Which is better, hotter water or higher flow

d0eby

I have an outdoor wood burner, with a 150 gallon tank. When outside temperature gets down below 20, it cant keep up heating the house(at suggested water temp 185) So my question, is which is more efficient, New pump that will go from 17 GPM to 31 GPM. or increasing the water temperature to 199, but will burn more wood to keep higher temperature. So basically which heats better, more flow or higher water temperature?

Zman

You are likely being restricted by the tubing running to the house. What is the size and length of the tubing? What circulator do you have and what are you considering. Heat exchanger size can sometimes be important as well.

d0eby

i was worried about that, it is 1 inch buried pex with a taco f5 007 (17 GPM) about 40 feet and looped back out. Flat run, not up hill at all. Inside is another taco to pump it threw the house when thermometer calls for heat. 3/4 pex to just copper pipe with the heat fins on as radiators threw house. 60 foot inside house. I was looking at same model taco pump(010), just upgraded to 31 GPM. I was going to put that on the wood burner side, to have the water flowing in the loop faster. Keep the same taco pump for the in house flow.(actually 3 pumps inside for the 3 different floors, but only heat 1 floor) I didn't know if the upgraded pump would help, if i turn the wood burner temp up, it goes threw wood like crazy.

HomerJSmith

They are interactive. One is restricted by the size of the piping as to flow because of pipe erosion and noise. Go the largest flow rate for your size pipe. Pex is more forgiving about pipe erosion and the pex needs to be insulated. So, increasing the supply water temp is the way to go for increased BTU output. But then again, you have to have the radiation capability to use that higher heat energy.

Jamie Hall

Thermodynamics rears its head again. In order to get more heat out of a radiator, the radiator must be hotter. The heat output from a radiator is more or less proportional to how much hotter it is than the space it is in.

Now it's important to remember that that is the average temperature of the radiator. In your case here, it is possible that more flow will produce more heat, but only because the water temperature won't drop as much going through the radiator. One usually aims for about a 20 degree drop -- but there is nothing hard edged about that.

Worse, the temperature which you quote (185) is already pretty hot, considering that water boils at 212. Raising it to 199 would get some more heat, yes, but if the system is already struggling in won't make that much difference. I'm concerned that you simply don't have enough radiation available to heat the structure -- and there is no helping that, short of putting in more radiation.

In answer to the direct question: higher flow makes little or no difference. Higher temperature makes a real difference. Neither will help if there isn't enough radiation.

hot_rod

does the the boiler reach 180 and shut down under those inadequate heat conditions?  If it runs non stop and loses temperature, you don’t have enough 🔥 

d0eby

It does cycle, i actually had it at 190. The wood burner would burn more often than normal, but it gets so hot in the big burn area, it was off and on. Just a little more frequently than normal. That's why i think a faster pump might work better than cranking up the temperature.

Zman

Your actual GPM is not the max indicated in the brochure, it depends greatly on the resistance of your system. A 007 in your application is probably moving ~6 GPM.
The 0010 has a pretty flat curve and would not be a good selection.
A 0011 or 0013 would be good choices and would almost double your flow.
Increasing your flow would be a better idea than raising the temps for the reasons described above.
I suspect it is cycling because it cannot transport enough heat from your boiler to the house.

Jamie Hall

d0eby said:

It does cycle, i actually had it at 190. The wood burner would burn more often than normal, but it gets so hot in the big burn area, it was off and on. Just a little more frequently than normal. That's why i think a faster pump might work better than cranking up the temperature.

Try again here. If it is cycling off, that means exactly one thing: it is putting out more heat than the radiation can absorb. More flow would raise your return temperature, but unless your return temperature is absurdly low you simply don't have enough radiation.

Just moving the heat somewhere else is not going to help if you can't get rid of it where it's going.

hot_rod

If I were a betting man, I say the problem is in the piping between the boiler and building. It's very common to see that pipe undersized. The ID or 1" pex is smaller than copper, and insert type fittings add more pressure drop.

You may be able to pump your way to a better output. instead of replacing that 007, put a second in series to double the head. The 15-58 shown below is close to what you have for performance spec.

Ideally if you replace that pump, get one more specific to those heaters, Taco has a model, or any of the two or 3 piece type. B&G PL series is another choice.

A heat load calc would help tell the story of how much heat energy you actually need to move.