Pacowy wrote:My advice would be to include plenty of excess boiler capacity above whatever the computations show is the load corresponding to your installed radiation. As discussed in other threads, many factors can cause coal boilers to come up short relative to their nameplate capacity (e.g., substandard coal, boiler in need of cleaning), and if you don't make enough steam to build a positive pressure in your system you are likely to be quite unhappy with the results. This issue is compounded if the unit is going to produce DHW, support hydronic loops or other future expansion, run with the jackets off to heat the basement, etc.
Above and beyond making sure the system actually works, oversizing the boiler also may have efficiency advantages because big boilers tend to have larger heat exchange surfaces relative to smaller boilers. Using EFM's as an example, an EFM 900 has a heat exchange area of 59 s.f., while an EFM 1300 has a heat exchange area of 80 s.f. Even if you didn't need the full output of a 1300, you likely would use less coal to make the steam you need by running the 1300 at a jog rather than running a 900 flat out.
Our last 2 houses have been heated by steam with stoker boilers that can handle all of our normal btu requirements running at 60-70% of their rated capacity. Prior to that, we had an underpowered steam system. I vote for the excess capacity.
Hi Mike, I have a concern with oversizing for steam heat.. especially with a boiler where the water can cool down quite a bit.
Since an oversize boiler will not only have more heated surface [good for heating the water] but it will ALSO have a lot more water...
So when on a fall or spring day.. or virtually any day this winter.. the boiler sits and idles [assuming it's not set up for DHW with an aquastat keeping the water hot].. then the water cools quite a bit.. then the house thermostat calls for heat. and it takes a LONG time to get the water up to making steam, and once it makes steam,, the thermosat is satisfied easlily,, since it's warm outdoors.
It seems to me that an oversize boiler will cost a great deal more in fuel and 'wait for steam' time than a 'correctly' sized boiler ???
I've quizzed 'Townsend' extensively about his AA260, and it sits and idles.. he doesn't have a timer on it yet,, and the water seems to stay at 190* just from the fire in contact with the water jacket around the firepot..
I dont think a keystoker or EFM with a idling fire will keep the water that warm,, and the design of the Keystoker and EFM stokers, will also require more time to generate a full fire to create steam.. ?? so there will be considerable lag before steam and heat for the house is created.. and even MORE lag with a significant increast in water int he vessel??
Please correct me if I"m wrong,, maybe the steam boiler would be set up with an aquastat to keep the water nearer to boiling? so it has less lag? I know that if you have a DHW coil, it would have to have a minimum setting to provide hot water..
Anyway,, just thinking and wondering..