LP tank, tankless heater and duel-fuel heat pump

[merrill77]

The water heater thread reminded me that I've been putting off some decisions - so thanks to all for the info on tankless water heaters. I've been considering one of those for a couple of reasons, primarily energy efficiency. The unlimited hot water will be nice as our three boys start consuming more of it! I've heard the new gas units are so efficient that they use PVC for the exhaust vent. Our water heater is about 12 years old. It was replaced under our home buyers warranty, so I am sure it was the lowest quality available...as a result I figure I'm getting near the end of its life expectancy. I didn't know where were outdoor units - that might work for me.

Has anyone run across data on reliability/lifespan of the tankless gas heaters? They are pretty pricey - should they outlive a conventional tank?

BTW, have gas, so I'm also considering putting in a propane tank behind my house. My first floor heat pump is also near (past) its life expectancy...so I was considering a duel-fuel pump for better efficiency in the winter. The heat pump is also in a closet in my workshop, so the gas line doesn't have to run very far. It is also right below the kitchen, making a gas range a possibility. I'm trying to consider them all together, since (I assume) there is considerable expense in adding a tank and associated lines.

I'm open to any advice - I've only barely researched this at all.

As an aside, the current water tank is in my workshop...so a smaller tankless unit means a few more sq ft in my small shop

 

[junquecol]

I have had a dual fuel system for over 30 years now. Original equipment is still going strong, but is on the "to be replaced list." Instead of propane, look at fuel oil. Propane and oil cost basically the same per gallon, but propane only has 96,000 BTU's per gallon, while fuel oil has 146,000 BTU's per gallon. A 65% effic. oil furnace would be cheaper to operate than a 100% effic. propane furnace. Of course there are no 100% effic. propane furnaces, but a lot of oil furnaces that are more than 65% effic. If you are looking at the gas tankless heater, then look at a hydronic coil piggybacked on top of the HP air handler.

 

[mkepke]

I agree with Bruce that you need to consider the cost per BTU for the various fuel sources available to you.

When I replaced my LP gas dual-fuel package unit, I went with an all-electric heat-pump. I do not believe a dual-fuel LP heat-pump is a good choice - the dual-fuel package units are limited to 80% efficiency and the HP is typically somewhat less efficient than a split-system HP too. If you want a dual-fuel with high efficiency furnace, you will pay an *extra* arm and a leg for the pleasure of a split-system with a high-eff. LP furnace.

The other problem with dual-fuel systems is that they operate on an all-or-nothing principle. If you need 1 BTU more than the HP can provide, you need to switch 100% of the heating load over to the dual-fuel source. In contrast, a HP with heat-strips will continue to run the HP while those heat-strips are run to provide the missing BTU. LP is way too expensive to run it any more than necessary.

By going to an all-electric split-system HP, I was able to:
i) get a 15 SEER unit for the same cost as a 14 SEER package unit
ii) put the air-handler in my crawlspace, thus protecting it from the weather and increasing its longevity
iii) eliminate the unsightly hulk of the package unit
iv) and oh yeah..stop burning propane at $3.50/gallon !

Regarding tankless WH: I took a serious look at these before discovering my furnace needed to be replaced. Now that the Govco tax credit has expired, there is no financial payback possible in my case. Run your own numbers. Remember that to install a tankless, you probably need:
i) to have your gas regulator replaced and gas piping upsized (since tankless need ~2.5x as much gas as a tank heater)
ii) new venting installed
iii) a 120V circuit run to operate the tankless electronics
iv) electrical and plumbing permits

If you want to relocate the unit you are looking at extra gas piping (and not the cheap black-pipe either !) and more hours for the plumber to relocate lines.

When I replace my 50 gal tank, I plan to get another tank heater with 2x the insulation, crank the tank-temp to 135F to create lots of reserve hot-water and install a thermostatic valve to limit all the hot lines to 120F, so there is no scald danger.

-Mark

 

[cptully]

Mark,

You wrote:

If you want to relocate the unit you are looking at extra gas piping (and not the cheap black-pipe either !) and more hours for the plumber to relocate lines.

