What size HP...what size 2 stage furnace...what size modulating furnace?...I will assume furnaces are nat gas fuel...and the inclusion of the HP condenser is due to the 8 KW solar setup...I hope this has been analyzed correctly...

just curious, what is your cost of nat gas/therm and cost of electricity/KWH?

IMO

Tigerdunes, thank you for the reply.

80k furnace in both modulating and 2 stage, both natural gas. Believe heat pump is a 3-ton but will double check.

Inclusion of HP condenser is due to the 8KW setup and having extra capacity outside summer (use the electricity rather than sell it back). Cost per Dth is 7.91 per Dth under 45dth/month and 6.82 Dth afterwards. Cost for power is 8.8 cents until 400kwh, 11 cents until 1000kwh, then 14.5 cents afterwards. Solar is calculated at a 90% offset so any used would be at the lowest tier worst case scenario.

Modulation for cooling, for me, is mostly about humidity control. I don't think that you need that. For heating, it is more about efficiency.

Great choices in an HVAC system, the biggest practical difference I see between the 2-gas furnaces is greater control of cfm and comfort in heating mode – by quite a difference with the 59MN7 series. I don’t think you can go wrong with either furnace.

I personally would choose the 59MN7 series for myself.

IMPO

SR

Ionized, I had read the lower stages of heating are actually less efficient - is that incorrect?

Sr, thank you for the recommendation. Do you think the multiple stages of heating through the heat pump + the furnace create redundancy? As I understand it the furnace would only fire underneath the set point (40*f).

If you are planning on taking full advantage of your solar setup. I would want a 4 ton size in the HP var speed model. Weigh the cost difference between the 2 stage and modulating furnace. The two stage would have an output of around 50K low and 75K high stage. I personally think this is more a cost difference issue and that the modulating furnace would be overkill.

dealer has provided written load calculation both heating and cooling?

IMO

I know less about modulating fire furnaces than I do boilers and variable speed compressors. I could be missing something, but I don't see how the furnace situation could be fundamentally different from the boilers. In both cases, you have the same considerations, burning fuel, heat exchangers and low temperature output (at the heat exchanger coolest end more fundamentally) is needed for condensing performance.) Lower output must mean greater ability to avoid "overheating" the exchanger cutting into harvest of+ the heat of vaporization.

Figuring how to most efficiently run this system might take the skill of a good engineer. You have the tradeoff of fuel costs and efficiency of the systems at the maintained temps as well as set-back recovery. That is hard enough all by itself, but you have the added complexity of the difference in price and cost of electricity sold vs. bought depending on your net metering situation. I'd say that the efficiency of heat pumps in partial output, if you go that route, and the net meter differential conspire to keep you out of daytime setbacks even if you are typically gone most of the daylight hours. Normally it does depend on how well your home is sealed and insulated your home is. A factor that is often forgotten is that furnaces typically have a lower lifetime cost because they last longer and are less costly than heat pumps. that means that as the clock turns on their run time, the dollars and cents out is faster with the heat pump not considering fuel cost.

I think Tigerdunes makes a good point. The furnace is part of a dual fuel system and will operate at temperatures below an outdoor set point. In that situation a 2-stage furnace is adequate.

Do you know how much electricity your solar panels will generate in the winter when the sun is lower in the sky? The angle of the sun has a big influence on how much electricity is generated. The panels may produce 8 KW in the summer, but in the winter it is going to be less. You may be buying more electricity than you think.

If were my house, I would not buy a heat pump, put the saved money towards the modulating furnace, and sell the excess electricity in the winter. The extra wear and tear of the heat pump running in the winter will shorten the life expectancy. That is something else that needs to be factored into all of these equations.

I have to agree with Mike...I would drop the HP idea altogether and go with the high end modulating furnace paired with either a 2 stage AC condenser or the var speed AC condenser version...again I will ask, were you provided a written load calculation both heating and cooling?

IMO

Without knowing the net metering agreement and other hard info. , it is hard to say how much benefit using up the PV panel output with a HP can be. If there is relatively little loss in selling power during the day and buying it back at night, there is better benefit in other directions. Every state has different net metering requirements. The worst thing is that the agreements can change quickly and often after homeowners invest in generation systems. There ought to be a period of several years that gives them stability. No one would build an industrial generation facility without some some sort of long-term purchase agreement, why should homeowners not have the same security for their investment?

Revisiting your situation particularly with regard to your cost of electricity, you might be better served with an all-electric Greenspeed HP & Infinity Fan Coil. The COP of an Infinity Greenspeed puts the cost per kW of heat delivered into your home at a very low price. Moreover, the range of cfm of the FE4 series fan coil is far greater than that of either the 59MN7A or the 59TN6A series.

As well, with the FE4 there’s no need for venting, chimney or a gas line. You would just need to have a 200amp entrance. The only real down side would be that the gas furnace would require a much smaller backup generator during power interruptions.

All electric would require less maintenance and longer life cycle of the indoor equipment.

