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clemkonan 09-20-2007 11:45 PM

Tankless Heater for DHW and basement heating
My hot water tank crapped out I am having to reset it about 4 times per week to get hot water,. The beast is a rental and I am thinking of going tankless.

My problems are:

1) Which is better gas or electric I know gas is more expensive

2 Would it be a good idea to use one unit to supply hot water for potentially 2 showers at the same time plus heat a small basement ( hydronic) 25 by 20?

My original plan for the basement was a small gas stove about 20,000 btu

NateHanson 09-21-2007 10:22 AM

Definitely install a gas water heater, whether you go tankless or storage. They'll cost a few hundred more upfront, but you'll recoup that cost fairly quickly. Electric prices are sooooo high lately!

I don't know whether you can use the same tankless unit for DHW AND hydronic heating. I'm interested in that too, so hopefully someone with info will chime in.

clemkonan 09-21-2007 05:17 PM

Tankless water heater
Here in Ontario gas prices have been spiking I think we are losing the advantage of gas over hydro. The gas unit requires a vent adapter which is pricey and the efficiency of gas peaks out at 85% , for electricity is efficiency rates are higher.

That being said I probable would install a cartridge on the water line to remove scaling elements otherwise the electric unit could crap out.

dmaceld 09-21-2007 06:40 PM


Originally Posted by NateHanson (Post 64021)
I don't know whether you can use the same tankless unit for DHW AND hydronic heating. I'm interested in that too, so hopefully someone with info will chime in.

Simple answer, yes and no!:):( The no first. The International Residential Code, and common sense, do not allow hydronic water to be mixed with domestic water, so you should not (must not) connect the piping so water from one system also flows through the other system. Now for the yes. There is no reason you cannot use the same heat source to heat both hydronic and domestic. All you do is run the water from one system through one side of a heat exchanger and the water from the other system through the other side. I'm not sure how practical this will be with a tankless heater. I have my doubts that tankless buys you anything at all for hydronic heating except higher cost, more complication, and possibly a more temperamental system! In fact I'm not sure how much tankless buys you for DHW unless it's installed right at the point of use.

I really don't know much about tankless but what I will say is if the heat input into the water can be varied to fit a variable load, i.e. the faucet is part open or full open, then they probably work OK. If the heater is built so that the heat input is either full on or full off, then there has to be some sort of storage volume in the heater to smooth out the heat input to fit the heat load.

NateHanson 09-21-2007 06:46 PM

My understanding is that the tankless heaters vary the flame to keep the output temperature constant, regardless of flow or input temperature.

The use of a heat exchanger sounds inefficient here, and I don't think it would work at all with a tankless heater because the tankless only operates when the water is flowing, so you'd have to run your shower whenever you wanted the floor to heat up. So you really can't use a tankless to do both.

However, I'm interested in hearing from pros about the idea of using a tankless to do hydronic heating alone. I'd love to add hydronic to our bathrooms, but our central heat is FHA, so no hot water to pump around.

dmaceld 09-21-2007 06:47 PM


Originally Posted by clemkonan (Post 64102)
That being said I probable would install a cartridge on the water line to remove scaling elements otherwise the electric unit could crap out.

You referring to a cartridge filter, or something like a water softener? If a filter, that won't work. The scale is caused mostly by chemicals dissolved in the water and filters won't stop those. A softener type system that uses chemical reaction to remove iron, or whatever, molecules will work. In a storage hot water tank a magnesium rod helps to keep the bad molecules from building up on the heating element.

redline 09-22-2007 12:25 AM

Do you have a boiler or hot air furnace?

Clutchcargo 09-22-2007 08:25 AM

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You could probably figure out a way to make a single tankless do both DHW and hydronic heat but it wouldn't work well. DHW runs around 110F while heat runs around 180. The only way I can figure to make this work efficiently is if you use it to run an indirect water heater.

Cache 09-24-2007 12:43 PM

When I lived in Korea we never had anything but tankless. They were always LP. I suppose certain places also had natural gas but propane was more common. Anyway, for this discussion they are similar.

The tankless heaters were very good at heating water for both showers and floors. Asia is really big on hydronic floors. Forced air is incredibly uncommon in homes. Anyway, two different heaters connected to the same fuel line. I don't think trying to figure out a way to run both systems from the same heater is worth the effort.

We never had a problem getting hot water/floors as long as we didn't run out of fuel. I've heard the newest electric models are even better at heating efficiently as the heating elements are directly in the water. I also think that the logic behind electric ones is better, but I haven't been able to find a direct unbiased comparison of the two. Gas is cheaper but the unit is only about 80-85% efficient. Electric is nearly 100% efficient at the tank (but you pay more because the process that provided the electricity in the first place was less efficient).

I suppose there are many variables that factor into a direct comparison between tank heaters and tankless, but type of use and location are probably the most important. If you use hot water 20 gallons at a time at frequent points throughout the day, then I see a tank heater as being the better choice. Running 20 gallons of cold water full throttle through a tankless heater is the most inefficient way to use it. The unit is trying to maintain constant line temperature so running the hot line at half throttle is better, especially with gas heaters. Pulling 20 gal out of a tank heater is no big deal and the unit is set to supply much less fuel to heat the incoming water as warming doens't need to be instant. The problem of course is that the tank heater also wants to maintain that water temp during non-use.

Also, where I live the winters get down to -10*F for extended periods. This means that ground water is getting really close to freezing. It takes quite the tankless heater to convert 45* water to 120* instantly if it is used by a large family and always under high loads. Gas tankless heaters are more powerfull than electric and might be required depending on the circumstances. Gas units cost more in terms of installation and are often installed in less desirable locations because of venting requirements. This increases their inefficiency. In Korea they were always located outside for safety reasons. The hot water needed to be piped to the faucet and would lose much heat in the process. Electric heaters are designed to be placed very close to point of use which increases efficiency and reduces installation costs.

A large family should probably consider a tank/tankless combo. The tank heater is at a very low temp setting to maintain 75 gal or so of warm water. When the faucet is turned on, the tankless brings the pre-warmed water up to temp instantly, but the load on the system is much lower. Not hard for a tankless to bring the temp up 30*F. The problem with tank systems for a large family are that few units are even capable of heating water fast enough to keep up. After a couple showers or a load of laundry/dishes, the water coming from the showerhead will be lukewarm at best. The tankless units never run out of hot water, but at these levels will be expensive to run. If you have a 75 gal. tank preheating the water to 100*, the tankless unit will bring it up to 130* quite easily, and the last person showering will never be stuck without hot water. The tank unit won't be wasting so much energy during non-use either.

Anyway, there are obviosuly many variables, but my personally opinion is that nice electric units are better. There are many ways to obtain electric energy that don't consume natural resources, and many of these are becoming much less expensive. I think that solar is the only viable fuel source long term. Within 5-10 years it should be very affordable too. The debate between gas/electric will be over at that point until over supply of fossil fuels causes prices to plummet. And I think that tankless or some type of combo system will win the other debate too. It just makes more sense.

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