Are electric water tank boosters an efficient/cost effective option?
I’ve been investigating hot water tank boosters (Rheem’s booster and the Steibel Eltron Megaboost) and found little information about their effect on DHW efficiency.
We (couple with a 5 year old) recently bought an all-electric bungalow in British Columbia with a three-bedroom basement suite. The home came with a 182L (40 imp gallon) electric resistance tank.
The tank is undersized by any listed definition. We previously rented to a couple and had no issues around hot water shortages. The home has two new model dishwashers, two laundry pairs we run on cold water only, and three bathrooms (two upstairs) with 1.5 GPM shower heads.
However we are soon renting to a party of three. A booster seems like an ideal solution to running out of hot water, given that the number of renters will likely fluctuate between 2 and 4 over the coming years. We can keep a smaller tank that seems to suit five occupants just fine but leaves room should we sometimes rent to larger groups/families. Theoretically we’d save money from reduced standby losses over the long term.
Both boosters I’ve investigated are power hogs. The Rheem model run on the same circuit and controls the hot water tank – and requires a bump to 30 amps from 20. The Megaboost IST way worse – needs a dedicated 40 amp circuit.
Would the standby savings offset the booster juice? Which setup would be the cheapest to operate?
(Before we get into heat pump water heaters – I’m hesitant because we heat the home off two single-head heat pumps and their capacity is already borderline given the home’s age and need for more air sealing (in progress.) Our utility room is also quite small in order to fit in the suite.)
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Replies
It won't have much effect on efficiency either way, compared to the electric resistance tank. It's probably a little higher efficiency compared to running the tank hotter and using a mixing value to reduce the temperature. Best efficiency would be to replace the thank with a larger heat-pump tank. If there's not enough room for that, a smaller heat pump combined with the booster would give you most of the benefit of the heat pump efficiency.
Electric resistance is considered to be 100% efficient -- all of the input electrical energy is converted into heat. It takes a rather lot of energy to heat water though (that's why water works so well as a coolant, and to move heat around in a boiler system), which is why you have heavy electrical demands for those units you're looking at.
I don't see any reason those units would NOT work, but you may find you're better off with a second tank-type water heater instead.
Bill
Adding a second tank could work - what would be the advantage over a booster? I would assume the standby loss of a second tank would make it a costlier option.
The tank-type water heater will be lower maintenance. A larger, single tank-type water heater would also help, and would have lower standby losses than two smaller units due to less exterior surface area per unit tank volume.
Standby losses will increase operating costs, but not much -- standby losses are pretty low.
Bill
Since a decent electric tank has an energy factor above 0.95, there is very little energy saved with a electric tankless unit. Tankless units are also not zero maintenance as they need to be descaled, this can be an issue if your local water is very hard.
If you need more capacity, the simplest is to crank up the temperature on the existing tank and add a mixing valve (anti scald valve) if there isn't one already. With a resistance tank the energy penalty of running it hotter is minimal, just a bit extra standby loss.
If that is not enough, than replace the existing tank with a larger unit. The cost of this would be less than plumbing/wiring/install of a booster.
About the only time I would consider a booster is if you absolutely cannot fit a bigger tank. In that case, I would add one that can carry your full expected flowrate but downstream of the existing tank. Have the tankless temperature set point set a bit bellow the water tank, that way it will only run if the tank runs out. This would reduce the scale buildup and should be no/minimal maintenance.
Any water heater should be run at 140F minimum, and tempered back to 115-120F with a mixing valve. This is for protection from Legionella.
The cost difference between a new tank and booster depends on a couple of factors. If you can do the work yourself, the booster will be far cheaper. Even if you're hiring someone, a bigger tank may require rerouting of pipes which would likely put the new tank cost above the booster as well.
If possible with the current layout, consider a drain water heat recovery pipe. It would have minimal maintenance, add shower capacity, and reduce energy use. Adding a booster, tank or tankless, can only claim the additional capacity.
The first question is how big a booster are you considering? 240 volt 40 amps that might do something 120 volt 15 amps not much at all.
Do you have room in the panel for the extra breakers for the booster?
I don’t see a booster helping much. If you put it after the water heater it does nothing until the tank is putting out cold water. If you put it before It will run a lot but it is likely to only change the incoming water a few degrees will that extend the tank much I think not.
Akso idea to crank the tank up to 160° and mix that water back down to 120 is likely to be the solution with the lowest first cost and the highest operating cost. This is a win for the landlord at the expense of a tenet paying the utility bill
If you are paying the utility bills the smart move is to install an appropriately sized tank or set of tanks and consider upgrading to a heat pump water heater
Walta
A hot water tank runs out of water gradually, it does got from hot to cold. Once the cold water coming in the bottom starts to mix with the hot water at the top, you've still got quite a lot of lukewarm water available. It takes a lot less energy to get water from 90F (an uncomfortable temp for most) up to 105F (at least adequately warm for most) than from 50F. If you set the turn on temperature appropriately on the booster, it will definitely extend shower times. You would never install a booster before the hot water tank, as it would run every time hot water was demanded, and have minimal effect.
“A hot water tank runs out of water gradually, it does got from hot to cold.”
Ideally there would be little mixing as the cold water enters the heater thru a dip tube so the cold dense water is inserted at the bottom and the hot water is lass dense in the heater so it would float on top of the cold water with little mixing.
It seems the work that way in my shower when you run out of hot water the temperature of the water seems to change from normal hot to cool water in a matter of seconds. If there were much mixing the change would be gradually.
Walta
Rliebrecht,
Even though BC is a temperate climate, it still has a heating season from September to April, so standby losses aren't really losses for much of the year. If they are a concern, you can almost completely eliminate them by using one of the well insulated Rheem resistance heaters: https://www.homedepot.ca/product/rheem-marathon-50-gallon-18l-lifetime-electric-water-heater-4-5kw-240v-/1001070393
This may not be possible with your existing water supply layout, but I'd consider separating the two suites hot water pipes and providing a heater each. That completely eliminates concerns about volume. Each unit becomes the source of their own hot water use problems.
40 gallons is fine for a 3 room apartment, is ridiculous for more.
Perhaps biting the bullet and installing a properly sized tank for your unit and leaving the 40 gallon for the apartment would be the wisest option. Inspect the plumbing runs and see how difficult it would be to separate.
Most rental units eventually get separate utilities, so this could be a start.