I agree with boiler man, I hadn't saw where you'd said it was poorly I insulated. However, I can't imagine what your going to be dealing with if it does actually get cold this winter. Where are you located? A 22 degree night should be a walk in the park for any outdoor boiler, let alone one holding 500 gallons of water. Which once again, water storage does not does not does not equal greater efficiency, I can't stress that enough.
I'm still not buying any draft issues as your wood is obviously burning very well. The bad part is that operating your boiler at such low temps is gonna doom it to an early grave, the moisture in the wood inside the firebox is normally sent up the stack because temps inside are to hot for moisture to gather, but since your temps are so low, that water is allowed to stay in the firebox to mix with ash and cause serious damage fast.
I know hicks is made in warmer climates which is kinda troubling and backs up issues I hear of folks around here that has them. For them to have such small fireboxes, yet such large water capacities makes me really question there knowledge about how a furnace should operate. Like one guy I knownhere needing 40 loads per winter, blows my mind.
You see the same thing with knock off brands advertising there 1/2" fireboxes and all that, I was approached by a stove company that's on here quite often with there big mammoth stoves and I let them spill, asked if they had any proof that there 1/2" fireboxes were more efficient and so goof, well of course they didn't, and there thoughts were based more on feelings and emption vs facts, but I can give you mountains of info showing anytime heat transfer is involved, thinner is better, but you have to go thick enough for it to be durable, and 1/4" is plenty for that to be durable.