Adding electric water tank to a combi boiler system

znack

I have an oil fired boiler the heats my hot water and baseboards. I don’t use the baseboards as I have wood heat and oil last winter was $150/month just for hot water for a 1 person household. I think it would be more cost effective to switch to an electric hot water heater. I want to keep the boiler as backup both for water and heat. Where do I install the shut off valves? Referring to the picture would it be just replace old shut off valve (#1) and install a new valve somewhere around where I have the arrow (#2)? I would obviously shut off main water supply to the boiler but would there be anywhere else I have to shut off to install the second valve? Also would it do any damage to the furnace if I shut off the power supply to it permanently except for turning it on 1-2 times a year to make sure everything still works?

Hot_water_fan

That’s an inefficient setup, but should be nowhere near $150/month. Something else is using your oil. 

znack

Canadian dollars by the way. Impossible to have anything else using the oil.

Hot_water_fan

What about a water leak? I’d expect usage 5-20% of that $150/month. It’s inefficient, but not that inefficient. 

znack

Nope, well pump would be running when it shouldn’t if there was a leak.

Hot_water_fan

I’d figure out why you’re using so much oil first. For reference, I use about 100kwh per month for two people using an electric water heater. That runs me like…$14 a month.

Jamie Hall

Don't be stubborn. That much oil usage, just for hot water, is simply not right.

Let's just consider the numbers for a moment. Assuming you are paying a insane price for oil, let's call it 40 gallons a month. That's 4,000,000 BTU per month, more or less. Now let's assume that your incoming water is at 50 F, and your hot water at the tap is 110 F. 60 degree rise. That works out to your using about 270 gallons of hot water every day.

Really?

znack

Guys I don’t know what to tell you. There has been no noticeable increase in water consumption over the 14 years I’ve lived here. I would see any leaks as it’s an an unfinished basement so almost all leaks would be seen. The boiler itself was replaced less than 10 years ago and again no increase in water consumption. The oil tank was replaced 4 years ago and it’s an inside tank so I would smell oil if there were any leaks. The only thing that has changed is the rising price of fuel and will further increase when the government adds in the carbon tax. So despite the fact that electricity rates are also increasing it will still be cheaper for me to switch to electric hot water. I just need a little guidance with regards to the plumbing aspect of the job so I can save some money.

Hot_water_fan

Let’s take a step back. How many gallons of oil did you use last year? If you’re truly using $150/month worth of oil, then you’ll probably end up using the same $150/month for electricity, so you would have gained nothing. 

Here’s how you compare prices: 
electricity: $/kwh x 1,000,000/3412 /efficiency 
oil: $/imperial gallon x 1,000,000/138,500 x 1.2 / efficiency.

So if you’re truly using 40 gallons per month, you can find out how much that costs in electricity. 

random12345

@znack I am going to disagree with the others here for the moment and assume that there is nothing wrong with your calculations. I’m in MA in the US and my last oil bill was $4.19/gallon, electric is $0.403/kWh. We use 1100 gallons/yr for heating and hot water. I have a new steam boiler with tankless coil. I’m curious what you pay for oil and electricity up there? How many gallons/month are you using for your hot water?

znack

Electric is 0.16354/kWh. Oil is $4.84/gallon. I don’t have any way of accurately tracking total water consumption/month but would guesstimate 40-60 gallons.

Hot_water_fan

It's less important whether oil is cheaper by 5% or more expensive by 5%, I think your oil consumption for hot water is just super high. Do you have a huge tub? A car wash? Are you positive the baseboard zones are off? Valves break, hot water could be heating your house even if the thermostat is off. I think switching to electric is fine, but there's probably another problem.

random12345

If that's Canadian dollars, your oil price is relatively high compared to electric. I'd consider either a heat pump water heater or electric tank if gas is not available. HP energy factor is probably around 2.7 or quite possibly significantly lower if it gets really cold in your basement. Looks like it's nicely insulated though. Don't know where you are in Canada either. Oil boiler with tankless coil EF is anywhere from 0.4-0.6, with the lower end being more likely since you're only using it for hot water and never for space heating. Electric tank EF is 0.945. It could go either way depending on hot water usage. In the grand scheme of things, at these prices, you might save a couple thousand dollars over 10 years after equipment costs and assuming you are doing the install yourself. If somebody else is doing it, I question whether it's worth it. Not in the trades myself, just a homeowner, sorry I can't answer your original question.
https://efficiencymaine.com/at-home/water-heating-cost-comparison/
https://bnl.gov/isd/documents/41399.pdf See Appendix 1.

