Hi,

Hopefully an easy question. Probably a dumb one, but I want to get this right. I'm looking at electric boilers and trying to understand how the manufacturers quote energy usage. If a unit is described by "50,000 Btu/hr" and "27 Kw," does this mean it consumes 27 kilowatts for every hour it runs at full capacity?

Thanks.

 

50,000 btu/hr equates to aprox 16 kwh. I have no idea from where the 27 kw is coming.

 

There are approximately 3400 BTUs per kilowatt so a 27 kilowatt boiler would put out approximately 90,000 BTUs (assuming about a 2% loss in energy conversion) for every hour it was operating at full capacity. It would consume 27 kilowatt hours for every hour it was operating at full capacity.

Note that there would be a slight amount of heat (BTUs) lost to the surrounding atmosphere through the boiler insulation and also some losses from the connecting piping.

A boiler rated at 27 kilowatts or 50,000 BTUs per hour is obviously not able to run at 100% capacity indefinitely.

 

Okay. Thanks for those clarifications! I think I can go back to and properly evaluate the specs now.

 

A nice rule of thumb is to calculate the heat load in MBH and then divide by 4.

I think most here would agree that the actual heat load calculations are overstated. My guess is that the true heat load is around 70% of what the Manual J calculations say.

So to still be conservative, I'd split that 30% difference in half. So assuming that the true heatload is 85% which gives you some play, the math is simple. 3.413 MBH/KWH / 0.85 = 4

If the Manual J heat loss calculation is 60 MBH then you need a 15KW boiler. Likewise a house with a 40 MBH heat load would require a 10KW boiler.

 

Ok. I think I have it. That's pretty handy.

So if I'm replacing a ~40 year old hydrotherm boiler that is supposed to be capable of 100MBH (in a house that has had insulation and new windows added), then I'm going to need a 25Kw electric boiler (or less).

Is that right?

I'm also realizing that the efficiency of the electric unit, aside from the conductive losses Furd mentions, are near zero. Hadn't thought about that.

P.S. I'm realizing that I mixed Kw and BTUs from two different units in my original post. Sorry to Grady and others for garbling the facts --didn't fool them, though.

 

Your existing boiler probably has a combustion efficiency between 70 and 80 percent. A new, non-condensing boiler would be around 80 to 84 percent and a condensing boiler around 90+%

These are NOT the AFUE efficiencies that you see in the advertising but real-world efficiencies of the combustion only. From these you need to subtract the losses of start / stop operation, radiation (from the boiler to the ambient air) losses, various standby losses and, of course, your piping losses.

The truth of the matter is that in the real world you will be doing quite well to achieve an overall efficiency in the high 70 to low 80 percent range with any fuel-fired boiler.

The electric boiler, while also having the radiation and piping losses will not have the stack losses and the conversion loss from electricity to water is minimal; about 2% as I stated earlier. This apparently higher efficiency is definitely offset by usually higher costs for electricity than for fossil fuels.

Depending on the type of radiant floor system you have you may be able to run water heated by only solar panels and have it work quite well. A standby electric boiler may work well for those times when solar doesn't supply enough heat by itself.

As always, the first order of business is to close up the building envelope to the point where it needs the least amount of supplemental heat, regardless of the source of that heat.

 

Your boiler rating means nothing. You need to do a heatloss calculation.

 

Hi again Furd,

Your point about sealing things up is well taken. The windows, for instance, have made an enormous improvement. Me and caulk are starting to get a reputation...

So do this mean that because the electric boiler will likely be substantially more efficient than the natural gas one, I could get away with a smaller boiler still?

You mention the solar thing (we've been chatting about that aspect in a separate thread). I'm thinking about installing photovoltaic panels (PVPs), as opposed to passive/hot water solar panels. To size the PVPs properly, I need to figure out how many watts this new boiler will need. An important detail is that I definitely do NOT need to meet the peak demand of the boiler. We have net metering that balances PVP-generated watts I put onto the grid with the watts that I take off the grid. This occurs ANNUALLY, which I find pretty interesting. I can use the summer watts to power the electric boiler in winter (if I'm understanding the slightly vague answers I'm getting from Xcelenergy.com). So all I need is to size my PVP system to meet an annual budget. I'm still a bit fuzzy on how they work the accounting if they're still charging me on a monthly basis.

