tank or tankless?

[p51]

What would be more efficient ,30 gal. 3500 watt tank type eiectric water heater or a 19kw electric tankless W/H, with the same water usage. Typical 2000 sq. ft. house, 3 bed room ,2 bath.

 

[eric7379]

This is really a difficult question to answer.

First off, when you say "same water usage", it is not really indicative of the demand on the hot water heater. You could have only one person living in the house and still have the same water usage. In that case, I would think that a tank-type water heater would be the best route to go, because the single person would probably never see a ROI (return on investment) because of the high cost of a tankless water heater.

If there is hardly any demand for hot water, then the tankless water heater will not require all 19kw. Yes, it MIGHT cost less to operate, but the cost of the installation is another thing to consider.

Instead, what is the maximum flow rate of the tankless water heater? The number of 19 KW is not really enough to give an answer. Is it rated as a "whole house" water heater(meaning that you can have the shower running and doing dishes and laundry at the same time and never run out of capacity)? Another thing to think about is that some of the electric tankless water heaters that I have looked at in the past required a 150-200 amp feed to it (the number really depends on the size of the water heater; the exact number may be lower). How many houses do you know of that have that much extra capacity in their service to their house, let alone in their panel?

I had a gas tankless water heater installed in my house. I don't have any complaints at all about it and yes, it has made a noticeable difference in the gas bill. I had one installed that had enough capacity for the whole house (running more than 3 devices at a time) I went with the gas water heater because of not enough capacity for the electric (the ones that I looked at required a 200 amp feed).

 

[broadgage]

The tankless heater should be slightly more efficient, since no power whatsoever is consumed when no hot water is being used.
A hot water tank will cycle on from time to time, consuming power, even when no hot water is being used, owing to the already heated water cooling and being reheated.
In a cold climate the heat lost is not truly wasted since it helps to warm the house.
In a hot climate it is a complete waste, and may add to energy used for A/C

The tankless water heater is probably more expensive to buy, and certainly more expensive to instal if a service upgrade is required, as is likely.

The large and variable load of a tankless heater may cause undesirable flickering of lamps, especialy if the POCO transformer is relatively small, or the service entrance conductors are long.

 

[steve66]

I would say tankless by far. You don't have a big tank giving off heat all night long when everyone is asleep, or when everyone is at work.

Tankless only uses power when there is a demand for water.

But I also think Natural Gas is a lot cheaper than electric for heating applications, so IMO, the best thing would be a nat. gas tankless.

Steve

 

[techntrek]

Something to consider is how much sediment and minerals are in your water. I considered tankless (I have 400 amp capacity and LPG so I'm covered either way) but I have minerals and calcium in my water - some people find they clog the baffles fairly quickly. To get the quick heat transfer the water is divided into many small tubes, and that is what gets clogged.

 

[Den]

I have a marathon fiberglass water heater in the house and it is real effeciant. I can shut off the power to it and be gone for 3 days and still take a shower when I get back because it did not loose the heat so I don't really buy the idea that they are running all the time just setting there. I also have an 90% effeciant lp gas tankless boiler running the floor heat for my new shop. It is 40x64x15 and it runs extremely effeciant. It takes less gas to heat the shop than my house which is a lot smaller sq'. The shop is insulated better. The boiler is a dual purpose one and switches to heat domestic hot water on demand. I don't have the shower hooked up yet in the shop so I don't know but it is supposed to keep up with 3 things at once. I really like it and would recommend a gas if you are going tankless. The dual purpose boiler cost 1500 on sale and it takes the place of a water heater and furnace. It can be fitted to any type of heat needed and that is the only way to go for me. I don't like the power demand that tankless electric heaters need, especially the whole house units.

 

[Den]

The tankles heaters have filters at the inlets that will clog with lots of minerals. they are supposed to stop the crap from getting to the elements and I was told that they need cleaned or changed regulalry but that it doesn't hurt the actual heaters but might be a maintenance issue????

 

[AV ELECTRIC]

One other option during an interruption in water supply do to a natural disaster a tanked water heater could supply water for the household .

 

[mivey]

Your most efficient unit by far will be a heat pump water heater, especially since you are in south Florida.

As for the two you mentioned, there is very little difference in a high efficiency tank vs tankless, but the tankless will have higher maintenance costs so I would go with the high efficiency tank unit.

IMO, the tankless is ideal for something like a utility sink in a shop that rarely gets used or for somewhere you need a constant stream of hot water for time periods approaching infinity.

For regular use, the tank type will be the least cost in the long run and nothing else can even come close to a heat pump type for efficiency.

 

[petersonra]

I think you will find that the efficiency of a tankless design is a little higher but its not as much as the hype would suggest. For less than $100 you can buy a water heater blanket and a timer to shut off the water heater while you are not there (like a setback thermostat) and this system will approach the efficiency of a tankless design at a much reduced cost.

