shop with a Modine HC121 unit heater controlled using the DHW function and the customer is ecstatic about it because he keeps the shop at 50* with the floor at all times and then bumps it up to 65* with the UH when he's in there. I actually do the same in my own shop and would never have it any other way