Tankless Water Heaters.

[David B Rivera]

My class have been doing some calculation for single family dwellings. The new Tankless Water heaters take a lot of power, some have two or three circuits. Using the standard calculation (Fixed appliance) we used the 75% demand and the service looks acceptable for the loads. When using the optional calculation for new or existing applications the service is reduced to a point where I would not be confident it would work properly. Does anyone have any experience with using the optional calculation with these water heaters? Just looking for some insight, I teach that most of the code is the Minimum.

 

[Sierrasparky]

Welcome to the forum!:thumbsup:

Are you speaking a whole house tankless water heater?

Are you trying to make it work for a 100amp service?

Most of these units the maker specs a minimum Electric service to the home. usually 150 amps the larger ones a 200 amp.
What would be the 4 or more appliances that you would include with this 75% reduction?

I would add the standard demad for SFD. as trying to reduce the service size for these type units does not seem practical to me.

 

[mivey]

as trying to reduce the service size for these type units does not seem practical to me.

I would agree. The customer is not going to be happy with the smaller whole-house units.

 

[electrimate]

You use the nameplate rating of the heater for alternate calc as per 220.82(B)(3)(d). It's not just an appliance, it's a heater.
Those heaters draw the nameplate amps while they are heating, which is only while the water is running. Derating the service is fine, as long as you follow the rules.

Kinda off topic but, I thought I wanted a tankless until I did some research. At four times the cost for the unit, it would take me ten years to break even. And don't forget to add the cost of a recirculating pump. If you want to really save on electric water heating, put a timer on a conventional heater.

 

[David B Rivera]

On our standard cal we have a 300 amp service, if we tried to apply 220.82 B 3 d we got a 200 amp service. (10KVA @ 100% and the remaining @ 40%). Anyway I agree that they are not worth the price and there warranty is much shorter than a standard water heater. I install one at our local college lab, which more makes since if they need a constant supply of hot water. Looking at any exisitng service I wouldnt try to under size it, but part d is a water heater. Thanks you for your replies.

 

[Sierrasparky]

You use the nameplate rating of the heater for alternate calc as per 220.82(B)(3)(d). It's not just an appliance, it's a heater.
Those heaters draw the nameplate amps while they are heating, which is only while the water is running. Derating the service is fine, as long as you follow the rules.

.

What derating are you reffering to?

 

[electrimate]

Not really derating, just referring to the 40% of the load over 10kva in the service calc.

 

[mlnk]

Tank water heater with Intermatic WH timer-good idea. Electric tankless drawing 80 amps or more? Bad idea and it costs more.

 

[John120/240]

One thing about tankless water heaters is they take up very little space. I still don't see the

advantage tho.

 

[kwired]

Tankless gas fired water heaters make more economical sense to me, until I possibluy learn more about them anyway.

Electric tankless in this part of the country does not make much sense. The biggest marketing thing is the fact that you save energy by only heating water as it is used instead of heating up a storage tank and then you lose that heat (no matter how well it is insulated you lose some heat) and have to keep using energy to maintain tank temperature even when no hot water is being used. During heating season this heat is not really lost it is just a little less work for the heating system. It does increase load of cooling system during cooling season though. Enough I have been told you need about 1/4 ton of cooling capacity for every tank type water heater when doing cooling load calculations. So there is more to think about than just how much install costs and how much energy is directly used by the water heater.

But in this part of country ground water is around 55^oF. That means you need to be able to continuously raise water 70⁰ at the rate of use if you want a tankless and 125^o output temp. That means a lot of service capacity to be able to do that, where a storage tank can do it with a smaller electric service but needs more time to do so.

 

[iceworm]

Q: How does one keep the water temp constant at the shower?

I looked at putting in few. The only method I could see was to put the shower on it's own heater. I saw one where the unit had a flow meter control, but it just kicked elements in and out - no modulating SCRs. Unless the shower is pulling hard enough to keep all of the elements on, I suspect the temp is going to move around.

My hesitation comes from the only one I ever used. Even when no one was using water, the shower temp still changed. I suspect it was the changes in flow rate from when the well pump kicked on - pressure variation was from 55 - 70psi.

I can see they have uses, I just can't figure out how to make them work well.

ice

 

[kwired]

Q: How does one keep the water temp constant at the shower?

I looked at putting in few. The only method I could see was to put the shower on it's own heater. I saw one where the unit had a flow meter control, but it just kicked elements in and out - no modulating SCRs. Unless the shower is pulling hard enough to keep all of the elements on, I suspect the temp is going to move around.

My hesitation comes from the only one I ever used. Even when no one was using water, the shower temp still changed. I suspect it was the changes in flow rate from when the well pump kicked on - pressure variation was from 55 - 70psi.

I can see they have uses, I just can't figure out how to make them work well.

ice

IDK, but IMO if you live in the northern half of the country the electric tankless (even though would make more money for me on installs) is not the way to go. Initial cost and maintenance I believe still are too much to justify using a storage tank type heater, tankless gas units I don't know enough about to make much comment on, if they are low maintenance they are probably worth installing, if they are not low maintenance then they are not worthwhile either.

Will a tankless unit use less energy than a tank type unit - yes. Again in the northern half of the country where the heating season is typically longer than the cooling season this energy is not as wasted as some think, it is just deducted from the demand of the space heating equipment.

