I've signed a contract to install a Generac 22.5 kW whole house generator and transfer switch in our home outside Boston.

Our home has Carrier Infinity 3 ton heat pumps installed on the first and second floors. The first floor has hydronic backup heat and the second has electric backup heat.

In order to avoid overloading the generator, the transfer switch sheds individual heavy loads when the total load gets too high. I talked to my HVAC dealer about how to implement the load sheding feature for the heat pumps. Initially we discussed the utility curtailment feature which allows utility companies to shed a heavy load in a home when the load on the grid is so heavy that it strains the utility's ability to maintain power. However it turns out that this feature works for cooling but not heating, which won't work here because both heating and cooling loads must be shedable. That leaves the conventional approach of removing power from the heat pumps and electric backup heat. This is done by Generac System Management Modules (SMMs) that are connected to each load to be shed and disconnect the power to the load when needed.

The HVAC dealer consulted with their Carrier distributor rep who has said at times that "generator applications should always be designed by an electrical engineer". Also at one point he recommended that the heat pumps be disabled when the generator is running, which of course would defeat the main purpose of the generator.

I'm concerned by the comments from the Carrier rep. It may be that he's just being extra cautious but I can't be 100% sure. Has anyone used a whole house generator with in Infinity heat pump? Have they implemented load shedding by disconnecting power to the heat pumps? I just want to make sure I'm not doing something that will cause problems in the future.

Thanks, Mike

  • I have a 16Kw Generac system here in FL. I am running my 5 ton Carrier Infinity Heat Pump with no issues, but I only have 1 system, not 2 and have minimal backup heat strips. I don't use load shedding, I just did not hook up double ovens or electric dryer to the generator. It ran well for 2.5 days after Irma last year.....kept house at 74 degrees 24/7 – Limo DRIVER Jul 20 '18 at 13:26
  • How large are the heat strips on the second floor system? – Limo DRIVER Jul 20 '18 at 13:27
  • Is the hydronic heat fuel-fired? Can enough hydronic heat get up to the 2nd floor to keep the pipes from freezing? If so I don't see where you really need the heat pumps and the very hungry resistance heat. – Harper Jul 20 '18 at 15:09
  • @Limo DRIVER can the heat pump handle the switch-over if the compressor is running? Is there a momentary loss of power when this switch-over occurs? – Jim Stewart Jul 20 '18 at 15:11
  • @Harper Limo DRIVER is in south Florida. The backup resistance heat is prolly never used. – Jim Stewart Jul 20 '18 at 15:13

I worry your service might be much larger than your generator

Since I hear you are in the snowbelt, and have oil for heat only (never heard of an oil fired water heater or range), and 2 x 50A circuits just for the emergency heat (24kw right there) -- I'm worried that your house may be set up as an "All Electric Home". What's often done in the snowbelt is to supply the house 400A service (aka 320A service) - commonly as two 200A panels side by side. One panel entirely for the heat pump and emergency heat, and the other panel for "everything else".

If so, a 23KW whole house generator will be totally inadequate to cover the entire house. Even the transfer switch will be expensive at 400A. I don't know what you have or what your loads are, but straight-up, 400A is 100kw. Even after removing the emergency heat from the picture, your other electric appliances can also overwhelm the generator. You will need a lot of load-shed circuits, or a much bigger generator. A 23kw generator is appropriate for a 100A house.

If my theory is correct, and you haven't said, then I'd say that one of us is missing something fairly significant about the discussion with the generator salesman. Perhaps the salesman has sold you something that hits your price point, and is indifferent to the actual usability of what results.... it may take months or years to uncover any deficiencies, by which time they gotcher money.

Load shed is the same basic tech

Regardless of utility or generator, the mechanics of shedding load are the same - either a big contactor that disconnects the load, or say, heat or A/C can be shed by manipulating the 24V thermostat lines; intercepting the thermostat's "call for heat" or "call for A/C". That also allows the furnace and A/C to sequence itself normally if that matters to the unit, such as preventing short cycles.

So what's the business about A/C being sheddable and not heat? That's the power company talking, and they have a different business agenda than you. The fact is, power demand goes up and down, and the grid must cope with this by spinning up and shutting down very large 50+ megawatt generators. 200MW coal plants and 1000MW nukes Do Not Like to be idled, so it falls to more expensive generators to be "peaking units", typically gas turbines or diesel engines. Solar just happens to be a fabulous peaker. Because the highest peaks all year are in summer afternoons, and the main component is air conditioning caused by solar load.

