Hydronic baseboard/panel heater for small attached greenhouse?

ajs317

I have an small (8' by 10') covered screened patio attached to my house that I use as a greenhouse in the winter by attaching twinwall polycarbonate panels to the three sides of the patio that are open to the outside (the fourth side is the exterior wall of the house and it has a roof). In the past I've heated this using a small electric radiator. The space warms up quite a bit during the day especially on sunny days, but overnight and when it's very cold, I need the heater. I don't need to keep the space that warm, just not freezing--the heater is there to keep it from dropping below 40 degrees or so.

Could I heat this space using a fin-tube baseboard or a small panel rad? I have a manifold set up at my boiler, so I could easily run a PEX line out there and mount it to the exterior wall of the house, and control it with its own actuator. I figure this might be cheaper than using the electric space heater. I thought there might be a concern with the boiler short-cycling given that heat loss would be much higher than an interior space, but this demand relative to the interior spaces would be offset by a much smaller delta-T needed.

Any thoughts on why this might not be a workable idea?

STEVEusaPA

I'd use a flat plate heat exchanger and run glycol in that loop.
If you have oil, you're probably short cycling anyway.
You might actually need/use enough btu's as it's going to be a pretty big zone-btu wise.

ratio

If it's not zoned separate it wouldn't short cycle—in fact it would tend to lengthen cycles. If it only needs to keep it above freezing, add a HX & a small glycol loop on constant circ, or maybe on an outdoor stat set to like 40°. Adjust things to keep things warm enough & Bob's your Uncle!

Note: While certainly cheaper to operate, this will be significantly more expensive then buying an electric heater. I suggest a cost/benefit report be generated prior to commencement.

HomerJSmith

You might find insulating the ceiling with 2-3" polyurathane Celotex or R-max might benefit you. Tighten up your heat loss.

ajs317

Thanks everyone, this gives me lots to think about/research. I have some initial questions. Can the glycol loop be run as a circuit off of a manifold that also shares other hot water circuits?

I gather that the HX works by connecting a hot water circuit between the manifold/boiler and the HX to a glycol circuit running through the output. Would the HX best be placed near the boiler with a longer glycol loop or near where the glycol loop would exit the house (so a longer water line with HX near the output device)?

Would I need a separate circulator for the glycol side of the loop? If not, how does the glycol get moved through the output? Would any other materials be needed other than the HX and PEX loops?

HomerJSmith

Yes, you need a flat plate heat exchanger between the boiler and the anti-freeze circuit unless you put anti-freeze in the whole sys. The anti-freeze loop needs an expansion tank, a tridicator, a pump (possibly a Taco 003) and webstone fill and drain valves. I also put in a balancing valve, too, to regulate the flow, but if the pressure loss and the pump is closely matched, it can be eliminated . The boiler side of the flat plate heat exchanger needs a strainer on the input.

Also, you need some way to regulate the temperature in the room, turn the pump on and off.

It doesn't matter where the HX is placed. Economy and ease of installation should be your guide.

You can do this, but it is really a job for a professional.

ajs317

Thanks, Homer. I'm seeing why one of the previous folks mentioned a cost/benefit analysis. This sounds more complicated and costly than I think I need given I have the electric heater solution.