Comments on: Heat Pumps and Hydronics – A Great Team for High Performance Homes

By: David Butler

David Butler — Mon, 05 May 2014 00:16:08 +0000

@Bob, it's impossible to make
@Bob, it's impossible to make specific recommendations in a forum like this.

HAVC selection depends on many factors including the design load, room layout, management of peak glazing gains, local utility rates, and last but not least, homeowner preferences & expectations.

Although 3rd party HVAC design is atypical for residential, it makes sense for high performance homes because so few mechanical contractors understand the nuances of designing for better-than-code homes. Plus a 3rd party spec allows homeowner or builder to get competitive apples-to-apples bids on the equipment configuration that's most appropriate for the home and individual circumstances.

By: Bob Hall

Bob Hall — Sun, 04 May 2014 22:37:04 +0000

David, I am currently having
David, I am currently having a 2K sq/ft home being built in N.Scottsdale. 2x6 walls foam insulation, 2 people & 1 small dog. Any suggestions on my HVAC system?

By: David Butler

David Butler — Tue, 25 Feb 2014 22:33:02 +0000

Tom G wrote: "Allison,
Tom G wrote: "Allison, getting your hands on this graph would probably make for an interesting article"

If you can locate a copy, please share. I recently removed an old Trane dual fuel kit from a client's home. The original paperwork was taped to the enclosure, but no graph. BTW, I wrote the article, not Allison.

you wrote:
"I've always wondered from a practical side, who would update these settings over time to accommodate future rate changes."

Well, people like Tom C who are even aware to ask the question. But for inquiring minds, the math is easy once you understand how to do it. What would be helpful would be if manufacturers would publish heat pump COP graphs.

you wrote: "basing the control strategy on the ability of the HP to maintain the requested interior t'sat setting is lame and could cause higher then needed operational costs, especially considering the price of natural gas right now."

Actually, just the opposite is true if the economic balance point is significantly lower than the thermal balance point, which is usually the case with propane or electric supplemental heat.

But if the economic balance point is above the thermal balance point, I agree that a control strategy based on ODT is the way to go.

By: David Butler

David Butler — Tue, 25 Feb 2014 00:25:01 +0000

@Tom, great question. The
@Tom, great question. The economic balance point calculation requires that you have energy rate details and the heat pump's COP data.

First, determine the marginal cost of natural gas. Include all variable costs, but exclude any flat fees that don't depend on usage. If rate is tiered by volume, use the appropriate marginal rate. Depending on how gas is metered, the rate must be converted to therms (100,000 BTU). For propane, convert the price per gallon to therms by dividing by 0.916. Finally, divide by the seasonal efficiency of the boiler or furnace. For example, if the marginal rate is $0.75/therm and boiler AFUE is 90%, then the cost per therm of heat is $0.83. Add 5 to 10 cents per therm for the blower fan, depending on whether it's ECM or PSC.

Now take the marginal winter electric rate and multiply by 29.3 to convert to cost per therm. Divide that by the marginal gas cost. That's the break-even COP for the heat pump. Watch the decimal places. The result should be between 1 and 10.

If you know the heat pump's COP at 47F and 17F (some mfrs publish this), you can draw a linear COP curve based on the two points. Although that ignores the 'defrost knee' at ~35F, it will get you close. For more precision, you can calculate the COP curve from the expanded data table. You just need a couple of points on either side of 35F. COP = BTUH output divided by BTUH input at a given outdoor temperature, so you have to convert watts to BTU's. Total power is compressor + outdoor & indoor fan watts multiplied by 3.414.

The economic balance point is the temperature at which the heat pump's COP is equal to the break-even COP, as calculated above.

Given the small incremental cost of a heat pump vs. straight cooling, it may make sense to use a heat pump even if the economic balance point is as high as the 30's.

By: Tom Gillman

Tom Gillman — Mon, 24 Feb 2014 20:40:23 +0000

Back in the 80’s I worked as
Back in the 80’s I worked as a designer for a HVAC firm and installed a “dual fuel” air-source heat pump with a LP forced air furnace in a new construction commercial bank building. In the “dual fuel kit” from Trane there was a fairly simple graph that allowed you to plot the economic relationship between the cost of fuels and the temperature that you set the backup furnace to operate. Allison, getting your hands on this graph would probably make for an interesting article and bring some meaningful conclusion to the numerous references to operational economics discussed in the comments above. I think basing the control strategy on the ability of the HP to maintain the requested interior t’sat setting is lame and could cause higher then needed operational costs, especially considering the price of natural gas right now. Also and to answer Tom Cooper’s question, I’ve always wondered from a practical side, who would update these settings over time to accommodate future rate changes. Experience tells us that a high percentage of customers won’t keep up with the manual temperature setting adjustment. Tom, your unit would need to be hooked up to the Web and monitoring fuel market prices to make this adjustment automatic. Tom G

By: Tom Cooper

Tom Cooper — Mon, 24 Feb 2014 02:52:45 +0000

This is an interesting and
This is an interesting and useful article. In particular, you wrote "But with natural gas prices the lowest it’s been in decades, it’s important to determine the economic balance point for hydronic heat – the temperature where it becomes more economical. Thermostats are available that can lock out the heat pump below a given temperature." This describes our new Trane system, but how does not calculate the temperature at which the natural gas fired boiler is more economical than the heat pump? Tom

