Installing a tankless water heater

[Daox]

Whilst looking at the power vent option and installing it and the price of ~$300, I also started looking at new tankless water heaters which are around $800 for a comparable unit. These new models sport some very nice features like electronic ignition (no pilot light), direct venting (a dedicated air intake so the burner doesn't use house air), and for ~$1200 you can get higher efficiencies in the 94% range and thats boiler range efficiency...

I also have to take into account that this summer I'm planning on putting up the solar hot water panels I got last fall... I was hoping to only install a boiler and have that as my backup to the solar for DHW and home heating. However, I'm still not sure how it'll all tie together. Apparently boilers can't directly heat DHW, I'm not sure why, but on demand hot water heaters can. Doesn't make sense to me.

[Piwoslaw]

Can you retrofit your heater with spark ignition? Pilot light is a waste!
I think that a smaller flue with power vent and maybe a damper would be the way to go. Remember that there will be a draft even when the heater is not working, sucking lots of air out of the house 24/7... A closed combustion chamber (intake air straight from outdoors) is the way to go if you are serious about sealing the house.

Originally Posted by Daox

I also have to take into account that this summer I'm planning on putting up the solar hot water panels I got last fall... I was hoping to only install a boiler and have that as my backup to the solar for DHW and home heating. However, I'm still not sure how it'll all tie together. Apparently boilers can't directly heat DHW, I'm not sure why, but on demand hot water heaters can. Doesn't make sense to me.

Normal boilers operate in a closed loop, so after the first time the system is fired up there shouldn't be any scale left in the water. DHW is a continous flow of new water getting heated, i.e. a continous flow of new scale to get deposited. On demand heaters are probably made to be less susceptible to problems in such an environment, while boilers would choke up pretty fast if used to heat DHW.

I'm not sure if these are popular in the US, but in Europe dual function boilers are quite common. They provide both heating and hot water, with the latter getting priority (central heating goes on hold while using hot water). I'm not sure exactly how this is done, but I don't think that it is two different heat exchangers over the flame, and it definately doesn't share the same HX since that would cause mixing.

EDIT: After studying a few diagrams, I believe that is a HX between the central heating and DHW plumbing.

[AC_Hacker]

Originally Posted by Daox

Apparently boilers can't directly heat DHW, I'm not sure why, but on demand hot water heaters can. Doesn't make sense to me.

I suspect that boilers use single wall heat exchangers for higher efficiency, and DHW requires double wall heat exchangers in the US.

Originally Posted by Daox

I was hoping to only install a boiler and have that as my backup to the solar for DHW and home heating. However, I'm still not sure how it'll all tie together.

On the Bosch 125 models, the unit achieved temp control by varying and balancing pressures... it didn't really know what the temperature actually is.

At the time I bought mine, there was also a model that was compatible with solar that would sense temperature. It was more expensive, so I didn't buy one.

I'm know that there is a Rinnai that senses temperature and is approved for solar. I'm pretty sure that every major maker of tankless has a solar approved model, too.

EDIT: After studying a few diagrams, I believe that is a HX between the central heating and DHW plumbing.

This would create a double wall heat exchanger (because there are two heat exchangers).

-AC_Hacker

[Daox]

Originally Posted by AC_Hacker

On the Bosch 125 models, the unit achieved temp control by varying and balancing pressures... it didn't really know what the temperature actually is.

Thats good to know. I'll have to do some research tonight to see, but I'd imagine most of them do this now.

[Daox]

Originally Posted by Piwoslaw

Can you retrofit your heater with spark ignition? Pilot light is a waste!

I was thinking of doing that, but knowing it won't work with variable temperature inlet water is a deal breaker for me, so at some point its going to need to be replaced. If I can figure out a way to get it working well enough for a few months until the solar is up, that would be ideal. However, I'm having difficulty figure out how to do that without spending much. I'm thinking I can use a blower fan I have and make my own power vent for it, but I'd need to get on that asap.

Otherwise, a guy at work said he might be able to get me a used gas tank heater for pretty cheap to hold me off until I get the solar in.

[AC_Hacker]

Originally Posted by Daox

If I can figure out a way to get it working well enough for a few months until the solar is up, that would be ideal.

If you can be assured that the outside temp will not go below freezing, you could mount it outdoors... I could get away with it where I live, but your weather patterns are way different.

Your Bosch unit isn't worthless by a long shot. It could be used to heat a tank to a given temperature... for instance a hydronic storage tank...

A temperature sensor could sense when the storage tank temp was at the cold set-point, and activate a circulation pump which could create a pressure differential, which would trigger the tankless... when the storage tank temp hit the high set-point, the circulation pump could be turned off, which would stop the Bosch.

-AC_Hacker

[Daox]

Thats true, however if I'm going to do this, I think I want the best or at least better components that are more so made to do what I'd like them to do.

[Daox]

A guy at work here suggested this boiler unit. He said his uncle has one and loves it. It does heating for both DHW and space heating like Piwoslaw suggested. I haven't seen others that do this, but I haven't looked a ton either.

PE110-NG - Triangle Tube PE110-NG - 86,000 BTU Output Prestige Excellence Combo Boiler - Space Heating & Domestic Hot Water (Natural Gas)


I'm also not sure if this would be any better than buying a much less expensive boiler with the same efficiency and just having my own ~15 gallon water tank with its own circulator pump on it and super insulate it.

[AC_Hacker]

Originally Posted by Daox

I'm also not sure if this would be any better than buying a much less expensive boiler with the same efficiency and just having my own ~15 gallon water tank with its own circulator pump on it and super insulate it.

That's a pretty penny...

That unit certainly has all the latest features. It's really expensive for a DWH, but pretty fair for a high efficiency combo boiler.

How serious are you about radiant heating? That boiler would work fine with wall radiators, on the order of what Xringer has.

Are you considering trying to use solar heated water with radiant? If you're going this route, you'll need to seek out the most efficient radiant possible and wall radiators wouldn't do, neither would under floor hydronic.

How serious is your home insulation? What would $4K worth of insulation do for your house?

Lots of factors to balance,,,

-AC_Hacker

[Daox]

I'm very serious about hydronic floor heating, I just don't have all the floors put in yet (very few actually). Same goes for the solar hot water system.

At the moment, my insulation is about standard on the first floor and sorely lacking on the 2nd. Again, just haven't gotten to that yet.

After talking with a friend who does HVAC work he said I would be better off just going with an external water tank and a normal boiler to heat that tank up. The savings would be substancial.

After a lot of searching, I was thinking of picking up a cheap water heater again just to get me on my feet again. I was thinking probably go for a 40 gallon gas tank heater (~$300) since it'll pretty much drop right in. When I go ahead with the solar and the boiler I'd sell it.

However, the other option I am looking at, and starting to like a lot, is a small tankless electric water heater. These are even cheaper (<$200), should work with solar hot water, and could be repurposed after the boiler is installed to reduce heat up times on the 2nd floor of the house.