Using a gas on demand DHW for space heating?

[Daox]

So, I have a Bosch Aquastar 125 natural gas on demand water heater. I thought I was going to use it for DHW and have since decided not to (doesn't work well with solar hot water). It is rated for a max gas input of 117,000 btu (@ 80% efficnency gives me 94k btu output) which is more than enough to heat my house (gas furnace is 80k). As I start converting my house over to hydronic heat, I'll need something to backup my solar hot water setup that will be finished up this summer. The manual for the Aquastar specifically says 'for heating potable water only not approved for space heating'. So, my question is why can't I use it for space heating? What if I do use it as a boiler? Is this illegal? Is it just going to be a problem with insurance if something bad happens?

The second question I have is what are the implications if I start modifying it? As is, its only 80% efficient, I think I could definitely improve upon that with a few semi-simple modifications (dedicated air intake, additional heat exchanger). It also has a pilot light where I think spark ignition wouldn't be too hard to add, perhaps even from a later model with that feature.

[Ryland]

First two things that come to mind are that for space heating you want the water hotter then 120F 2nd thing that comes to mind is seeing the inside of a Bosch on demand water heater that was used for something other then DHW I don't remember what it was but it was running more then it should and the insides of it had warped and burned.
Also a boiler should be more then 80% efficient.

[strider3700]

Yeah my guess would be that it can't handle the longer more constant run times involved in space heating vs the quick hot shots necessary for a shower or bath.

[Xringer]

I've been told that it's 'illegal' in MA for homeowners to work on their gas heating or hotwater heaters.

It's not very hard to replace defective heat sensors etc, and I don't think
OEM part replacement would even be detectable by forensic analysis..
But, if you hack the ignition or HX, your insurance company isn't going to pay..
(If they found out).

~~~

Space heating (in this area) is going to have a duty cycle that's
about 80% greater than heating DHW for a medium family.
(At least during the winter time).
I'll bet if you read the warranty, it says Not for commercial use .
And stipulates 'normal home use' somewhere in there..


My guess is, Bosch thinks it's going to add on ten years of wear,
for every one year it's used to heat a home..

If you don't care about warranty stuff and plan to buy spare parts as needed,
then you should try it. It just requires some planing and plumbing..

With a lot of good baseboards installed, 120 deg F water will work for home heating.
It's not as effective as 180F, but it will work for much of your normal needs.
When it's super cold, you might need to bundle up and burn some wood..

[AC_Hacker]

Originally Posted by Daox

So, I have a Bosch Aquastar 125 natural gas on demand water heater...rated for a max gas input of 117,000 btu (@ 80% efficnency gives me 94k btu output).

Yes, it is possible... The insurance issues previously cited are valid concerns. Also, not cited is that if you hack a gas appliance, you are taking your life, and your family's into your own hands. But you also do that when you repair the brakes on your family car. Personally, I prefer to do this kind of work myself, because since my life depends on the quality of the work, I am very motivated to do it right, and it gives me peace of mind to know that the work was done right.

Originally Posted by Daox

The manual for the Aquastar specifically says 'for heating potable water only not approved for space heating'.

This is about the pressure regulation of the Bosch 125X. We have discussed this at length before.. no need to do it again.

They (Bosch) do (did?) make temperature controlled versions of this heater. It is possible that you could get your hands on those components and retro-fit them in.

Originally Posted by Daox

So, my question is why can't I use it for space heating?

You can.

You could build a sensor/solenoid-valve setup that would electrically do what the mechanical sensors of the temp-controlled Bosch did. You would want to be very mindful of all failure possibilities, and build it so that it ALWAYS failed in the off state (fail safe). You could do this with the Arduino knowledge you have already earned.

Originally Posted by Daox

What if I do use it as a boiler?

It would work.

Originally Posted by Daox

Is this illegal?

Probably illegal in most industrialized countries.

Originally Posted by Daox

Is it just going to be a problem with insurance if something bad happens?

Of course there will be problems if your hacks fail... there is also the possibility of loss of life... But you take that chance when you drive the car to the grocery store, too.

Originally Posted by Daox

I think I could definitely improve upon that with a few semi-simple modifications (dedicated air intake, additional heat exchanger).

Most likely your improvements would be very small, unless you developed a way to condense out the moisture in the stack gas. Then you could expect a 15% jump in efficiency, especially if you used that to pre-heat water that was incoming to the heater.

Originally Posted by Daox

It also has a pilot light where I think spark ignition wouldn't be too hard to add, perhaps even from a later model with that feature.

Bosch had a pilotless (electronic) igniter that was contemporary with your unit. I know, because I have one.

Originally Posted by Ryland

First two things that come to mind are that for space heating you want the water hotter then 120F

This would be no problem, but you would want a buffer tank for your Bosch to heat up, that would be the heat reservoir for your hydronic system.

Originally Posted by Ryland

...seeing the inside of a Bosch on demand water heater that was used for something other then DHW I don't remember what it was but it was running more then it should and the insides of it had warped and burned.

To do this, the gas valve would have to be stuck on (very, very unusual) AND the water flow would have to be off (very, very, very unusual). Someone would have to force the unit into this condition, there are safeguards built in to prevent this from ever happening.

Originally Posted by strider3700

Yeah my guess would be that it can't handle the longer more constant run times involved in space heating vs the quick hot shots necessary for a shower or bath.

As long as you have water and gas flowing into the Bosch it will keep running. They are built pretty tough. Some of the sensors can fail and if they do, the Bosch will fail into the off state.

As a long-time user of a Bosch 125, I can say that you should not be worried about this possibility. I have taken showers longer than you can believe and the outcome was total relaxation on my part, just another day for the Bosch.

All this being said, the most compelling reasons to do this conversion would either be an overwhelming love of hacking or utter, grinding poverty, with no chance of earning axillary cash.

You would probably be better off financially, getting an extra job in a burger joint, and saving the money to buy a Condensing Water Heater (about $2000) that has been discussed in detail elsewhere in EcoRenovator.

Then you could sell you Bosch on ebay, and move your electric water heater to a position right under the kitchen sink, where it will still be somewhat useful.

-AC_Hacker