 

[jmellc]

I think water heating is the niche where solar can do its best job. I have seen simple solar heaters built by writers & artists that heat ample hot water in warm seasons with no real insulation or prime placement. With a little work, they can harness the free heat of the sun and save a lot of energy and money.

Now to put my $ where my mouth is and set up one for myself. I have been threatening to do it for a lot of years.

 

[John120/240]

I think water heating is the niche where solar can do its best job. I have seen simple solar heaters built by writers & artists that heat ample hot water in warm seasons with no real insulation or prime placement. With a little work, they can harness the free heat of the sun and save a lot of energy and money.

Now to put my $ where my mouth is and set up one for myself. I have been threatening to do it for a lot of years.

Well OK, jmellc here is that kick in the seat of the pants . We have a long weekend coming up.

Install one & please report back with your results. Cost, advantages, disadvantages, ROI.

Thank You John

 

[mivey]

I think water heating is the niche where solar can do its best job. I have seen simple solar heaters built by writers & artists that heat ample hot water in warm seasons with no real insulation or prime placement. With a little work, they can harness the free heat of the sun and save a lot of energy and money.

Now to put my $ where my mouth is and set up one for myself. I have been threatening to do it for a lot of years.

Especially good if you do it without an energy conversion like with a PV array. They have had solar water heaters for a very long time that are about as simple as a huge coil of black pipe on a roof. The hot water runs out quick. Brings to mind the benefits of being early to the showers at some remote mountain parks.

 

[jmellc]

I thought many times while crawling through scorching attics that there must be a way to use this heat for something good. Here in the south, hot summers are the 1 weather feature that changes little, except for a few mountain areas. I would not put a standard water heater in an attic but unbroken water lines could coil through or a "penthouse" built on the roof to hold several tanks. With steady sun and 90+degrees most days, a lot of water can be heated. Imagine 3 or 4 55 gallon drums, with water lines running in a PVC chase from crawlspace. As water is used, 1st drum loses hot water but gains hot water from 2nd drum. 2nd gets hot water from 1st, partly diluted by incoming cold water. With any average user, this would give hot water 24 hours a day all through June, July & August.

I also had the thought that many buildings could be built to use cold weather in a similar way. Imagine a restaurant in cold winter zones. Walk in coolers could be built on the outside walls, partly using cold outdoor air to cool the space, naturally with backup mechanical cooling. Intake vents could also bring in cold outside air.

I did a job in a glass walled office building in mid winter. Mostly about 20 degree daytime weather. Come in at 7:00 building like a freezer. Soon as sun hit any section, it was hot. Go to shady side, nearly as cold as outside. Many of you have seen similar, I'm sure.

 

[iwire]

I also had the thought that many buildings could be built to use cold weather in a similar way. Imagine a restaurant in cold winter zones. Walk in coolers could be built on the outside walls, partly using cold outdoor air to cool the space, naturally with backup mechanical cooling. Intake vents could also bring in cold outside air.

In a well designed refrigeration system you use the heat generated by the cooling to heat the building and hot water. Almost all the supermarkets I work do this.

The problem with using outside air to cool the coolers is humidity control and icing up.

 

[T.M.Haja Sahib]

In a well designed refrigeration system you use the heat generated by the cooling to heat the building and hot water.

I think there might be a minimum size for the refrigeration system below which it would not be economical to do so and so it may not be relevant in all cases.

 

[kwired]

I think there might be a minimum size for the refrigeration system below which it would not be economical to do so and so it may not be relevant in all cases.

And where is this line where it is no longer economical to do so? I have a small room out in a shed that I used to have a refrigerator in there. That refrigerator kept the room fairly warm especially at this time of year when it gets cool at night.

For a supermarket there is typically enough refrigeration equipment it is a no brainer - lots of heat is taken from the interior of the store by frequent opening cooler doors and is pumped outside by the cooler refrigeration sytem. Hot water is used year around, and during winter months why dump that heat outdoors when it cost less to pump it back into the store than to purchase more energy for heating when some of it will be pumped right back outside by the coolers?

I even seen small dairy farms that have heat reclaim system for heating wash water. The heat comes from the milk storage coolers. They also build a small room with the condensing units and open louvers to the outdoors during summer months and during heating months they close those and open louvers to building interior for keeping that heat, quit simple but effective. It is "free heat" that was going to be there no matter what.

 

[T.M.Haja Sahib]

And where is this line where it is no longer economical to do so? I have a small room out in a shed that I used to have a refrigerator in there. That refrigerator kept the room fairly warm especially at this time of year when it gets cool at night.

For a supermarket there is typically enough refrigeration equipment it is a no brainer - lots of heat is taken from the interior of the store by frequent opening cooler doors and is pumped outside by the cooler refrigeration sytem. Hot water is used year around, and during winter months why dump that heat outdoors when it cost less to pump it back into the store than to purchase more energy for heating when some of it will be pumped right back outside by the coolers?

I even seen small dairy farms that have heat reclaim system for heating wash water. The heat comes from the milk storage coolers. They also build a small room with the condensing units and open louvers to the outdoors during summer months and during heating months they close those and open louvers to building interior for keeping that heat, quit simple but effective. It is "free heat" that was going to be there no matter what.

It is the arrangement of ducting, insulation etc., that may cost more than the return from the heat recovery from small size refrigeration system. I base this from heat recovery scheme for exhaust gas from diesel generators, which is an economical proposal for gen sets of size 500 KW and above.