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ropeguru
Premium
join:2001-01-25
Mechanicsville, VA

Heating/Cooling opinions...

I am going to be changing out my current heat pump this summer so I can have separate zones for main floor and my basement.

I have a nice open attic to have the upstairs air handler installed along with all new ducting for the upstairs. Current ducts are in the floors and come from the basement. I am in Virginia and think that an 80% efficiency gas furnace will be fine. I have about 1200 sq. feet to heat and cool both on the main floor and the basement.

In the basement where the utilities are, I am kind of limited on space in addition to limited space on the side of the house outside for any discharge vents. In the basement I am thinking of either a 50 gallon natural gas water heater with a hot water supplied air handler or an on demand water heater with the hot water supplied air handler.

Each unit would of course have cooling coils installed for the summer cooling.

Anyone dealt with using hot water heating such as this and pros/cons of using a larger water heater or an on demand water heater?



leibold
Premium,MVM
join:2002-07-09
Sunnyvale, CA
kudos:10
Reviews:
·SONIC.NET

That is an interesting question.

I know of boilers being used for hot water and heating and I know of using excess heat from heat pumps in cooling mode to be used for water heating. However so far I haven't heard about using hot water from a water heater as supplemental heat for a heat pump in winter.

I wonder whether a water heater would be able to provide a sufficient amount of heat. Just a quick comparison of BTU ratings for typical residential water heaters and furnaces shows that the furnaces tend to have much higher BTU ratings. Since you are planning to use a large water heater it might just be feasible.

Tankless (on demand) water heaters do have high BTU ratings, but what would you do with all the excess warm water after running it through the air handler ? Most tankless water heaters explicitly prohibit warm inlet water and will shut down if you try to circulate the warm water back into it.
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ropeguru
Premium
join:2001-01-25
Mechanicsville, VA

The hot water isn't even a supplement for a heat pump. It is the primary heat source and there is just an additional cooling coil.

»www.rheem.com/products/integrated_systems/

I know several people that have the hot water setup I am looking at and they seem ok with it. They use the larger hot water heater instead of the tankless though. I am just trying to expand on more opinions than two.



pende_tim
Premium
join:2004-01-04
Andover, NJ
kudos:1
Reviews:
·Comcast
reply to ropeguru

Why not 1 system with 2 zones? A thermostat controlled damper would be relatively inexpensive. This way you could use the existing ducting and save a bunch of money.

If you run water to an attic mounted AHU coil, freezing could be a real problem in the winter.

Have you considered using electric strips as the supplemental heat source? Run the efficiency numbers for your climate, and you may be surprised at the relative operating costs and the benefit is that when the system defrosts, you are not hit with cold air.

Has anyone done a Manual J load calculation yet? Don't forget to have a blower door test done to find leaks, you may be able to save a "ton".
--
The difference between genius and stupidity is that genius has its limits.



ropeguru
Premium
join:2001-01-25
Mechanicsville, VA

said by pende_tim:

Why not 1 system with 2 zones? A thermostat controlled damper would be relatively inexpensive. This way you could use the existing ducting and save a bunch of money.

If you run water to an attic mounted AHU coil, freezing could be a real problem in the winter.

Have you considered using electric strips as the supplemental heat source? Run the efficiency numbers for your climate, and you may be surprised at the relative operating costs and the benefit is that when the system defrosts, you are not hit with cold air.

Has anyone done a Manual J load calculation yet? Don't forget to have a blower door test done to find leaks, you may be able to save a "ton".

I am NOT running hot water for the unit in the attic. That will be strictly an 80% efficient natural gas heat source.

I am going to have the contractor, when I decide who that is, do a Manual J and blower door test. I am also going to be replacing the cheap vinyl windows with better ones over the next year or two as I can afford it.

Zach1
Premium
join:2006-11-26
NW Minnesota
reply to ropeguru

A couple of things come to mind....