By: Keith

Keith — Thu, 20 Dec 2012 07:01:53 +0000

Dear David,
Dear David,
I'm a home owner in New Jersey with a recurrent, major problem with my heating system that is driving me nuts. I'm hoping you might be able to shed some light on my problem:
My basic system design is a 4.5 ton heat pump with a hyrdronic heat exchanger, fired by a high efficiency gas boiler. This system was in place when I bought the home, except the I replaced an old cast iron boiler with the high efficiency gas boiler. My heat pump has a stuck reversing valve (A/C works fine), so I have been using the Aux Heat (heat exchanger) as my primary source of heat for these three years. My first winter (3 years ago), the original heat exchanger, mounted in the duct work, ruptured along 2 of the bends in the copper. I assumed the heat exchanger was old and simply wore out. So I replaced it, with a new unit, same size, same btus... and in my second winter, it blew out again.
I assumed it was a faulty exchanger, but the manufacturer guaranteed me it was too much pressure in the line. So, upon replacing it again, my contractor installed a small expansion tank, and a check valve just before the intake of the heat exchanger. Unfortunately, last Friday, the new heat exchanger also blew up. That makes 3 and my contractor can't think of why this could be happening. I am very frustrated but can't figure it out either. I have decided to get a new, high efficiency heat pump, and my contractor is suggesting the Aux heat consist of heating strips (electric) instead of the gas fired hydronic heat exchanger.... which I understand are much less efficient. I don't want to give up on the hot water exchanger as it's more efficient and they system is already designed that way... but, I don't want to keep "feeding" it heat exchangers.
The high efficiency boiler also provides heat to several hot water baseboards, an indirect water heater without any problem.
Also, a common thread with each "blow out", has been that it happens on a mild day, after it has been below freezing at night. This year, it was 29' at night, and warmed up to near 50' during the day. I specifically remember this happening on each occurrence, and I'm not sure if it has any bearing on pressure or anything else, just thought I'd mention it.
Any ideas would be greatly appreciated,
Thanks in advance,
Keith White

By: Rick

Rick — Wed, 12 Dec 2012 20:30:35 +0000
David, thank you for your
David, thank you for your input. It gives me some basis for my calculations. Where I plan on building 30K of water would cost $225. However I expect cooling requirements would be half or less so it still looks attractive compared to installing and maintaining central air. This is municipal water which varies very little in temp because it is delivered through a piping system buried 9 ft. or deeper. We live in a moderately dry climate so the warmed water could often be used for garden or lawn irrigation, essentially lowering the cost to near zero. excess water could simply dumped to the wastewater system. Although I haven't checked the specific water system it is typical for water in the area to be alkaline (about PH 8) and only moderately hard. Normally there is virtually no scaling except in hot water systems.

I think your comment on on over sizing the heating coil is correct. One of my motives was to reduce airflow and noise. However I am probably biased by exposure to so many over sized hot air systems.

I agree that tie ins of solar or other sources would probably not have a reasonable payback. The reasons for doing it would have to be environmental, conservation of resources or simply to provide a backup source of heat.

It is a little off the subject but I have read an article of yours on HRVs and it has made me question the economies of one even in our climate. I believe mechanical ventilation is a requirement by code in our area for new homes as well as an absolute necessity for humidity control in winter in our super tight houses. Do you know any way to calculate the payback for installing HRV. One problem seems to be that all the units available seem to be vastly over sized for my needs.

Thanks again for your input so far.

By: David Butler

David Butler — Wed, 12 Dec 2012 12:57:21 +0000
@Rick, your project sounds
@Rick, your project sounds very interesting. Are you talking about pumping water from your own well or drawing from a municipal water system? Assuming 200 cooling hours annually @ 2.5 gpm, you're talking 30k gallons. Where I live, that costs about $40.

You mention 40F incoming water. Is it that cold during peak cooling season? You can cool directly with ground water up to about 50F.

But just like an open loop GSHP, you'll need to discharge in a separate well or pond. Most water requires treatment (you don't want to pump hard or acidic water through your fan coil), so be sure to check local environmental regs regarding discharge water quality and permitting requirements. Some jurisdictions allow a small amount of chlorine or salt.

You mentioned grossly oversizing your H20 coil. You can manage the return water temperature more effectively by varying the storage temperature, flow rate and airflow.

As for solar thermal tie-in, I'm not sure you'd see a payback unless you do the labor yourself and have access to low cost storage tanks, especially considering the small size of your design load, and the fact that you have access to natural gas.

Let me know if I can be of assistance.

By: Rick

Rick — Mon, 03 Dec 2012 21:27:44 +0000
I am currently planning a new
I am currently planning a new home in central Alberta where our climate can be extreme. The envelope will be very efficient. I am leaning toward the use of a dual coil air handler for heating and for a/c if needed. It could also function as a heat pump. Hot water for the heating and domestic use would be supplied by a tankless water heater. Air conditioning could be supplied by the water coil as well if the cost of water is not too high. In this area it normally enters the home at about 40 degrees F. Natural gas is usually inexpensive and electricity is comparably expensive. Heating requirements are estimated at around 30,000 BTU per hour but I will likely grossly oversize the coil as I believe the condensing water heater would be more efficient at larger temperature drops in the circuit. On of the reasons for this method is the ease of adding supplemental heat from sources such as solar or an outdoor wood furnace simply by raising the temperature of the water as it enters the hot water heater. This could be accomplished with a water tank(s) for heat sinks and heat exchangers(probably just coils in the water tank. I would appreciate any comments or ideas.