Water Heaters - Tank, Tankless, Condensing - And Radiant Hydronic Heat

I've got two NATURAL GAS water heaters in my house that will need replacing in the year to come. 

1) Domestic Hot Water Rheem 40gal, 34,000 btu. - My original plan was to replace this with a similar size unit. Swap in swap out. 

2) Hydronic/Radiant loop Apollo 5010 commercial unit 65,000 BTU 50gal. 

Both are 20 years old, and likely not overly efficient. 

Research makes it seems like when comparing convention tanks, high efficiency tanks, condensing tanks, basically it's a wash in terms of 13 year overall costs. Condensing units are more expensive up front, but cheaper monthly. High efficiency units are only mildly more expensive than conventional but a little bit more expensive monthly. 

That leaves tankless setups. My worry is that for the hydronic heat system, this thing will be going all the time. It's my understanding that tankless units are only more efficient when they are used occasionally. 

Any thoughts or advice on the topic of the tanks vs tankless water heaters for radiant heat? 

Re the tanked ones, have you looked into the heatpump versions if you have an electric water heater? They're more expensive but if the alleged savings materialise should be worth the extra money.

Not sure if they're also available in gas versions.

I just went through this. Decided to go conventional (Rheem).

K.I.S.S. was my conclusion.

Tankless draw a ton of electricity or gas-as in you may not have the service to support it. The others cost a ton, and it takes years to amortize the cost difference.

I have one electric tankless heater now, and had a smaller under-sink electric unit back in IL. The both have the same problem - if the hot water supply isn’t turned up high enough, or something causes the pressure(and thus flow) to drop, you lose all hot water until it starts flowing enough water again. 

With our previous under-sink Bosch unit, to get it to even turn on I had to max out whatever the adjustment was. Even then, sometimes it wouldn’t turn on at all. 

With our current much larger unit that’s installed in our master bath, if one of our neighbors flush their toilet we lose all hot water until their tank has filled. I’ve had a plumber out to check it & other than installing a normal water heater - which I’d prefer, there’s just no good way to do it - we’re stuck with “shower lotto”.

pheller said:

High efficiency units are only mildly more expensive than conventional but a little bit more expensive monthly. 

More expensive to purchase and more expensive to run? 

That's no bueno.

z31maniac said:

pheller said:

High efficiency units are only mildly more expensive than conventional but a little bit more expensive monthly. 

More expensive to purchase and more expensive to run? 

That's no bueno.

 

That's got to be wrong.  The only monthly cost would be gas, and if a high efficiency uses more than a conventional... 

Don't be surprised if those tanks last another 20 years.  They are being used for heat, so the water isn't constantly being replaced with new mineral rich/acidic water.  Its the new minerals that react with the metal tanks and rust them away.  Gas is still relatively cheap.  Repairs on wall hung water heaters are not only expensive, but just finding a plumber willing to spend the time to figure out what is wrong, track down the parts, and return to fix it is the real problem.  There is too much work out there now for tradesmen.  Why would anybody want to waste half a day on a job that may end up in failure where you can't even charge the customer after spending time diagnosing the issue.  Standard chimney vented gas water heaters are the way to go.  Power vented slightly more efficient gas water heaters are strictly for homes with no chimneys.  The energy savings do not justify the purchase/installation costs.  Wall hung units have their place in this world, but more often then not, they are not worth any long term savings you may or may not see.  Consider looking at a combi unit.  Rinnai and others make small wall hung units that make both hydronic heat and domestic hot water.  Like most wall hungs, they are only good for around 15 years thou.

z31maniac said:

pheller said:

High efficiency units are only mildly more expensive than conventional but a little bit more expensive monthly. 

More expensive to purchase and more expensive to run? 

That's no bueno.

I think he meant slightly more expensive to purchase than a conventional unit but slightly more expensive to run than a condensing unit.

As the owner of a natural gas tankless domestic water heater, I absolutely would NOT use one hot hydronic heating.

Tankless are designed to generate a lot of BTUs quickly for a short time, to raise a sudden volume of cold water to hot water immediately.

Tanked heaters can have a slower recovery time because they're not trying to generate 100% of the hot water needed every time there is a demand.

On a hydronic circulator system, 130d water is going out to the radiators  and 95-100d water is coming back for a touch up, continually, until the space is warm and the thermostat is satisfied.  That's going to happen regularly throughout the day.  So all that water mixes with what the tank is recovering and it all maintains temperature much more steadily and efficiently.

I've used a Tagaki TK Jr. (tankless gas HW heater)  for closed loop hydronic heat since 2005. I have literally never had to service it in any way other than periodically cleaning the strainer on the inlet.  Reid Fleming, Worlds Toughest Milkman (aka Duke) is right that the flow control and burner modulation control strategies of a tankless are technically incorrect for a hydronic heat application- a tankless is set up to maintain the set point output temperature and will throttle the water flow (there's a electronically controlled flow control valve)  and crank the burner up to max BTUs when it first fires up. Having said that, in my setup it settles down to about a 20-25 degree Delta T (difference between inlet and outlet temps) after it runs for a little while and the burner modulates down to a reasonable simmer. Takagi used to provide installation notes for installing their tankless as a heating only boiler (even though it is NOT building code approved for that application.) Unfortunately, the TK Jr and its replacement, the TK Jr 2 are discontinued and the current model specifically says not to use it in a heating only application. I'm willing to bend the building code a little, but if the manufacturer specifically says "don't use it for this" that's where I draw the line. I wish I bought a spare TK jr. back when they were available. I suspect that the new low NOx emission versions may have a different heat exchanger design that's not up to this application. 

In reply to flat4_5spd :

Good catch on the Reid Fleming bit!

I am surprised that the tankless has worked out for you that way but it sounds like it has.