Steam heat efficiency

Steve Garson

All things being equal, i.e. an 85% efficiency hot water boiler at 180* with no outdoor reset or a 85% efficiency steam boiler, will they both consume comparable amounts of fuel? In my reading, it seems like steam has a better ability to transfer its heat. But the HW boiler needs less water to accomplish it's goals.

I would like to understand the physics of this process, if anyone is versed in it. A steam system is so much simpler once it works correctly.

Constantin

Efficiency of what...

... is the real question. You seem to be referring to the AFUE rating, which is a arguably arbitrary test condition that your boiler may or may not experience most of its lifetime.

Not all home heating systems need to generate water at 140°F+, not all of them benefit from low temperature return temperatures, etc. What about electrical energy consumption? Hot water systems require pumps, many steam systems do not. What about the control strategy (outdoor reset, for example), modulation, standby losses, etc.?

To give you but one example, I think it was Scott Milne that mentioned his Vitola ran at 3% over the AFUE rating when it was attached to radiant floor system. We've had several reports here of low-mass, condensing, modulating gas boilers saving their owners between 25-40% in fuel over the CI boilers they replaced even though the AFUE rating only indicated a 14% advantage between the boilers.

So, I think it all depends on the system and what it is meant to do. A boiler is but a part of it.

Steve Garson

My question pertains to my home which is now heated with steam. I've taken what I have learned on the wall and gotten it to work very well. Now the decision to install a modern steam boiler to replace the 70 year old boiler.

My radiators are thin tube with enough EDR to heat with hot water if I install new feeds and returns. But the required water temp will be higher than a condensing boiler would likely be most efficient.

So the question pertains to replacing my steam boiler or putting in the investment to install new plumbing to feed my radiators with water.

Uni R

Condensing iron rads...

Steve, don't forget that not even Weezbo has many near design days all winter long and the shoulder seasons may be quite long depending on your you live. Additionally it must be remembered that with a modulating system, the water is being heated more to an average required maintenance temp rather than cycling up and down between limits and therefore will be lower on average. The efficiency that you think will be out of reach may not be. Have you done a heat loss?

Steve Garson

Yes. I've done the heat loss and calculated the EDR of my present radiators. That's how I determined that the thin tube radiators would do the job on a design day with 180* water.

Constantin

I doubt you'll pay off a conversion on economic grounds...

Several luminaries here have shown how converting from one type of heating to another is usually economically not justifiable, unless you take a very long view. Frequently, the above-ground steam piping lasts forever yet is found to be unsuitable for HW use due to the higher pressures that HW systems exert on the piping.

Thus, it is my understanding that many steam systems undergoing HW conversions require a complete re-pipe. If the walls in the home are open anyway (gut jobs for example), the marginal cost of replacing the piping with PEX is pretty low, otherwise, things get expensive rather quickly unless you do it yourself. Very efficient HW appliances may also require a new chimney liner, new infrastructure (gas pipe) etc.

I would take a hard look at how much each option will cost vs. what you expect to save in heating costs. Even if you take a optimistic 40% reduction in energy consumption, the payoffs for insulating and weatherizing the home are usually higher. I would start with the envelope and work backwards. Get the house tight, well-insulated, then consider your heating options.

Steve Garson

My house is as insulated and sealed as is possible. Sounds like a new steam boiler is the way to go.

Steve

t. tekushan

steam boiler

I, too would opt for the new steam boiler. A new steam boiler will have much less water content than your old one so that bringing that small volume of water to a boil is not a big deal. Conversion to HW is a means of downsizing. If you balance the steam system well so that all radiators heat at about the same rate, they need not reach full saturation to heat the house. One of the things I really like about steam is that the use of a set-back thermostat is very practical because of steam's ability to recover indoor temperatures very quickly and efficiently. When the system runs long enough to purge all of the air from the system the vacuum created within the radiators frees up the pick-up load allowance in the boiler sizing to be used for heating.

Personally, being in a building or home where the steam heat is going full bore to recover indoor temps is a thing of awsome beauty, and knowing that its at peak efficiency while doing so is the icing on the cake!