Add tankless natural gas to existing electric tank water heater

[Raj Jas]

Hello everyone,

I current have a 240L electric hot water tank and am using roughly a whooping 70kW/h a day, because of this high usage anything electricity usage above 1600 kW/h in a 3 month period is being billed at $0.15198/kWh. On the other hand Natural gas is being billed at $13.264/GJ (33.41 / 2.5 GJ) because of this I was thinking about switching to a gas hot water heater, but am also curious about the possibility of using a tankless natural gas hot water heater to feed the existing electric hot water heater. Any input would be greatly appreciated.

 

[Standardairconditioner]

Your idea sounds feasable from an industrial point of view, where we prime up cold water from the tankless, then letting the electric hot water heater tank do the rest and also store it, but I have never seen this set up among Gaz Métro setups in my town.

Best case scenario, your idea won't match the efficiency of a modern gas hot water heater tank. It might get close, most likely be less efficient, but it won't match nor exceed it.

If natural gas is indeed in your neighborhood, I would take advantage of that. Natural gas doesn't have full area coverage, those that desperately want it, may never get it.

Nothing heats faster than something that "burns" and NG is going to stay very cheap as long as we still have oil (that is another topic all together).

 

[Sponsor]

 

[Stuff]

That is a large tank but you must have other large electric loads. A tankless gas water heater feeding an electric tank seems like the worst of both unless you are in Florida. For most people a natural gas tankless would not warm the water enough so you would still rely on the electric tank to bring it up to temperature.

If you do anything then get rid of the electric load and just get a traditional gas tank water heater.

 

[Dana]

If the high hot water usage is in the form of showers (rather than tub baths), you'll probably get more value out of a gravity film type drainwater heat recovery heat exchangers than from converting to gas:

Key factors for getting the performance out of them is that they must be installed vertically, and the output of the heat exchanger needs to supply both the cold side to the shower and the cold feed to the hot water heater. It's often easier to just let it feed the entire cold water distribution system to the house. The output of a ~50% efficiency heat exchanger will be roughly room temperature or a bit higher, which isn't usually a problem for other cold-water draws while someone is in the shower.

Most of the manufacturers are Canadian, and most will sell retail-direct from their website. Natural Resources Canada maintains apples-to-apples third party tested performance numbers to compare one vendor's model to another's. The tallest and fattest one that fits usually has the fastest pay-back, since the higher performance is better than the marginally higher price for the bigger unit.

If hot water use is high because you are filling spa-sized tubs drainwater heat exchangers won't do a thing for you, since the drain has to be flowing concurrently with the potable water for the heat exchange to happen.

 

[Raj Jas]

Thank you everyone for your help.

I think the best option for me going forward will be to install a Natural Gas tank to replace the electric tank which is now about 10 years old. The current existing electric tank is 284L, what size natural gas replacement would you recommend? I am thinking a 40 Gal tank would work for me. What would you guys recommend as a replacement tank, is there a specific model you guys would recommend. Also what do you guys think about ordering online?

 

[Dana]

To come even close to making a recommendation I'd want to know:

How many people live in the house?

How many bathrooms with showers or bathtubs?

Any large soaking/spa type bathtubs? If yes, what volume?

Do you heat with a gas boiler?

Is the hot water heater in a basement, or somewhere else?

Do you have a basement that uses a dehumidifier?

What is the price of electricity below that 1600 kwh in 3 months price break point?

If gas, is there already gas lines plumbed to where you would install the hot water heater?

A heat pump water heater with a US DOE EF test rating of 3.0 uses less than 1/3 the amount of electricity of a standard electric hot water heater, and will be cheaper to install than a gas water heater assuming the gas lines are not already there.

 

[Standardairconditioner]

Also what do you guys think about ordering online.

Have fun re-boxing it and paying it ship it back when they ship you the wrong one or missing parts or incorrect fittings or the fact the Ups guys will have dropped it on hard concrete about 250 times as it is roughed between New Mexico to Canada. But that's what warranties are for.

 

[Raj Jas]

How many people live in the house?

There is two people living upstairs and 2 people living in the first floor suite. At max 5-6 people would be living in the house at the same time.

How many bathrooms with showers or bathtubs?

Three bedrooms and two full bathrooms and two bedrooms with one full bathroom downstairs. So five bedrooms and 3 showers in total, also there is a dishwasher and laundry upstairs as well a laundry downstairs.

Any large soaking/spa type bathtubs? If yes, what volume?

Nope.

Do you heat with a gas boiler?

Natural gas central forced air.

Is the hot water heater in a basement, or somewhere else?

284L electric tank is located downstairs in the utility room next to the natural gas hvac system.

Do you have a basement that uses a dehumidifier?

No, dehumidifier in the house.

What is the price of electricity below that 1600 kwh in 3 months price break point?

Directly from my bill:

Bimonthly Residential Conservation Rate
Rate Period 01/12/16 - 03/11/16 60 days
Basic Customer Charge 31.23
Energy Used Block 1: 1,600 kWh @ $0.09845/kWh 157.52
Energy Used Block 2: 2,582 kWh @ $0.15198/kWh 392.41


If gas, is there already gas lines plumbed to where you would install the hot water heater?

