Stuck with oil boiler, but go forward with pipe changes ?

Dave Carpentier

So, looks like we're not getting gas this year. I even had a contractor on board to help, darn it. But.. now Im thinking I should get his help to re-pipe my system so that an eventual gas modcon or maybe an a2w pump can just be swapped in (piping wise).

Any concerns running the oil CI like this ?

One option I can see is running the primary circ during every heat call. The extra thermal mass in the boiler and pipes could help extend run time and cool the boiler off more between runs. Im not sure if Im looking at that the best way though.

Bonus question.. recommend a mixer for the slab heat ?

Jamie Hall

Generally OK. I'd use a thermostatic mixer on the radiant floor — several good makes.

And yes, run the boiler with an aquastat on the buffer tank, and run the primary circulator any time the boiler is running.

Dave Carpentier

For sure run the primary circ when the boiler is running, but what of a heat call in the infloor but the buffer tank aquastat is currently happy ? Should I circulate in the primary loop just to add more thermal mass ? The boiler is going to shut itself off when it hits its own aquastat high temp, so I think I want to suck some btu out of it so that when it does run, it has a longer run time ?

Jamie Hall

I don't think, with modern boilers, that there is much point in worrying the residual heat in the boiler, unless your are a down to the last microWatt fanatic about energy conservation. And adding that much water and metal into the equations won't add enough heat capacity to lengthen buffer tank run times much.

At least that's my take on it…

Grallert

I guess it not likely to be an issue but I would conscious of cooling the boiler off too much. Again not likely to be an issue with modern boilers.

Dave Carpentier

What qualifies as 'modern' ? This one is 24 yrs of age. Appears to be aging well (knocks on wood).

I run it as 'always hot', but did add a Ranco to extend the delta to 25 deg.

hot_rod

so total load is 70K?

One rule of thumb is the low temperature load should not be more than @10% of the boiler capacity. That is to prevent the boiler from extended cold run times as it will see the radiant return temperature. At design the radiant may be returning 95F to the boiler (SWT 105-10 delta) If the boiler is keeping up or gaining on the loads that radiant return temperature blends with the boiler operating temperature, so that RWT is a moving target. You could calculate that number knowing flows and temperatures. You want to assure the oil boiler return is above 130 within 10 minutes or so of run time.

If the buffer tank is hot and any call for heat comes on, it will just pull from the buffer tank. The boiler and its circ will just run when the tank aquastat drops low enough to call. I'd guess you would run the buffer 140 or so + the 25° diff, so up to 165. That may be cutting it close to assure warm enough return to the oil boiler, worse case you run it 150- 175. Calculate that, or sit and observe for an hour or so.

Do so until you reach thermal equilibrium. That being when the boiler is no longer adding heat to the system, SWT and RWT are staying put. Heat input and heat output are perfectly matched at this point.

The hotter you run the boiler the less efficient, but the hotter the buffer tank, the longer the drawdown time. So tradeoffs there. Obviously a non condensing boiler has a min imum return temperature requirement, 130 seems to be the industry agreed number.

Any addition heat input, mod con, heat pump, solar, etc, just ties into the headers off the buffer tank. You could have a multi, multi fueled system, selecting the most efficient heat input device at any point in time.

More info and formulas here.

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