The power company wants to shave peaks any way they can, so they can build fewer peaking stations. (they're expensive; it's not just the $ per W, it's that the mortgage must be paid everyday even though they only run 27 afternoons a year!) Therefore they are interested in shedding peak loads - air conditioning! They couldn't care less about shedding heating load, because that doesn't occur at peak times.

So the switching hardware that the power company uses can work for your generator, but the decisions of when to shed load need to be switched when you switch power source. When on utility, the power company should decide when to shed A/C. When on generator, the generator's control system should decide when to shed A/C, water heater, range, electric heat, etc.

ThreePhaseEel says there are some implementation challenges in designing load sheds that work for all modes. Ok. However most thermostats are very simple creatures, and it should be possible somehow. However this may be a "bridge too far" for a COTS (commercial off the shelf) system. Mind you a thermostat interrupt may not shed load instantly on demand, if the furnace controller wants to run awhile longer to avoid a short-cycle etc.

Elegant design goes far

You may have gathered that simply brute-force attaching a generator to an all-electric home is a hard undertaking. It helps quite a lot if the house can be "designed to be off-grid" by elegant choices in design and appliances.

A great example of this (requires propane or methane, not oil) is the "Empire style" wall furnace. This unit requires no electricity whatsoever so it licks the problem of pipes freezing, without even needing to start a generator. The double-wall version is a 50,000 BTU brute, which is the equivalent of about 15,000 watts (60A) of emergency heat. This machine is only sold in places like California, and is unobtanium anywhere in the rust belt, which makes no sense whatsoever.

This is an "Achilles' Heel" of almost every heating system in rust-belt homes: they all are designed to require active pumping to circulate heat or cool around the house. They must pump fluid to radiators or mini-split heads, or forced air. This requires far too much non-thermal power, and makes "your pipes not freezing" totally dependent on large amounts of electricity. (it's even a bit much for solar, especially in winter when you really need it, since you may not be there to broom snow off the collectors).

They come in other form factors, including direct-vent through-the-sidewall types. They completely work on passive convection, so smart placement of the furnaces is essential so heat will be circulated around the house. They use external "millivolt" thermostats, but can also be operated with a relay from a 24V thermostat, and can use both methods at once. So for instance one might set the millivolt thermostat for 50 degrees F, so it only calls for heat if the 24V thermostat system has totally failed due to power out and the house has chilled. Now your generator has less load, and is not as critical.

Of course anything you can do to make a big electric appliance "not electric" also helps ease generator load. At extremes, a house could be engineered so onsite solar panels could keep the house entirely livable and fully functional with grid power lost. But in your case, I would focus on that very, very hungry emergency heat. Two big Empire furnaces (or several smaller ones) would go far to assure your house can't freeze.

  • Thing with an Infinity system is that you can't intercept the 24VAC side for load shedding (the system will complain if you try). Normal utility curtailment uses a dedicated set of terminals on the compressor controller, but those only work when the unit's in cooling mode... – ThreePhaseEel Jul 20 '18 at 22:19
  • I was unclear re the breakers. Each heat pump has a 50A 240V breaker. The house originally had oil heat and hot water and conventional AC before we converted to heat pumps. Our service panel is 200A. The heat pumps are variable speed so they rarely run at full speed. The controller is computerized and does not use the conventional 24V relay closure to call for heat or cooling. The breaker for the electric backup heat is 50A 240V. The installer has lots of excellent reviews but is inexperienced with heat pumps. I've signed a contract so I don't know what will happen if I back out. – MikeA01730 Jul 21 '18 at 9:57
  • @MikeA01730 -- how practical would it be to extend the hydronic system to the 2nd floor (presumably with a hydronic coil fitted in a reheat position relative to the existing air handler, and the strip heat there removed/blanked off)? – ThreePhaseEel Jul 21 '18 at 12:50
  • @ThreePhaseEel I could convert to hydronic on the second floor if I thought it was worth the money, but I don't think is. If it turns out that the electric heat is shed a lot, even most of the time, that's not terrible. The electric heat is only used when it gets very cold. The hydronic heat on the first floor and the heat pump on the second should keep the second floor warm enough to be livable and certainly will prevent freezing. – MikeA01730 Jul 21 '18 at 13:07
  • Keep in mind if it is too cold "outside", the heat pump will be dead as a doornail. This has two impacts: #1 you will need the electric heat. And #2 you do not need to provision both both heat pump and electric strips at once. I put "outside" in quotes because nothing keeps you from building a gazebo around the heat pump outside unit, closing it in with insulating panels, and running a fuel heat source in there, like I don't know, a diesel generator's radiator and exhaust :) – Harper Jul 21 '18 at 17:33

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