Yes i believe there is, I have included pictures of my setup.

https://imgur.com/a/bnPsY



Also there is the following rebate available so it would be great to get a qualifying tank:

https://www.fortisbc.com/Rebates/RebatesOffers/EnergyStarWaterHeaterProgram/Pages/default.aspx

 

[Dana]

A typical gas fired tank will average maybe 65% efficiency after flue & standby losses, for every GJ of gas going into the burner only 0.65 GJ ends up as usuable hot water.

A typical electric tank averages 90% after standby losses, so for every kwh going in, only 0.9 kwh ends up as usable hot water.

A heat pump water heater with an EF of 3.0 delivers 3 kwh worth of usable hot water.

Normalizing to 1000 kwh of hot water, it takes 1000/0.9= 1111 kwh to deliver 1000 kwh of usable hot water heat. At the marginal $0.15198/kwh that costs ($0.15198 x 1111kwh= ) $152.

With the heat pump hot water heater it only takes 1000/3.0= 333 kwh to deliver 1000 kwh of usable hot water. At the marginal $0.15198/kwh that costs ($0.15198 x 333 kwh= ) $50.62

A typical gas fired tank will average maybe 65% efficiency after flue & standby losses. One GJ=278 kwh, so for every GJ of gas input only 278 x 0.65= 181 kwh ends up as usable hot water. Normalizing that to 1000 kwh of hot water, it takes (1000/181=) 5.52 GJ of gas. At $13.264/GJ that costs (5.52GJ x $13.264/GJ= ) $73.21.

So, typical gas hot water heater's marginal operating cost is slightly less than half what it costs to run an electric hot water heater, but a heat pump water has a marginal cost a bit less than a third. If you normally run a dehumidifier in the basement to control humidity/mold, running a heat pump water heater would take a large share of that dehumidification load as well.

The GE GeoSpring series has fairly lousy warranty coverage in Canada, and a reputation for high failure rates. A Stiebel Eletron Accelera 300 has more favorable reliability figures, but it's bigger, fairly expensive (CDN $2,879 purchase online through Home Depot), and only has an EF of 2.2, making it comparable in marginal operating cost to a gas-burner.

If you heat the house with a boiler you'll probably get better net efficiency out of installing an indirect fired hot water heater operating as a zone off the boiler. The size depends on your hot water needs, and to a lesser extent the size of the burner on the boiler.

A 240L electric tank is bigger than the average. With a standalone gas hot water heater you'll probably need a 50 gallon (190L) version to have similar overall performance.

 

[Dana]

A bit of cross-posting going on here.

With three showers in the house you'd drain a 40 gallon hot water heater pretty fast, and even 50 gallons would be a stretch if it has a typical sized burner. A condensing 50 gallon unit with a 76,000 BTU/hr burner like a Vertex would probably do it just fine, but they're pricey. A combined 40 gallon plus a 3" x 60" or 4" x 40" (or bigger) drainwater heat exchanger would deliver comparable showering performance at somewhat higher net efficiency.

Looking at the approved list, the HTP Phoenix Light Duty (model PH76-50 ) would be a good choice. It's considerably more expensive than the roughly equivalent A.O. Smith Vertex (model GPHE50 ), but it's a considerably better value too. The all-stainless construction will give you 20+ years of service, whereas the glass-lined Vertex may not last the full warranty period. The "Light Duty" is only in relation to their commercial hot water heaters. The Westinghouse badged version of the PH76-50 (which is also on the list) is the same water heater, carried by Home Depot in the US for about US$2600 (yeah, I know OUCH!), but through distributors the original HTP version is typically a bit under US$2000. That's expensive to be sure, but it'll outlast a $1700 Vertex by 2x, and will have an operating cost comparable to a heat pump water heater (without the reliability concern.)

Since a condensing tank can also be vented using plastic vent pipe (like your condensing hot air furnace) The total installed cost may not be much higher than with a non-condensing tank.

 

[Raj Jas]

Wow Dana awesome information, thank you so much for taking the time out to help me. I have started researching a little on the drainwater heat recovery heat exchanges and would be very interested about installing one in the near future. However right now I think I should spend the money replacing the electric tank itself as it is nearing close to being 11 years old. I have read up a little bit about heat pump water heaters and don't think it would be a good fit as the tank would be enclosed in a small room utility room and colder Canadian winters. Looking at the Condensing natural gas tanks I thing the GPHE50 would be a great fit considering that we may be selling this house in the next 5-10 years. What Canadian distributor would you recommend purchasing from?

 

[Dana]

"...colder Canadian winters..." ??? Colder than what?

I assume a FortisBC customer would be in southern B.C. /Okanongan valley region, which is pretty temperate compared to the midwest, and a decent climate for heat pump water heaters. They work fine in my area, which has a 99% temperature bin 2-3C cooler than in Kelowna or Penticton

Not that it matters...

You're opting for condensing gas, which is a pretty reasonable. Even if you're only planning to be there 5-10 years it's worth pricing a PH76-50 against a GPHE50- sometimes the quote for a professional installation isn't very big, and may fall in favor of the stainless Phoenix.

Alas, I don't have insight into which Canadian distributors would carry & support either of them.