Boiler vs forced air – energy efficiency
I own two rental buildings:
Building #1: 3 units – 1up/2down . Approx. 1100sf footprint. Flat roof with 4″ rigid board insulation and epdm membrane. No insulation in walls. New storms but old windows. New exterior doors all the way around. Stucco exterior.
Built in 1925. One gas fired (converted) boiler. Hot water heat. Boiler is original. If the efficiency is 60%, that’s pushing it.
Building #2: up/down duplex. Approx. 950sf footprint. Gable roof with some rock wool in attic. Some insulation in walls. Stucco exterior. ALL new thermopane windows. All new exterior doors.
Built in 1900. Installed new Ruud 100,000BTU 80%+ efficiency gas fired furnace last summer.
Why, oh why, do my heating bills for Building #1 consistently run 1/3 less than for #2. Same city, same therm gas cost. Thermostats at both 68-69deg F.
Is a hot water boiler (even an ancient one), so much more efficient at heating a space than a forced air furnace?
Thanks all.
Replies
I would guess that air infiltration and occupant habits are the big swingers here.
The old windows with storms are likely to be better than the new thermopanes, depending on installation details.
And it's not unusual to have an occupant who always has the window open (perhaps because the building is unevenly heated).
Another thing to check is whether the forced air system is causing air to be drawn in from outside, or is leaking air to the outside. Any ductwork that isn't in the conditioned space needs to be carefully sealed and possibly insulated.
1. How sure are you of the thermostats? If the your renter has control of the temp it might be 68-69 only when your are on site. (I've had this problem)
2. 1900 built balloon framed and more air leaking out the top. The air tightness of the buildings is the most likely player in your fuel bill.
3. One thermostat in 1900 built duplex might be in the cold room and others are at 75-80. If the thermostat is on a cold wall the room could be warmer. (I've seen this one too)
4. Check the boiler's rating. It might be more efficient than you think. There should be a plate on it with an input BTU and output BTU do the math and see what it is rated. (some will have different outputs for steam and hot water)
Hope that these will give you some answers
Bill
Thanks for the thoughts. The thermostat is on an interior (not cold) wall in the lower unit. The tenant in the upper unit has mentioned on occasion that it is warm up there (as in too warm). So now I'm wondering?
Should I run a t-stat wire up to the second story unit and place the thermostat up there (instaed of the lower unit)?
I may be off on the sf of the duplex, but the furnace was sized by a heating contractor who ran through some calculations based on sf of structure, age, windows, insulation, etc. This building is in Minnesota.
Just installed a 70K furnace in our Rochester area home, roughly 1900 sq ft. Could have gone with 60K, but the brand we bought bottomed out at 70K. But our house is 1970s with significant "tightening" since then. I wouldn't be surprised if 100K was required for an older building.But the reason for that higher requirement is largely air infiltration differences (though insulation -- which you say is lacking -- is important). Tight construction didn't really take off until the 60s or so when insulated windows became common, making other infiltration problems relatively more significant.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
100kBTUs for a 1900 sq foot home?
You're probably close to double the size you need to be. That's not good for efficiency at all.
Don't get me wrong, I'm a wethead for sure, but I wouldn't want to compare such a stacked deck. And hey, your boiler may be grossly oversized too, who knows? but that's what jumped out at me first.
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
So you're guessing that a home built in the 1920's is going to perform at a 25 btu/sf level? I would disagree. For either of these homes, if they were in the midwest, the heating plant would be well sized at 50 btu/sf, if they are constructed similar to other buildings of that era with which I have direct experience. Your ROT doesn't apply to old buildings.
Good catch Tim, for some reason I missed some of the building descriptions. You're probably right.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
There is a world of difference between a hot water baseboard/radiator system and a forced hot air system (FHA). Hydronic uses a heat sink - baseboard or radiator - so that once it reaches room temperature and stops circulating, you still have the "heat sink" in each room still radiating heat. This gives a more uniform, less drafty heating and folks can be comfortable in a lower termostat setting.
FHA systems do not have a heat sink - they are either blowing hot air or they are off - heat comes in bursts, creating drafts/air currents that are felt once the blower stops as room temperature is met - and folks then turn up the termostat a little more to keep that warm air flowing.
That's why hydronic systems are really considered the best - but they are also the most expensive to install and do require some plumbing maintenance like periodically "bleeding" the system for trapped air, circulator pump maintenance, etc.
And don't get trapped into comparing thermostat settings - the typical wall stats can vary plus/minus 3 degrees from the acutal temperature. And your estimate of 60% efficiency - is that from the boiler maintenance person? The service person has all the equipment to check out your boiler from draft to overall efficiency.
For a boiler built in 1925 60% efficiency is amazing.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
Hard to say - there a lot of old timers who love the old boilers, castings better than today some say. And it's really the burner that is important - no metion of how old that is - a fairly new burner well tuned with a solid old boiler can do a good job.
DIA,
What other appliances use gas in these buildings? You're missing something.
The comparisons should be as you expect. Either there is a great difference in the occupant's use of heat, a gas leak, or a great deal of heat loss in the smaller buiding your not accounting for. It doesn't make sense for a reason. I can't tell you what that reason is without a thourough survey of the building and its use. None of the other experts here can either. In most detailed energy survey/investigations that I have performed there is something going on, or some configuration that shouldn't and when corrected, real energy consumption will approach the expected.
Tim
Edited 5/2/2006 7:56 am by Tim