Will the proposed water heater also provide domestic hot water? If so, you will most likely have to use a NSF approved heat exchanger to keep potable water and the loop used for space heating separate. Since codes and/or their enforcement vary widely from location to location, YMMV.

Also, some manufacturers of tank type water heaters don't recommend and won't warranty units used for space heating. Again YMMV.

Otherwise, using hot water and heat exchangers for forced air heating works well and is done all the time. I use one here to supplement the geothermal and under-floor radiant. However, the hot water is provided by a wood-fired boiler which also provides our domestic hot water during the winter months.
--
Zach


lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1

1 edit
reply to ropeguru

said by ropeguru:

Anyone dealt with using hot water heating such as this and pros/cons of using a larger water heater or an on demand water heater?

I have to replace the original (1989) gas furnace and water heater later this year. I am going to skip the A/C for now since we don't have that many cooling days in Ottawa.

But our family can't do without hot water ... lots of it. I am going to finally install a basic solar water heating system but we will still need a gas water heater for colder months. This is where I started thinking of using hot water based air handler to eliminate the gas furnace ... just like you. I collected brochures and manuals of several commercial systems.

However, I am seriously considering building (actually getting built by someone more handy than myself) a much more efficient water-air heat exchanger using copper pipe spirals embedded in carbon foam. It should be light enough to hang from the floor beams. I will use a very quiet DC blower running at moderate CFM.

A few different system configurations are shown in the installation manuals of any commercial system. Potable cold water is always kept separate but the hot water could actually be reused depending on regulations and system design.

The gas heater will use less energy to heat up the luke-warm water coming out of the heat exchanger during winter (for space heating or domestic use) and also during summer (for just domestic use).

*** I put some comments on the LifeBreath diagram to show what parts I will change. The system will become 1/2 the height and 1/4 the weight. I hope it will also cost less than 1/2 of their list price but I have not started looking at component pricing.


ropeguru
Premium
join:2001-01-25
Mechanicsville, VA
reply to Zach1

I would certainly be following all the guidelines, practices and codes. This will be a professional install and not a DIY at this point.


lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1

Very good article written by a Canadian who used on-demand gas water heater for space heating.
»www.consol.ca/downloads/Dual_hea···stem.pdf

Because of regulations you may have to use water-water exchanger first to separate domestic hot water from space heating water. This US company makes both types: »www.brazetek.com/



cowboyro
Premium
join:2000-10-11
Shelton, CT
Reviews:
·AT&T U-Verse
reply to ropeguru

Do your math regarding the cost of gas and electricity for heat pump.
Being in VA the efficiency of a HP is fairly high (warmer weather -> higher COP) and AFAIK the cost of electricity is fairly low.
You'd have to compare the cost of ~10,000+ BTU/kWh to 80,000 BTU/therm (given the efficiencies).
For example if you pay $0.12/kWh your break-even gas cost is $0.96/therm....
Keep in mind that the difference between a regular A/C and a heat pump is only ~$500 so IMHO it's not worth locking yourself into gas-only as heat source regardless of current costs.



Msradell
P.E.
Premium
join:2008-12-25
Louisville, KY

said by cowboyro:

Do your math regarding the cost of gas and electricity for heat pump.
Being in VA the efficiency of a HP is fairly high (warmer weather -> higher COP) and AFAIK the cost of electricity is fairly low.
You'd have to compare the cost of ~10,000+ BTU/kWh to 80,000 BTU/therm (given the efficiencies).
For example if you pay $0.12/kWh your break-even gas cost is $0.96/therm....
Keep in mind that the difference between a regular A/C and a heat pump is only ~$500 so IMHO it's not worth locking yourself into gas-only as heat source regardless of current costs.

You beat me to it. I'm betting in Virginia standard high-efficiency heat pumps with electrical strip backup will be the most economical solution, especially when you consider operating costs. Having the water coils, providing heat for the basement unit is just asking for trouble in the long run because of maintenance issues. Also in very cold weather, the availability of hot water for other purposes would be severely limited unless the water heater was significantly oversized.