Residential one-pipe system cost control question

McKern

In our circa 1920 condo building (6 units/3 floors, plus garden unit) a programmable thermostat on the first floor controls the boiler in the basement. Assuming the system is properly functioning and that the unit with the thermostat is as weather-tight as possible, will heating costs be significantly effected by, for example, some missing storm windows in the other units?

One technician says yes because the temp of the water returning from a colder unit would be lower and, therefore, would cost more to reheat.

Another tech doubts that there would be a perceptible difference in the bill, noting that even the most careless condo owner/occupant will--for the sake of comfort--ensure that the temp stays above 50-55 degrees, adding that the water doesn't linger in the unit long enough to cool down.

Other thoughts, opinions, and/or the definitive answer?

Brad White_9

Count me in with the others....

Depending on what your definition of "some" storm windows which are missing and assuming that the prime windows are relatively tight, you should not see much difference. The biggest wildcard is air infiltration.

Hard to quantify but if a window has a 1/16" effective crack around just the lower sash (say 30 x 30 inch sash), that is 7.5 square inches open between the great outdoors and your underwear. That hole is as big as your fist.

A little wider a crack and no weatherstripping will increase this accordingly.

The other part is glass transmission. A 30 x 60 inch double-hung window with triple-track storm windows (at 70 degrees F. inside and 0 degrees F. outside), will transmit 595 BTU's per hour by transmission. Without the storm window this is about 910 BTU's per hour, a difference of about 315 BTU's. That is worth about half a section of a 38" high 3-tube steam radiator. See how much radiator you have left? Plenty I would hope!


As for the retention time and colder condensate, pay that no mind. It ought not linger in the radiator and you could probably not measure the difference in return temperature from one apartment to the next. Besides, the greater capacity of steam to heat is the latent heat or heat of evaporation (what it gives up to serve you). That is about 960 BTU's per pound. Your condensate if heated from room temperature of 70 to the boiling point requires only 142 BTU's per pound. How exciting. Chances are the condensate returns at about 180 degrees anyway, so only 32 BTU's per pound. Alert the media...

Not that you asked, but I would suggest investing in TRV vent valves for the radiators and perhaps a Tekmar 269 steam control setup, rather than relying on one unit having a thermostat. They should never go on vacation you know!

Insulating your mains, venting well and lowering the operating pressure into the range of ounces not pounds, are all worthwhile things to do if you have not already done so.

McKern

Thanks, Brad & follow-up re residential steam cost control

Thank you so much, Brad, for your helpful and very thorough answer to my question.

RE your suggestion that we invest in TRV vent valves for the radiators and perhaps a Tekmar 269 steam control setup: the Tekmar system appears to be more than we can afford at this time. RE the TRV valves in particular, as I understand things (this is all new to me, BTW) these valves would allow unit owners to control overheating by shutting down individual radiators when the room reaches the temp on the thermostat.

Would this also (in theory, at least) result in lower fuel bills, or is this basically a more precise and elegant substitue for covering the radiator with a big ol' quilt?

Would the fact that the thermostat is set at 68 during the day, 62 at night ("too cold" is a bigger issue than over-heating) effect your recommendation regarding the TRV valves?

Finally, are "varivents" a kind of TRV valve? Last year a couple of unit owners purchased these for their radiators and installed them independently.

Great website--I appreciate the assistance. I'm wondering how many questions I am allowed to ask before I need to apply for an apprenticeship?

Patrick Mullaney

buy a brick

then ask lots of questions...

I will be corrected if I give bad advice, but on a steam system, it is better to leave the thermostat at a constant temp setting. I cannot give you btu transmission rates to back up that statement, it is from experience.