how low can you go

Dave_12

Most boiler manufacturers recommend a minimum of 140F to avoid flue gas condensation.

Since this requires fairly high boiler outlet temperature, we routinely recommend the use of a "Thermic" valves (preset at 140F). When properly sized and piped, these valves will keep the boiler return temp hot, yet allow the system temp to be much lower.

The valves are a three way valve with a thermostat similar to an automotive radiator valve. They should be installed in the return line back to be boiler, with an inlet from a by-pass line connected to the hot outlet side of the boiler. They are best when used with primary-secondary pumping. Various size of valves are available. We generally recommend that the valves be sized using the Cv flow rate, which is the GPM at a 1 PSI (or 2.31 FT) of pressure drop through the valve. This pressure drop needs to be considered with regard to the pump selection.

Management of return water temperature is critical. The use of these thermic valves can be very, very useful in applications where outdoor reset can drop water temperature, or with large cast iron systems where water temperature never gets high before the thermostat is satisfied.

Add a post if more information is needed.

Good luck.

Robert_15

how low can you go with an o.d.r

without condensing on c.i.boiler, say having the high temp. at 170 low at 110.

Ron Schroeder

Hi Scott, it depends mostly on the fuel type and excess air. What fuel are you running.

By the way, 110 is too low for gas, propane or oil.

Ron

Robert_15

n.g. we have the termastat set to 69f on both zones and it hold s the temp we have copper baseboards & the house is very comfortable

Ron Schroeder

I don't have the exact figures in front of me but natural gas has the highest condensation point of the normally used fuels. If I remember correctly, you need to keep the return temperature above 127 degrees to prevent condensation. Since the differential subtracts 10 degrees from the Low Limit setting, you would need to keep the Low limit above 137 degrees to prevent condensation.

Probably one of the worst things that you can do to a boiler is to have it running in condensation durring standby.

Ron

Robert_15

so if i have a return temp higher say of 135 on standby meaning when the boiler is not firing like during the day from soler gain is fine? so set the honeywell aq475 at 135 low and what about the high end say 165 to 170 to achive lowering the gas usage? or would it be better to have a longer span between the two say 180 high low 120 and the return temp never getting below 135?

John McArthur_2

What effect does excess air have on flue condensation? Logically, it seems that more air would equate to more moisture. Just learing the fine points of combustion testing.

Also, what range of excess air will cause a negative effect?

Ron Schroeder

Excess air dilutes the exhaust gasses. One part of the exhaust gas is H2O produced from combining the Hydrogen in the fuel with the Oxygen in the air. Diluting the exhaust gasses with air reduces the percentage of H2O in the exhaust, reducing the temperature that it will condense out. Going from 0% excess air to 100% excess air will lower the condensing point by over 20 degrees. Moisture in the air will raise the condensing point but only by about 5 degrees from 0% to 100% relative humidity.

While more excess air will lower the temperature where condensation will start, it will also lower the efficiency of the boiler.

According to studies at Brookhaven National Lab, condensation will start with a RETURN water temperature of 7 to 10 degrees below the saturation temperature of the flue gasses. The saturation temperature with natural gas at 70% ambient air humidity and 20% excess air is about 137 degrees so you need to keep the sustained RETURN temperature above about 127-130 degrees. The RETURN temperature will depend on both the boiler temperature and the delta T drop thru your baseboards.

The AQ475 works differently from just a regular triple aquastat. From the documentation, the Boiler Min Temp. doesn't seem to have a subtractive differential so what you set is probably the actual low limit.

Probably the best way to set the Boiler Min temperature would be to strap a temperature sensor on the return pipe just before it enters the bottom of the boiler and cover it with insulation. Set the Boiler Min to 130 and watch the return temperature on several calls. If the RETURN temperature goes below 127-130 for more than about 2 minutes, move the Boiler Min setting up a few degrees until you get sustained return temperatures of over 130 degrees after the initial drop in temperature at the beginning of the call. Also, the SUPPLY temperature should not drop below 127-130 on a long standby.

I would start with a Boiler Max Temp setting of 160 degrees. If the system can't keep up with the load on a very cold day, move up the Boiler Max a few degrees at a time.

How do you heat your domestic hot water?

Do you have an additional High Limit or aquastat on the boiler for safety? What type is it and what are it's settings?

By the way, Propane and oil will operate about 7 degrees or 15 degrees lower respectivly than N.G. before condensation.

Ron

Robert_15

Thanks Ron, this does make sence to answer your question i have a stand alone water heater & have a honeywell high - low aquastat set to 170 or so with a 30 deg. differce.also having a bypass i should be able to heat the house with 160 supply with the low at 130 without condensation in the coldest of temps say 5-10 f and not adjust the settings with the change of temp. outside?

Ron Schroeder

Hi Scott,

If I understand the way the AQ475 works, I think you would set the aquastat Hi Limit to at least 10 degrees above Boiler Max Temp on the AQ475 and the aquastat Low Limit to about the same as the AQ475 Boiler Min Temp. Then the AQ475 will do the temperature control and the aquastat will be primarily the required safety Hi Limit.

Ron

Robert_15

aq475

Hi Ron, i set the aq475 to 170 high 135 low with the outside design temp to 20f it seems to keep the temp on the boiler above 130 & the termastat set to 69f all the time. the circulator runs after the boiler shuts off.But if were to set the high to 180 the aq475 would find the curve for me & say burn to 160-170 for me? this is based on 33-35f avg.outside low is 20f here on L.I. NY.when i had the outside low setting to 10f the circulator seemed to run most of the time until the the low limit hit then the boiler would fire up to the 170f & that was based on outside temp of 26f
Thanks