The 25VNA should provide all your heat down to about -5˚F; staged heat strips provide the rest. You would have to go to geothermal to better than that which wouldn’t be a bad idea either.

IMPO

SR

Disagree with above post. I don't like the idea of taking advantage of solar panels by purchasing a HP system whether DF or standalone. Keep things simple. Refer to my last post and go with modulating gas furnace paired with either a 2 stage AC condenser or var speed AC condenser. Sell any excess KWs back to electric provider..

and to previous poster, a nat gas furnace lasts much longer than a HP condenser.

IMO

Tigerdunes, you are correct in most aspects, but the selling back excess is the sticky point right now. The selling of electricity to the utility from renewable power sources by individual customers is governed by local PUCs. In some places, you get nothing or virtually nothing for excess. Furthermore, in places you lose any excess at the end of the month, it does not roll over. Others, maybe you can't roll over at the end of the year. Some net metering agreements require the utility to pay the customer if there is a chronic overproduction. In some of those cases, you only get paid when you disconnect from the utility. (When you move or die.) That is like a no interest bank account/retirement fund. The agreement is so bad in some areas that with the price of panels going so low, homeowners are better off buying batteries. That is something not dreamed of as economic not long ago. Installing big water heaters or freezing ice and using it to cool the home at time of high demand are alternatives.

Definitely consider that the lifetime equipment costs of running a furnace vs, a heat pump when doing calculations to determine cost and environmental impact. I don't know if the ability to recover retail cost of excess power can tip the balance to make a heat pump favorable or not, but it might.

Re: Tiger

I have not taken into account that the OP wants to use solar panels. I’m just looking at the merits on one HVAC system over another.

In my previous post I mentioned that the ‘indoor equipment’ i.e. gas furnace verses electric fan coil, that the electric fan coil will last longer and require less maintenance. We may have a difference of opinion here and that’s OK but I do believe the electric fan coil will likely outlast any fossil fuel burning furnace. Outdoor HP & AC condensers are another story, which among other reasons is why I prefer geothermal.

I do agree with keeping things simple.

IMPO

SR

if one wants a backup generator, it would be powered by nat gas and of course the KW required to power either of the nat gas furnace models would be quite small especially when compared to all electric HP system. I think most people would agree that a gas furnace has a longer life span than a HP system. I am not certain I agree that a fan coil (air handler) requires less maintenance than a high end nat gas furnace.

"Moreover, the range of cfm of the FE4 series fan coil is far greater than that of either the 59MN7A or the 59TN6A series." not sure I agree or understand this statement.

IMO

If the selling back of KWs is an issue, then why have solar panels. Homeowner should have a clear understanding of how this process works and the option to use KWs generated for his home use or sell back to electric utility.

IMO

Re: Tiger

"Moreover, the range of cfm of the FE4 series fan coil is far greater than that of either the 59MN7A or the 59TN6A series." not sure I agree or understand this statement.”

To clarify, this is from my Carrier Dealer Price List info

59MN7A080V17—14 Heating cfm: 620-1500, Cooling cfm: 490-1375

59TN6A080V17--14 Heating cfm: 1060-1500, Cooling cfm: 490-1375

FE4ANF003L00 cfm range 200-1400, as well, constant cfm independent of static pressure

FE4ANF005L00 cfm range 250-1600, as well, constant cfm independent of static pressure

Regarding the relative merits and life cycles of gas furnaces verses fan coils, it’s largely a question of what is likely to crap out first, condenser/evapourator or burner/heat exchanger assemblies?

IMPO

SR

Heat pump versus Furnace lifetime.

Some manufacturer's give life time warranties on compressor of any AC or Heat Pump. (models are higher end systems)

Some manufacturer's give life time warranties on Furnace heat exchanger.

While you could have other system failures that that do not invoke this kind of warranty from being used the same can be said of any other equipment made from 'other' manufacturer's who don't warranty their premium equipment this way.

So from one manufacturer to another a heat pump may not out live a gas furnace.

Normal gas furnace is 10 year parts and 20 year heat exchanger.

Normal Heat pump is typically 10 year parts and 10-12 year compressor.

Lifetime warranties are 'your life time' not equipment. They typically cap this warranty at 99 years. The other is that if you sell the property in which the life time system is installed the warranty reverts to 5 year part from the time of the installation.

Buy a better product that is protected by a better warranty and stay put. Then only allow competent people to work on and service it.

If you are looking for a 'lifetime' system, I service the Katy, Texas area.

For above poster...it is my understanding that on high end high efficiency nat gas furnaces, they carry a limited lifetime warranty on heat exchangers...not clear about the exclusions...

IMO

Good point to separate the PV question. It does influence equipment choice if it is getting so inexpensive that it affects “fuel” choice in concert with efficiency and equipment costs..

Stretching the scope of the thread a bit, the net metering agreements for utility customers can change any time and overnight. It seems very unfair compared to large industrial power suppliers an cogen agreements. There is some recognition of the problem, but very limited movement to increased security for the little investor in power production.