Is it right that to get an estimate of the KwH the electric boiler will consume, I shouldn't just convert the therms of gas used into KwH? I'm guessing I need to include the math behind Furd's points about combustion efficiency to accurately figure what will be a lower number of watts. Have I got that right?

Thanks for that, Furd and everybody else. I've been doing a lot of reading, just awash in facts. These conversations are really helping me get some resolution on things. My head is still spinning a bit.

 

At $4.50+ per watt good luck...

Someday it will be feasible.

 

My area of expertise is with commercial and industrial sized hydronic heating (and a few related areas) so my answers to residential systems must be understood to be more theoretical than actual practice.


Residential boilers have several different methods of rating. I think the one that is most useful on fuel-fired units is the D.o.E. (Department of Energy) rating. Obviously the usable output is what you are interested in.

An electric boiler is usually rated by its electrical input in instantaneous kilowatts. Since a kilowatt is by definition 3412 BTUs and an electric boiler is about 96 to 98 percent efficient (efficiency equals energy out divided by energy in) you can take the kilowatt rating and multiply it by 3400 to come to a reasonably close approximation of the BTU output.

As Who stated, the figures obtained by a standard heat loss calculation are a bit high and can usually be reduced. The better you can assess the integrity of your installed insulation and air infiltration along with finding a design outside temperature that is representative for YOUR house the closer the calculation will be.

The first thing to do is to figure the heat loss for your house as it stands today. Then go back and change the numbers for insulation and air infiltration to represent a best case scenario and calculate the cost to reach that best case. This is, as Xiphias calls it, "fuel you buy only once" and will continue to save you money as long as you live in the house.

Since this (or was it the other) thread started with a question related to radiant floor heat I have suggested that hydronic solar collectors are the way to go. Radiant floors, if the tubing is embedded in a heat sink or lies just under the finished flooring (i.e. Warmboard type systems) utilize relatively low temperatures that are easily obtained from the solar collectors.

On the other hand, retrofit radiant floors where the the tubing is located underneath the subfloor in a retro-insulated joist space require temperatures much higher and are really marginal for solar collected heat.

The above are, of course, design considerations and while important do not address the fiscal considerations.

Who stated a cost of $4.50 per watt. It has been several years since I did any pricing of PV systems but I think that today that price would not cover the installed costs. Probably something like $6.00 per watt generated on average would be much closer to real out-of-pocket costs for this project. At that rate an averaged one kilowatt system would be $6,000.

When figuring the output of a PV system you cannot go by the advertised output but MUST use an averaged output. This takes into account cloudy days, less than full sun days, the occasional cloud, abnormal weather, less-than-ideal orientation of the PV cells, inverter inefficiencies and the like. Generally speaking, a figure of 75% of the advertised output is going to be much closer to the realized output over time.

You must also factor in that PVs do deteriorate over time so that the output when first installed will be higher than the output ten years later. Dirt on the cells WILL have a tendency to lower the output and inverters do NOT last forever.

The main thing is that few people have the money to install a PV system just lying around in a can under the back porch and therefore need to borrow the money. The cost of this money (the interest on the loan) is a very real cost that must also be amortized over the length of the loan AND the life of the system.

Photovoltaic is a fascinating topic and perhaps someday it will become a viable source of electricity but in my opinion that day is still quite a long ways away for the average homeowner.

 

See my note in the other thread about PV vs. solar thermal.

Another thought: your PV system will need batteries. Probably lots. Otherwise, it will be difficult to run an electric boiler at night, when you most need the heat. Batteries add a whole 'nother level of complexity and cost. Particularly compared to a simple large thermal storage tank.

Do you plan to sell back PV-generation during the day, and draw from the grid at night? If you had time-of-use pricing, that might work. But you'd also need a low draw during the day, and time-of-use buyback rates to make this worthwhile.

I don't get how a PV-powered electric boiler makes sense for space heating. I like learning, though. What's the thinking here?

 

furd, something to keep in mind about using solar panels for domestic heating is that if you could ever harness enough energy in the winter to heat much of the house you would then have a very tough time dumping the even greater loads you'd have during the summer unless you start covering up panels.

Many people think that the grade of energy you get from solar now gives its best bang for the buck and for the material used by preheating water going into the domestic. Even with evac tube unless their price really falls and summer BTU gain can be better controlled.

The sun packs a mean punch. We just don't always know how to fully absorb it or deflect it altogether.