Need to heat hot water

[tom baker]

I was volunteered for an electrical job at my granddaughters. They have a jetted tub in a manufactured home, maybe 40 gallons. When they fill it from the water heater, they run out of hot water when the tub is half full. They would like to put in a small instantaneous water heater to heat the hot water when it runs out.
I looked at a spec for one that was listed at 3500 KW at 120 volts, it was a model 3.5 so most likely was not 3,500,000 watts.
The mfg home water heater is an electric storage type and no room for a larger unit.
Would this work to use an instantaneous water heater? I am thinking to size it, look at the incoming water temp, flow rate, and desired output water temp (105 deg F)

 

[LarryFine]

I'm thinking a 240v unit with a greater heating capacity; perhaps as a replacement for the existing heater.

 

[tom baker]

I'm thinking a 240v unit with a greater heating capacity; perhaps as a replacement for the existing heater.

Yes and perhaps not a greater heating capacity, but more storage.

 

[wwhitney]

I am thinking to size it, look at the incoming water temp, flow rate, and desired output water temp (105 deg F)

1 kW = 3412 btu/hr = 57 BTU/min. And 1 BTU raises 1 lb of water 1 degree F, while a gallon of water weighs 8.34 lbs. So a 1 kW heater can heat about 0.14 gpm to a 50F rise. Your 3.5 kW heater could do 0.5 gpm or so.

Cheers, Wayne

 

[__dan]

Look at the fuel choice first. If they have natural gas available, methane, I would be looking for a nat gas unit. Navien (?spelling) makes them, instantaneous condensing gas fired DHW, and / or heat + DHW.

Any other fuel will get a lot more expensive, and with electricity you will be limited on output (flow rate) and recovery time. Recovery time is the problem you are having now. Going to an electric fuelled instantaneous unit, flow rate I would guess is the problem you will be trading for. The tub is a high flow fill load. If you size the instataneous electric unit for the tub flow rate, the kW could be up there.

I would start there. See if nat gas is available and no problem. Otherwise, try sizing an electric unit for the tub fill flow rate, temp rise, and see what kind of kW is called for. Propane will do it (for a Navien) then the problem will only be fuel cost.

40 gallon electric storage unit is about the smallest standard size for a house, but it will usually fill a tub. I would be surprised if the house has something smaller.

 

[synchro]

Yes and perhaps not a greater heating capacity, but more storage.

Yes, a water heater with more storage would help.

Also, you could run the water heater at a higher temperature and use a mixing valve to dilute it with cold water and bring it down to a safe temperature. It won't solve the problem by itself but it may help get you there.

 

[tom baker]

It’s a walk in tub, so at least 40 gallons.
Nat gas not available
Propane is an option, with navion
I will confirm tub size, it’s much deeper than a standard tub

 

[tom baker]

Yes, a water heater with more storage would help.

Also, you could run the water heater at a higher temperature and use a mixing valve to dilute it with cold water and bring it down to a safe temperature. It won't solve the problem by itself but it may help get you there.

Didn’t mention they have foster children, rules regulate water temp, mixing valve might be simple solution

 

[LarryFine]

Yes and perhaps not a greater heating capacity, but more storage.

I meant tankless, not storage.

 

[__dan]

Another problem with instantaneous type DHW, when you heat water the mineral in the water precipitates out, usually sticking to the tubing of the heat exchanger. Could be a problem with really hard water.

In the early days of instantaneous gas fired units, they were not rated for condensate but ended up running in the condensing range, with a short lifetime of the boiler less than five years. Modern quality condensing rated units should not have that problem but I'm sure there are still cheap short lifetime units still on the market. The Navien should be a pro quality but it's beyond my expertise to recommend. If you had the footprint and load for it, gas condensing boiler with a standalone indirect DHW is what I would be looking for.

 

[tom baker]

I have a Navion, quality unit, mine is 180,000 BTUs on nat gas. I am going to recommend a tankless propane heater. Even if the 3.5 KW water heater worked, once the main wtr heater is depleted, then that would limit hand, clothes and dish washing

 

[Barbqranch]

When you look at the size you need, remember that the existing tank will serve much like an instantaneous heater once the hot water runs out, so it will be pre-heating the water for you electric unit.

 

[Dennis Alwon]

I have a Navion, quality unit, mine is 180,000 BTUs on nat gas. I am going to recommend a tankless propane heater. Even if the 3.5 KW water heater worked, once the main wtr heater is depleted, then that would limit hand, clothes and dish washing

I agree. The electric instant water heaters are not worth it from what I have read on the forums. Some take 100 amps and the customers are not happy with it. Get a gas unit and don't worry-be happy.

 

[winnie]

How long does it take to fill the tub (water flow time?)

How large is the existing tank?

Maybe replace the tank with a same sized one that has higher power elements (a 'high recovery' water heater), and combine this with the temperature reduction valve.

Jon

 

[Fred B]

I agree. The electric instant water heaters are not worth it from what I have read on the forums. Some take 100 amps and the customers are not happy with it. Get a gas unit and don't worry-be happy.

Several years back I installed one for a customer for the volume needed it had 4 - 60 amp double pole to power it. When it came one the meter almost spun itself off the pole. Not sure if the technology has improved since then, but I will usually recommend gas on demand for that reason.

ps: with the push against NG and other fossil fuels not sure if the WH world has caught up for efficiency in the electric on demand units.

 

[winnie]

The efficiency of electric demand heaters is not the problem. They are near 100% efficient.

The problem is the huge amount of energy needed to heat water.

wwhitney gave the numbers in post 4. Pick reasonable numbers for fill rate, incoming water temperature, and output temperature and you easily need 10kW of power for this job.

If you want an electric demand heater for an entire residence, 20kW is small.

I suppose in theory you could have a heat pump demand heater for better than 100% COP, but this is not practical. Back to filling this tub: 320lb of water heated 55F (50 to 105) is 17600 BTU. Do this in 10 minutes and you are talking a 105K BTU/hr heat pump. 9 tons. More than my small house needs for a Massachusetts winter.

Jon

 

[James L]

If they have the space, a 2nd 40-gallon water heater might be the best option.

They can be plumbed in series and parallel using 5 valves, so they can run either one separately, or both.

 

[petersonra]

If they have the space, a 2nd 40-gallon water heater might be the best option.

They can be plumbed in series and parallel using 5 valves, so they can run either one separately, or both.

I like this idea. Plumb them in series and when you are not using the system to heat water for the tub shut off the the heater that sees the water first. That way you only use energy when you need it. Just turn it on an hour or so before you need it and it will be nice and warm for you. That way under normal circumstances cold water will just flow through the extra heater.

 

[drcampbell]

The efficiency of electric demand heaters is not the problem. They are near 100% efficient.
The problem is the huge amount of energy needed to heat water.

The bigger problem is that most Americans have little or no clue just how much energy is used for heating water ... or for anything & everything else. See, for example, "Man vs. Toaster"

What is The Toaster Challenge? (Cyclist vs Toaster) | Toaster Report

On June 3rd, 2015 a special YouTube channel uploaded three videos which have gone viral since. With this upload, they have sought to answer an age-old question: how much power can an athlete produce and is it enough to toast bread? Simply put, the video is centered around Robert Förstemann who...

toaster.report

... when you are not using the system to heat water for the tub shut off the the heater ...

I'm pretty sure that's what thermostats do. Once installed, further human intervention is unnecessary.

 

[Joe.B]

They're still pretty pricy but heat pump water heaters are the best way to go if you're not using gas, but they are not on demand.