Can I lower boiler aquastat to 160 deg given terra cotta liner?

D107

While I'm waiting to get a mod con installed--hopefully this year--I'd like to lower water temp of cold start from 180 to 160. Given that this 40K loss/77K mostly CI gain is piped in series, delta T could be more than 20deg--I'll have to check that. If I keep setback periods to 3-4 degrees, that should help raise the delta T for recovery periods at least.

Chimney is inside house, inside dimensions are 7x11. Height is about 33ft including a 10ft portion in unvented attic (only floor insulated now) and 2ft+ above roof. Chimney sweep said current condition is OK. Was coal fired from 1924, converted to oil probably 1950s, then gravity converted gas 1981. My concern is for the chimney and boiler but 140 degree returns should be OK, yes? There's no bypass but everything except risers is 3/4 --including the 3 zone return manifold (!)so flow is slow--less thermal shock I guess. Since I had blown-in cellulose last March, I noticed thermostats overshooting.

PS This is pumped on return before PONPC/expansion tank.

Thanks,

David

Bob Sweet

David

Being at best 2X oversized, and probably more than that for the majority of the heating season, short cycling is going to be the problem with keeping the temps above condensing range. What are your emitters, fin tube, standing rads, radiant. I'd be careful to use set back on high mass systems like radiant, especially if low return temps are a concern.

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D107

of the total 78mbh load, 62mbh is cast iron rads

main and 2nd floors. Basement 17mbh load is slantfin 15. I was hoping lowering the water temp would lessen short cycling since it would take longer to heat. Even though rads are in series loop, all of them get hot, just takes a little longer for the last ones--those rooms are where the thermostat is. Can't use TRVs since it's in series.

Been using around 1400 therms a year for heat, direct hw, drier and stove for the 40mbh loss. I'd guess that with the insulation I'll save 20%--and probably short cycle more. Probably went from a .9 ACH to a .5 or .6.

Being cold start, the return temps would be low until last rad gets hot.

Thanks,

David

Bob Sweet

David

you may find your delta T is less than 20*, depending on gpm and ft/hd and might be able to drop your supply temps even further depending on size of your radiation/thermal improvements.

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D107

so by keeping tabs on the return temp I might be able to

lower supply to 150 as long as return temp stayed above 140, yes? That's if delta T was 10 deg. (that's of course after the initial cooler flow-through on start up.)

thanks,

David

Jim Davis_7

Flue Temp more important than water temp

Water heaters run with temperatures lower than 160 degrees all the time. As long as we maintain a flue temperature above 275 degrees condensation should not be a problem. This temperature is the temperature right before it enter the chimney not at the boiler. Proper set up of the burner and venting should allow us to run evern lower temperatures. Indoor-Outdoor Reset Controls allow cooler water temperatures. Making the On-Off differential 25-30 degrees helps with the longer run cycle and chimney heating also.

Jim Davis_7

One additional thought

A properly controlled mechanical room should keep a small amount of heat rising up the chimney at all times, especially with a boiler, which keeps chimneys much warmer. If a chimney gets cold during off cycles it is because our mechanical room is in a negative pressure. Resizing flues or adding liners does not fix the real problem but does a better job a hiding it.

D107

Understood. How low a return temp can I have?

Thanks for your reply. Yes there is a 'vent hood' do they call it that permits room air at boiler to rise up chimney. Probably better if it wasn't cold start but hate to spend money on this old boiler with reset controls, etc. An FAI would probably help as well but basement leakage is still probably adequate.

Somehow I had gotten the idea that water returning at too low a temp would not only shock the boiler but also there would be inherent potential for vapors around that water to condense in an atmospheric boiler and thereby damage nearby piping, breeching or flue liner. What I understand from your post is that the more harmful condensation takes place in the chimney from the chimney getting too cold between firings. Lowering water temp might just keep the boiler running longer.

So until I can change that boiler--need to re-do all the basement piping unfortunately for reasons I won't go into here--I would just ask if there's a return temp I should not go below to thermally protect the boiler? (there is no bypass.)

Thanks again,

David

Jim Davis_7

Venthoods can seriously lower flues temperatures regardless of what water temperature you run. The only way you can shock your boiler with return water is if your burner comes on and the circulating pump doesn't and brings back really cold water. If the pump turns on immediately or is continuous, it is impossible to shock your boiler. Domestic waters heaters fired with 45 degrees street water coming in all winter and they survive. To save your boiler and chimney it would be worth it to have some modifications performed. This usually can provide enough savings to pay for the work. Have to look for NCI contractor in your area. Check contractor locater @www.nationalcomfortinstitute.com.