znack

I looked at heat pump hot water tanks but the cheapest I can get is $2700 cad vs $800 for electric hot water tank and heat pump tanks aren’t as serviceable if something breaks on them. I do have to get an electrician to wire the hot water tank but am hoping to do the rest of the work myself. I just need confirmation of where the shut off valves go and whether it’s safe to completely shut off the furnace. If I am in fact loosing water somewhere it has to be in the boiler system so powering it off completely would eliminate that problem.

random12345

@Hot_water_fan makes a good point. @znack are those prices for USG or Imp Gallon? That would change the math on this. Possibly little to no benefit to making a change.

znack

Us gallon

Hot_water_fan

Fuel costs are roughly the same and they'll change by the time next winter rolls around anyway- instead of dropping $1000+, why not just fix the excessive usage? It could be something extremely simple! My suspicion is that you're accidentally heating your house as well.

random12345

He's spending $150/month for hot water with a tankless coil that has an efficiency of maybe 0.4-0.6, and possibly as low as 0.25 or lower if it's scaled up inside and out, which means only $37.5-90 are actually spent on the water, at those prices, that comes out to 7.75-18.6 gallons of oil so hot water usage could be 59-142 gallons/day. A little high but not impossibly so. Haven't even looked at the efficiency of the boiler itself...What do I know, maybe there is leak somewhere...

znack

All baseboards in the 3 zones are always cool to the touch so they are definitely not producing heat. I have never found any water pooling anywhere the water pipes run. I know there are no leaks on the hot water side because the basement isn’t finished and the hot water runs from the furnace room to the kitchen and another branch to the bathroom, both runs clearly visible. Shower, dishwasher and washer(run on cool water) are all run on average once every 2 days. Any other suggestions what could be causing excessive usage?

random12345

Might be worth getting an invoice from your oil company and looking at what got delivered and for how much.

Jamie Hall

I personally think @random12345 's figure for efficiency for a tankless coil -- even an old one -- is low.

That said, the oil use is excessive, I agree -- but the heat which it represents has to be going somewhere (or the oil is -- not impossible, but a not very comforting thought), and I would very much recommend that you find out where and fix that.

znack

Each time was to completely fill the tank. $Cad
Date Product Location Qty Amount
(tax incl.)
12/02/2022 Furnace Oil 1 60.4 litres $103.31

11/01/2022 Furnace Oil 1 117.0 litres $232.06

08/26/2022 Furnace Oil 1 345.0 litres $590.11

random12345

Looks like 15-16 gallons of oil/month. Doesn't seem excessive to me. Long hot showers? You could check the flow rate of your shower head and time yourself in there. Check the cold and hot water temps. If your coil is at an EF of 0.4, temperature rise of 70 F, 31 days/month, that's 46 gallons a day. There's no shrug emoji on here, but that's where I'm at.

Jamie Hall said:

I personally think @random12345 's figure for efficiency for a tankless coil -- even an old one -- is low.

@Jamie Hall I pulled that .25 EF from the Butcher paper I linked to above.

Jamie Hall

Agree there with @random12345 . Not excessive. You'll need the same energy from electriity -- and it's mostly a question oof which is more expensive per BTU.

random12345

Looks like this is your boiler? https://kerrcontrols.ca/products/third-product/ Manual says this about shut-down and startup: It seems you have a steel boiler. The potential concern for a cast iron boiler would be that if cold water is continually moving through the coil, and the boiler is off, that will cool the block down so much that humidity in the surrounding air condenses on its surface and over time that can apparently cause rust. I don't know if the same thing would happen with steel. Not my area of expertise, sorry.

EdTheHeaterMan

I believe that using the unused water heating source as a passive preheater to the operating heater or tank is always better than abandoning a tank or coil outright. Especially if you want to have the option of using that as an alternate source in the future. By having the tankless coil as the first stop on the way to the electric water tank, you will benefit from passively pre-heating to boiler room temperature in the tankless coil from say the 50° groundwater temperature to say 70° room temperature. Then the electric water tank will use less electricity to get the water temperature to 110° or 120° for standby/storage. This will also keep the water in the tankless coil from going stagnant from sitting there for months on end.

Likewise if you use the tank to preheat the water when you decide to use the oil as the heating source, you will find the same savings. Here is a post with such a piping design from last year. https://forum.heatinghelp.com/discussion/comment/1711484#Comment_1711484

EdTheHeaterMan

Here is the updated diagram for a tankless coil from the 2022 comment above

znack

Thank you

john123

@ed Could you just run the output from the coil directly into the input for the electric tank without any loops? It would be a lot simpler and would help, I think, when you have a smaller coil. All the electric tank has to do, is maintain the temperature. As soon as anyone opens a tap or more importantly when people open several showers at the same time, the coil starts heating the cold water coming into the house--a little or a lot- whatever- but any little bit or a lot will help the electric tank. And the advantage is that the electric tank provides a full tank of hot water to bridge the demand for those several taps that have been opened.