Tankless LPG Water Heater for Hydronic Heat

[Stormcloud]

Hello, all:

I am leaning towards the installation of a propane-fired, on-demand (tankless) water heater to supply heated coolant to several heat exchangers inside my MCI 7 as a source of interior heat, via a closed (sealed) system. I think a Grundfos pump, small expansion tank, and several heater cores with fans behind should make a workable system, but I need more info......specifically....  in choosing the correct tankless heater, I have read that "the only heater in the world that will work is XXXX brand rated for zero pressure".

I know this system has been done before in someone's bus.

Did you use a 'zero pressure' tankless heater? Is a low pressure tankless heater workable?  Am I missing something?

Thanks.

Mark

[NewbeeMC9]

are you going to use it for hot water also?

youll need a Heat exchanger for that too, if you add a circ pump for the freshwater side, you can use it for instantaneous hot water also , and not heat the heating loop in the summer

make sure the heater has enough BTU to do what you want


I have rinnai on my house and have seen one on a bus that was only used for hw.

Mine need abut .5 GPM to work

[Stormcloud]

I may add a loop for domestic hot water in the future; at this point, the instant hot water will be only for heating the interior of the coach.
I'm sure the pump can provide 2 gpm or more; its the same pump I used in my shop for hydronic floor heat.
I'm just not sure about the 'zero pressure' thing.

Does there need to be a water pressure drop at the outlet of the tankless heater, or will the flowing water in the closed system (all at same pressure) suffice for the operation of the heater?

Thanks.

Mark

[Jerry32]

My Idea would be to put the pump discharge directly to the cold water side of the tank useing the pump pressure to activate the water heater? would not there be a differential of pressure between the suction and discharge.? Jerry

[Stan]

The tankless water heaters that I have worked on used a differential pressure cell tied across an orifice in the incoming water line. That type requires a significant pressure change to turn on the heater. This is how they determine the minimum flow rate and the corresponding maximum outlet temperature. I don't think a circulating pump would provide sufficient pressure differential on this type of heater. Since some people use tankless heaters for space heating, there must be a model (unknown to me) that will work.

[HB of CJ]

Is this tankless heater rated for long term/contineous duty cycles?  Wondering if the thing is designed to run any length of time.  Overheat/failure/fire/destruction/injury/death?  Dunno. 

[Stormcloud]

The tankless heaters I have look at have a 20 minute maximum on cycle, apparently to prevent toxicosis from the unvented heaters if installed in an occupied room.To reset the timer, it is a matter of turning the flow off, then on again. Other than that, these units can apparently be used almost continuously.

The heaters can become fairly warm, so much so that they are recommended to be installed on a non-combustible surface.
I intend to mount the heater in one of the bays, or perhaps the former A/C hole, for fresh air and to meet the exhaust requirements.

The smallest of the heaters are rated at maximum of about 35,000btu; even at 80% efficiency, I don't think I would need more than that.

Thanks.

Mark

[Jerry32]

You could also mount them in the engine room and use the vented heat to help keep the engine warm. Jerry

[Lee Bradley]

Mark,
Check out this article
http://www.houseneeds.com/shop/HeatingProducts/waterheating/takagi/takagitkjrmain.asp