The biggest advantage of a tankless design is you just will not run out of hot water, even if you decide to take a 3 hour shower.

As another poster said, get the gas fired unit if you decide to go this way. Very few homes have a service large enough to use an electric tankless unit and the cost of increasing the service might exceed the cost of the water heater.

 

[One-eyed Jack]

I think you will find that the efficiency of a tankless design is a little higher but its not as much as the hype would suggest. For less than $100 you can buy a water heater blanket and a timer to shut off the water heater while you are not there (like a setback thermostat) and this system will approach the efficiency of a tankless design at a much reduced cost.

The biggest advantage of a tankless design is you just will not run out of hot water, even if you decide to take a 3 hour shower.

As another poster said, get the gas fired unit if you decide to go this way. Very few homes have a service large enough to use an electric tankless unit and the cost of increasing the service might exceed the cost of the water heater.

I am with you all the way. The high maintenance and initial cost on the tankless is going to outweigh any savings you might get. The scale buildup will reduce effeciency in very short order. ie high maintenance.

 

[Varczar]

As another poster said, get the gas fired unit if you decide to go this way. Very few homes have a service large enough to use an electric tankless unit and the cost of increasing the service might exceed the cost of the water heater.

I would agree that if you are going to go with a tankless - get a gas fired one. One of the things that the POCO will also look at - especially in South Florida - with tankless water heaters installations is the voltage drop and flicker effects on not only the service to that home; but to other customers that are served off of the same transformer. I have had to upgrade the underground transformer that had eight customers on it because of a 28kW tankless electric water heater installation at one home. The customer had to pay for it. I have also had one where the tankless water heater customer had to pay for a neighbors service upgrade due to voltage drop and flicker issues ? they were served out of the same underground pedestal.

 

[LarryFine]

I have also had one where the tankless water heater customer had to pay for a neighbors service upgrade due to voltage drop and flicker issues – they were served out of the same underground pedestal.

I agree with the rest of your post, but there's absolutely nothing that can be done past the point of separation that can be done to compensate for a load-caused voltage-drop ahead of that point.

Added: In fact, enlarging a heavy user's supply conductors will actually increase the effects their large loads cause to others who share the same transformer or conductors.

The service upgrade didn't help. Only a larger feeder between the utility transformer and the pedestal can reduce the effects of voltage drop one customer's load causes on the other customers.

 

[Varczar]

Added: In fact, enlarging a heavy user's supply conductors will actually increase the effects their large loads cause to others who share the same transformer or conductors.

The service upgrade didn't help. Only a larger feeder between the utility transformer and the pedestal can reduce the effects of voltage drop one customer's load causes on the other customers.

You are absolutely correct. I did not specifically state secondary and service, I just said service. However, the secondary was changed out also. The secondary was upgraded from a 1/0-1/0-2 to a 4/0-4/0-1/0 and both services were put in as 4/0-4/0-1/0's.

Thank you for the correction!


Varczar, P.E.

 

[dereckbc]

I went through this drill a few years back when I built my home and would like to point out something to our northern friends. If you look at the specs on most tankless units. they can only raise the temperature about 50 degrees at the stated flow rate. otherwise flow gets restricted. Well here in TX winters are input temp does not fall below 55 so no problem, but you Yankees might have a problem especially with electric units. 40 + 50 = SMURF

Fortunately I have NG and two units that work very good.

 

[benaround]

It would cost the same amount of money to run a tankless for one hour as it would to

constantly heat a tank type for 5.4 hours.

 

[mivey]

never mind

 

[TOOL_5150]

I went with the gas water heater because of not enough capacity for the electric (the ones that I looked at required a 200 amp feed).

I just about fell out of my chair reading that. :grin:

~Matt

 

[petersonra]

It would cost the same amount of money to run a tankless for one hour as it would to

constantly heat a tank type for 5.4 hours.

Only if the BTU rating of the tankless unit was 5X the normal water heater.

Its all about how many BTUs you use for the most part. With a tankless design for the same amount of hot water it is the same amount of BTUs, thus the same amount of electricity.

Keep in mind that if the tankless unit is not near the point of use, you waste the same of amount of heated water to get hot water from the heater to the POU. The only difference in efficiency is whatever energy is lost from the heated water in a tank style unit while it is not in use. And that can be minimized in very cost effective ways.

 

[mivey]

The only difference in efficiency is whatever energy is lost from the heated water in a tank style unit while it is not in use. And that can be minimized in very cost effective ways.

Exactly. And the life cost of the tank-less could very well outweigh the minimal loss from the idle tank. When you throw in the cost savings you can get from using a heat pump tank system, it is almost a no-brainer to not use the tank-less if you live in a warmer climate. Also, the HP has a bonus: you get some free A/C in the garage.

For colder climates, I wonder if they can use a geothermal unit that robs heat from the ground instead of the air in a heat pump type application? I'm sure someone has thought of it already.