Solar PV + cast iron radiators?

[BoilerinBrunswick]

Hi all,

I've moved into a house with cast iron radiators and I'm interested in replacing a heating oil boiler with an electric boiler. The plumbers I've talked to think I'm nuts and that my electric bills will be nuts, the solar people think it's doable, but I can't seem to find anyone that actually has this type of system.

I've calculated how many BTUs I used last year and converted that to electricity, assuming a 70% efficiency for the current boiler, which is probably generous, and it's about what I paid last year for oil in electricity, before I factor in the solar electricity. I'm planning on insulating the heck out of the place also, but the windows are newer, and the house seems surprisingly tight despite its age. The boiler recommendations I'm getting call for 3 x 60 amp breakers and that makes me a bit nervous about the electricity usage.

Here are a few numbers:
BTUs that warmed the house last year (heating oil) assuming 70% boiler efficiency: 48,475,000
Conversion to kwh: 15,423
Other home electrical usage: ~4000 kWh/yr
Solar potential: 10,500 kWh/yr

I think via insulation and replacing a few energy sucking hole appliances I can greatly reduce my energy usage. I will also probably put radiant heat (also hot water) under the main floor of the house and turn off the radiators on the main floor in favor of the in floor heating.

So, any chance someone on this forum has this type of system or has some anecdotal or electric boiler usage to share? Am I a fool? Will I be sorry if I do this?

 

[goosegunner]

Also check with your utility and see if they have Load management for electric heat. Our utility cuts the rate in half if you go on their load management program. The last time I checked it was equivalent to use Lp at $1.25 a gallon. Not horrible by any means.

 

[Highbeam]

Lots of electric boilers out there. Especially in places with cheap electricity.

I think for sizing your eboiler you would want to assume a very good efficiency from your oil boiler. The risk is undersizing it. The eboiler will modulate down or cycle and deliver 100% efficiency.

 

[maple1]

I have one. For backup. I think it would cost about $25/ day to heat full time with it in the winter.

Does your utility have off peak rates? If so you could see big savings with storage and a timer.

And what size boiler are you looking at? Mine is 18kw and it's enough. It's on 1 100a breaker.

 

[BoilerinBrunswick]

No special utility rates for me 0.12 cents is the only rate for my utility. I initially thought I only needed ~61,000 BTUs (18 kw) , probably about the same size as yours (maple1 do you have cast iron radiators or some other type of radiant heat?), but my plumber thinks I need more like 100,000 BTU or the boiler will run all the time and never supply enough hot water. I do have some monster cast iron radiators on one floor but the radiators upstairs are smaller.

 

[Bob Rohr]

No special utility rates for me 0.12 cents is the only rate for my utility. I initially thought I only needed ~61,000 BTUs (18 kw) , probably about the same size as yours (maple1 do you have cast iron radiators or some other type of radiant heat?), but my plumber thinks I need more like 100,000 BTU or the boiler will run all the time and never supply enough hot water. I do have some monster cast iron radiators on one floor but the radiators upstairs are smaller.

A heat load calc would be a good step to narrow in on the actual load. slantFin and Turnham have free ones online.

Then use one of the online fuel cost programs to see the operating cost difference.

Big difference between 61 and 100K, one of predictions, or both is way off.

 

[maple1]

No special utility rates for me 0.12 cents is the only rate for my utility. I initially thought I only needed ~61,000 BTUs (18 kw) , probably about the same size as yours (maple1 do you have cast iron radiators or some other type of radiant heat?), but my plumber thinks I need more like 100,000 BTU or the boiler will run all the time and never supply enough hot water. I do have some monster cast iron radiators on one floor but the radiators upstairs are smaller.

I have run of the mill Slant Fin rads. Rather ordinary. Your cast iron rads should be a lot more capable than those, and should perform better with lower supply temps.

Type of heater/radiation is kind of irrelevant though - heat load is what counts. I'm in a conventional construction 20 year old two storey, 1500 sq.ft. down & 1200 sq.ft. up (with unfinished/unheated 1500 basement), on an open hilltop in a somewhat moderate Canadian climate. 100,000 is a pretty big heat load. For a bit more comparison, my wood boiler is 40kw/hr rated (translates to 120,000 or 140,000 btu - forget which), and it only burns around 6 hours a day average during the winter. I've been in Maryland before - isn't that a fairly warmish climate? I am kind of thinking the 100,000 figure is way out there - makes me suspicious of the plumbing advise you have been getting. Do these plumbers also do much heating for a living? Have they done any heat loss calcs? It's one thing to plumb components together - quite another to be able to figure out what heating components you need, and get it right.

[Napkin scratch math with your numbers: 15,423 kwh for a heating season over 6 months is only an average of 85kwh/day. Which is only 5 hours per day of operation for an 18kw boiler. Bounce that off your plumbers & see what they say].

As mentioned my electric boiler was only put in for backup - cheap & easy install, takes up next to no space. But is quite costly to operate, makes the meter spin pretty good. So is not a full time solution for me, and I am always mulling over what my next step will be when doing wood gets to be too much. I did have oil once, going back to that is a possibility again with oil prices being downish for a while now. What is the reason for wanting to replace your oil? I can think of some but not sure what your case would be. Any need or want for a/c in the summer? I would be tempted to investigate just leaving all the oil stuff in place & install some heat pump units - of the mini split variety. Given where you live (although still being a bit uncertain of your climate). Doing that is at the top of my 'what next' list, right now. Heat pump tech is also showing some promise for air to water units that might be able to use your existing system - I am waiting to see how that develops in the coming near future. But that won't do a/c.

 

[tom in maine]

Here is, perhaps, another option.
Using an air source heat pump that makes hot water (antifreeze).
There are several out there, one company near you is Chilltrix. We also sell one at (broken link removed)
Thermostatic control valves on each radiator and a heat pump (or two) might work well.
The number of units depends on the heat load of the building.
You will get 3x the performance of electric resistance heat and can still use PV's.

 

[Fred61]

I was thinking about this discussion while doing some yard work on this beautiful day only to find when opening that my thoughts have been offered by the geniuses here. Bob suggested a load calc. which entered my mind since I didn't recall the size of the space being heated nor whether it was mentioned at all. Numbers seemed high for a home in Maryland.

With the heat pump technology advancements today as suggested by Maple, I would not install any resistance heat. I would go with the solar and use heat pumps to supplement the oil.

That's what 'm doing. Installed 6 KW array last Feb. and am looking at a credit of $240.00 today. I've been using the mini splits on the cold evenings and morning and I can't even see a blip in extra usage on the graphs supplied online by my power company. Still making enough power to power the mini splits and all other electrical needs. It hasn't tapped the bank yet. After being spoiled with several years of in-floor radiant I can't say that I like warm air heat but it's free.

Several good alternatives offered by Tom.