SHORT BURN TIMES WITH TARM SOLO 60

Your burn sounds near perfect to me. Slowing the burn down will not get you to a higher temperature. Your load of wood only has approximately 5000 btu/lb of available heat. Burn slow, burn fast, that doesn't change. What's happening is that your load of wood does not have enough btu's to bring your tank to 190F. Two ways to achieve this goal. 1) burn wood with higher btu content per lb. But since wood can be quite variable, and you may not have higher btu content wood available, probably not a good approach. 2) weigh the amount of wood you burn to bring the temperature from your normal "discharged" temperature to your desired "charged" temperature. Assume 140 to 190F, and assuming no draw on the system during the burn/charge period. Then, you need (190-140) x 8.34 x 1140 / 5000 lbs of wood to reach your goal, or 95 lbs of wood. Start with this formula to calculate your wood weight and then adjust as needed. For example, if my radiant kicks on during a burn, for my 1000 gal pressurized storage I need to burn another 17 pounds of wood to reach the desired tank temperature over what I would need if there is no draw on the system during the burn.

The key to actually achieving the 190 with no boiler idling usually is load full and burn until wood load is burned down 1/2 to 2/3, then add the extra wood you need to reach your 190F high temperature storage goal, and as that burns down the burn and storage will coast to the 190F temperature. When I mean no idling I am assuming that with a delta-T of 20 your boiler does not idle at high burn, as that means you have sufficient flow to handle your boiler output. If you idle at delta-T = 20, then I would think your flow rate may be insufficient to handle boiler output.

After all of this, keep in mind that with 1140 gal of storage, the additional heat from 185 to 190F is 1140 x 8.34 x 5 = 47,500 btu's, and at 5000 btu/lb of wood that only amounts to about 10 pounds, which could be as little as 2 splits of oak. All in all, just as I started, if a full load brings your storage up to 185F you have a very sweet operation, as it is little more than load, lock, fire and walk away.

Those of us who are controls (or cashflow) deprived can manage our delta T manually with our three speed pumps. A reasonable target for most boilers is 20 degres delta T. In very general terms if you are seeing less than 20 you should slow your pump down. If you are seeing more than 20 you should speed your pump up.

I personally change my pump speed three times on most burns. I put it at medium speed for startup. I switch it to high when I throw in my first "load" of wood which usually takes me to bedtime. And then I usually put it back at medium when I load it before I go to bed (if required).

This week it's been in the 30's here in Michigan so I've been heating on less than one full load of wood per day similar to your setup.

When I mention delta-T=20, I mean the difference between the boiler supply temperature and the return from system after the Termovar. If the return after Termovar is 140, your boiler temperature should be 160 or higher (if it less than 160, then your flow rate may be higher than I think would be normal experience, if it is much greater than 160, then your flow rate may be lower that I think would be normal experience).

As return after Termovar rises, so should boiler output to maintain the delta-T=20 +/-. As return after Termovar rises above 160, I suggest gradually turning down the balancing valve to allow less boiler/return water circulation, and for the purpose of maintaining delta-t at least = 20. As boiler supply rises towards to about 185 or somewhat higher you should still be able to maintain delta-T=20, but as return rises above 165 or so you will likely will not be able to maintain delta-T=20, especially as return rises to 170 and above, and especially if boiler output is at the high end (lots of wood yet to burn).

This is where idling may start to occur, as boiler supply will be about 190. There are only two ways to prevent this of which I am aware. Either increase flow rate to increase the ability to move btu's, or let the fire burn down so boiler output is less. I use the second method, as by weighed wood burns the boiler has been loaded so that as tank temperature rises above 170F, the wood load is burning down, and the boiler kind of coasts downward, btu output falls, and tank temperature will continue to rise to about 190 (my normal target is 185 so I don't accidentally overshoot and idle).

Off hand I would not be too concerned that tank return temperature is higher than bottom of tank. It is very possible that the water temperature at the measuring point is different than the water temperature where it leaves the tank on the way to the boiler; in other words there may be a circulation loop in the boiler that is causing this. In my system the return from the tank fitting is 6" above the bottom of the tank, and especially as the tank nears full charge this temperature can be a good 10F or so above actual bottom of tank temperature. But with a horizontal cylinder tank like I have, there really is very little water in the bottom 6" of the tank, and when bottom of tank temperature actually rises it rises very fast. That very bottom of tank has little room to store btu's.

As you probably are aware, I have from time to time placed lots of sensors at various points and logged data. That has led to a pretty good understanding of actually what is happening, at least with my system. Good luck with yours, and don't be too disappointed if extreme high end performance may be a little less than you want. That extreme high end just doesn't amount to much, but it is exciting to achieve. I can achieve 192-193 top to bottom and no idling with very careful burning, but be assured that I am operating just shy of the idle point.