Why not the "cheap" black pipe? if you are implying that you have to switch to flexible pipe, I would question your source. I fully agree that a short flexible connection is worth while (I just redid the connection to my WH to eliminate a leaking rigid connection), but I see no reason that you could not use rigid pipe when extending/moving the gas supply for a new tankless heater. The only reason I can think of is a lazy plumber who doesn't want to cut and thread iron pipe.

Chris

 

[mkepke]

Mark,

You wrote:


Why not the "cheap" black pipe? if you are implying that you have to switch to flexible pipe, I would question your source. I fully agree that a short flexible connection is worth while (I just redid the connection to my WH to eliminate a leaking rigid connection), but I see no reason that you could not use rigid pipe when extending/moving the gas supply for a new tankless heater. The only reason I can think of is a lazy plumber who doesn't want to cut and thread iron pipe.

Chris

Sorry..just trying to say that the cost of the gas-fitting can add up, even if "cheap" black pipe is involved. Black pipe is cheap to buy, but not to professionally install.

Edit: In retrospect, the cost of the pipe will probably be minor-to-negligible compared to the cost of the unit.

-Mark

 

[drw]

Chris,
You asked about the longevity of tankless systems, while I do not have specific data, my system is warranted (parts, labor, everything) for 12 years. Additionally, during that 12 years the installer will descale the system once every there years at not cost to me. I was both pleased and surprised by this warranty and service; therefore my thoughts are if they are willing to provide this commitment in writing, it must be a fairly robust system.

Donn

 

[ACobra289]

RE: Dual fuel systems

About 3 years ago I replaced our HVAC with a dual fuel setup. I went with a gas furnace (92%) and a 14 seer heat pump. You can set the system to switch from heat pump to gas furnace at whatever temp you want. This recommended switch point is usually based on the efficiency of the heat pump at lower temps. I think we have ours set to switch at 32 degrees.

I love having the flexibility of not being tied to one fuel type. If the price of natural gas or electricity goes through the roof, I can easily set the system to only use one type.

We are saving quite a bit over our old 80% propane furnace, but that is to be expected with higher efficiency equipment. But where I really notice savings is during the summer. With our old AC unit, our summer bills could get WELL over 200 dollars. With the new heat pump, even during the hottest months, the bill is usually around $110. (With the temp set to around 73 or 74.) I like it cool. :gar-Bi Our house is a little over 2000 sq ft and decently insulated.

 

[mkepke]

RE: Dual fuel systems

About 3 years ago I replaced our HVAC with a dual fuel setup. I went with a gas furnace (92%) and a 14 seer heat pump. You can set the system to switch from heat pump to gas furnace at whatever temp you want. This recommended switch point is usually based on the efficiency of the heat pump at lower temps. I think we have ours set to switch at 32 degrees.
<snip>

Just to highlight the difference between my all-electric setup and Bill's dual-fuel, my HVAC runs the HP exclusively down to 20 degrees. Below 20 degrees, the electric resistance heat is allowed to augment the heat-pump (i.e. both HP and electric heat run).

I don't remember the lock-out temp for the HP..maybe 0 degrees ?

If you have a cheap fuel source like NG, that optimization of allowing the HP and the aux heat to run in parallel might be irrelevant. Heck, if I had NG, I might not have bought a HP at all.

I really wish PSNC would run a NG trunkline by the neighborhood..maybe in 10 years...

-Mark

 

[petebucy4638]

Just to highlight the difference between my all-electric setup and Bill's dual-fuel, my HVAC runs the HP exclusively down to 20 degrees. Below 20 degrees, the electric resistance heat is allowed to augment the heat-pump (i.e. both HP and electric heat run).

I don't remember the lock-out temp for the HP..maybe 0 degrees ?

If you have a cheap fuel source like NG, that optimization of allowing the HP and the aux heat to run in parallel might be irrelevant. Heck, if I had NG, I might not have bought a HP at all.

I really wish PSNC would run a NG trunkline by the neighborhood..maybe in 10 years...

-Mark

Natural gas is not as cheap as it used to be when compared to a good high-efficiency heat pump. Where the gas heat would come in handy is when the heat pump is defrosting or when it is extremely cold. One of the nicest options is a geothermal heat pump. Unfortunately the cost of installation is still much higher than it should be.

I think that you have made a good choice with the Heat pump/high-efficiency natural gas furnace option.

