Best efficiency of propane

BillR

I have purchased a Modine HD75 propane heater. It can be wired to run at either high, medium or low speed. This unit outputs 60k btu’s. This is being installed in a 13x26 greenhouse that I want to maintain 65 degrees F over this winter. It should take approx. 30500 btu’s to heat this space. Which speed would you suggest to run this heater to be the most efficient with the propane? I’m trying to make my 500 gal. tank last as long as possible. Thanks

Jamie Hall

This heater does not modulate -- it either puts out 60 K BTUh or it's off. So... what you want to do in a greenhouse is to make sure the heat is as even as it can be. This you do by choosing a blower speed which will distribute the air around the space evenly, or as evenly as possible. And this will depend on where the unit is mounted in relation to the space and how it's oriented.

Choosing different blower speeds won't make much difference in the amount of propane you use, unless the heating is very uneven.

ChrisJ

I would assume high would give the best efficiency, but I'm not totally sure. It probably won't make a huge difference.

Honestly, being they don't even appear to publish efficiency ratings on the HD ( Hot Dawg) series, and it looks like a budget priced unit, I wouldn't expect anything spectacular. If LPG usage was a big deal you really need to at least go with the Modine PTC series.

A PTC 55 would've been a much better choice.

ChrisJ

Any chance you can exchange the HD for this ?

https://www.supplyhouse.com/Modine-PTC55AS0121FBAC-PTC55-Effinity-High-Efficiency-Condensing-Gas-Fired-Unit-Heater-LP-55000-BTU

BillR

ChrisJ, no I think it’s too late to change units, I’ve already installed this one. I have it running off of a thermostat, so I think I’ll just have to run it at different speeds to see if there is any measurable difference in gas usage.

Jamie Hall

BillR said:

ChrisJ, no I think it’s too late to change units, I’ve already installed this one. I have it running off of a thermostat, so I think I’ll just have to run it at different speeds to see if there is any measurable difference in gas usage.

There won't be much. What there will be is a difference in air circulation in your greenhouse -- as I said earlier, set the blower speed for the best air circulation in the greenhouse. The efficiency of the unit is, I hope, quite secondary to getting the greenhouse growing properly.

captainco

The blower needs to be set for the amount of btus the heater is using. High fire in almost all cases is the most efficient. The best way to keep the heat even is to keep the blower running at all times. With a relay the blower can be wired to run at a lower speed during the burner off cycle to minimize stratification of the heat and keep it more even.

pecmsg

What outside air temperatures are we talking about. 

A heat pump will save a lot of LP usage. 

hot_rod

short cycling is what will cause inefficient operation and excessive fuel consumption. Ideally, size the unit close to actual load.
Of course the load constantly changes and the best heater would modulate with that ever changing load.