on-demand water heater

Applying the PH standard

on-demand water heater

Postby Dave Brach on Thu Dec 11, 2008 6:46 pm

I had an engineer suggest using an on-demand water heater linked to a coil in an air handler to heat a Passive House. To enter this in the PHPP I assume that I would pick "low temperature boiler" in the Boiler worksheet, correct? And I assume that I must account for the energy consumption of the fan/air-hanler in the Aux Electricity worksheet, but does anyone know where exactly?
Dave Brach AIA
Salt Lake City, Utah
Brach Design Architecture
Dave Brach
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Joined: Thu Jul 10, 2008 3:56 pm

Re: on-demand water heater

Postby Kat on Fri Dec 12, 2008 10:41 am


yes, if you have a gas boiler, you enter it and all the required specs in the boiler sheet. In regards to the fan coil, is your engineer suggesting that option because he realizes that your peak load is too high to be able to transport all needed heating through the ventilation system? Adding an additional fan coil and return air loop to the ventilation system to increase the air flow and therefore the ability to transport more heat is a solution that Gary Nelson and Mike LeBeau both have been discussing. That would avoid having to install additional point source heaters in the space. I don't think neither of them have actually attempted to install one. So, I am very interested in this suggestion. I am not sure how much the additional fan energy consumption (hopefully ECM fan) will add to the system. Also, the ducts now are getting bigger again. You will have to look at that. And yes, the additional fan consumption would go into the aux electricity. Also, under Heating system, there you will have to enter the consumption for the circulation pump as well as the electric consumption of the boiler. I think that one comes in automatically if you have an entry in the boiler sheet. The additional fan energy then goes into miscellaneous aux. electricity.

My concern with this system so far was, that in a passive house we are trying to simplify the HVAC system and by adding a fan coil back into that seems to back track. Passive house is based on constant low flow ventilation for comfort and low dust/noise purposes, with reintroducing a fan coil it seems to me that we are giving that up. So, I have been leaning toward small hydronic radiators as point source heaters under windows in a simple loop. This could include a coil in the air stream which can account already for 10 W/sqm of 1 W/sqft. The remaining energy, lets say at 17 W/sqm peak, 7 watts will have to be provided by the point source heaters distributed throughout the space. Those 7 watts could also be provided through electric in floor in bathrooms or electric baseboards in bedrooms. Most of the time the integrated coil in the vent system should be sufficient, just during the peak days the additional heat sources will have to kick in. In addition, it seems an added benefit, even though the local heaters are not on all the time, they do provide an option for zonal temperature control if desired.
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