Pete

 

[junquecol]

A geothermal heat pump can be installed using vertical loops, or a "slinky" coil. With the vetical loops you are going to need about 300' per ton of cooling. Holes have to be drilled, poly inserted and pressure tested before grouting. A real PITA is to have a hole full of water. Poly floats, so even if you fill it with water prior to insertion, you have to figure how to get it down the hole. Sometimes you just have to add a weight to the return bend at the bottom of the loop. With the slinky, you will need about 100' of trench per ton. Trench only needs to be about 6' deep and wide enough for poly slinky to fit in. One day (don't count on it) maybe the health dept will let heat exchange loops be installed in bottom of septic field. Two for the price of one.

 

[mkepke]

A geothermal heat pump can be installed using vertical loops, or a "slinky" coil. With the vetical loops you are going to need about 300' per ton of cooling. Holes have to be drilled, poly inserted and pressure tested before grouting. A real PITA is to have a hole full of water. Poly floats, so even if you fill it with water prior to insertion, you have to figure how to get it down the hole. Sometimes you just have to add a weight to the return bend at the bottom of the loop. With the slinky, you will need about 100' of trench per ton. Trench only needs to be about 6' deep and wide enough for poly slinky to fit in. One day (don't count on it) maybe the health dept will let heat exchange loops be installed in bottom of septic field. Two for the price of one.

My experience: geothermal only makes economic sense for new build construction, where the fields can be built into the site plan.

Drilling runs ~$10/ft.

Slinkies (coiled loops laid horizontally) need a lot of earth-moving. I did not have enough room on my 1.7 acre lot for 4.5 tons of slinkies. I have a house, outbuilding (shop) and a fair bit of natural areas - it would have fit if I was prepared to level a couple thousand square feet of natural areas.

The best quote I could get was a groundwater GHP system for $14.5K (4.5 ton) - but it depended on the same well as my drinking water. Add another $4K for a return well. I thought it too risky to share a well, mostly due to the risk of exhaustion. GHP water-source pumps a lot of water.

My experience only.

-Mark

 

[merrill77]

That was my similar to my experience as well. About 10k for sinking three vertical wells and installing the lines, plus the cost of the heat pump on top of that. That system would not, however, pump any water. It would have been a closed-loop system and use the well as a heatsink/source (so no risk of well exhaustion). The ROI on energy savings would have been close to 15 years compared to replacing with a 14 SEER air-source heat pump. Very hard to justify that...especially as efficiency of air-source heat pumps continues to rise.

My experience: geothermal only makes economic sense for new build construction, where the fields can be built into the site plan.

Drilling runs ~$10/ft.

Slinkies (coiled loops laid horizontally) need a lot of earth-moving. I did not have enough room on my 1.7 acre lot for 4.5 tons of slinkies. I have a house, outbuilding (shop) and a fair bit of natural areas - it would have fit if I was prepared to level a couple thousand square feet of natural areas.

The best quote I could get was a groundwater GHP system for $14.5K (4.5 ton) - but it depended on the same well as my drinking water. Add another $4K for a return well. I thought it too risky to share a well, mostly due to the risk of exhaustion. GHP water-source pumps a lot of water.

My experience only.

-Mark

 

[petebucy4638]

That was my similar to my experience as well. About 10k for sinking three vertical wells and installing the lines, plus the cost of the heat pump on top of that. That system would not, however, pump any water. It would have been a closed-loop system and use the well as a heatsink/source (so no risk of well exhaustion). The ROI on energy savings would have been close to 15 years compared to replacing with a 14 SEER air-source heat pump. Very hard to justify that...especially as efficiency of air-source heat pumps continues to rise.

The closed-loop system is the way to go. I have had clients request ground water heat pumps where they didn't use a return well. Talk about a mess, even on a large lot, not to mention the cost of water. Open systems suffer from the effects to the ground water mineralization and other contaminants. They can be difficult to keep running and the cost of maintenance can quickly wipe out any savings that they might provide at the electric meter. When advertising the energy savings of these units, the vendors never included the cost of running a pretty big water pump in the equation.

Closed-loop systems use treated water that is much less likely to cause mineral built up or sediments that can degrade the mechanism. These systems are efficient but very expensive.

Pete