Electric Reheat for Heat Pumps

[Designer_82]

Is electric reheat still typically specified / required for heat pumps?

Project is in New England area, Multi-family apartments.

The engineer is specifying 5 to 8 kw of electric reheat per apartment, but the building has a hard time upgrading electrical service.

Heat pumps I know can operate fine even down to 0deg or less, so can we remove all the electric reheat and still meet code?


Thanks

 

[MintJulep]

"can operate fine even down to 0deg" is not the same as "meet the heating load down to 0 deg."

Reheat or supplemental heat?

 

[LittleInch]

Is this also related to occasional required heating of the water to more than 60C for legionnaires disease prevention?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[HVAC-Novice]

Maybe ask the engineer who designed it?
Review the data of the units, the load, and the climate.
Most air-source heat pumps will have almost no heat output at 0°F, if they even work. COP will be really bad once you include de-frost.

 

[LittleInch]

Which "code"??

Your issue is more like getting sued by the purchasers of the apartments for having an inadequate heating system.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[AgMechEngr]

Ultimately, you need more heat. If electric capacity is an issue, consider gas. Heat pumps alone will be insufficient in New England.

 

[HVAC-Novice]

I assume "code" refers to what code prescribes down to what temperature the system is supposed to provide adequate indoor temperature. In my area that is -15°F. We do have very rare -20° or even lower.

If temps go below the temp, it is accepted that it gets a bit colder since that rarely happens. but note, this really only works for systems that are independent of ambient temperature. (like geo-exchange, or combustion heating) where the device still works when it gets colder. an air source heat pump will not only lose capacity, it will literally stop working that day it goes below the code-temperature. Then you don't just have insufficient heating, you will have NO heating at all. I wouldn't want to be the designer taking that call...

 

[nuuvox000]

I design in a cold climate and I would never specify heat pumps without electric heat. That's why almost nobody does it here; you have to pay extra for the heat pump and you still need electric heat so they just want to pay for the electric heat. I think we work for a lot of owners that build apartments and then sell them so they don't care a whole lot about the monthly electrical costs for the tenants.

Yes, the heat pumps will work most of the time but there will for sure come a day when the temperatures drop significantly and on that day when heating is needed the most, the heat pumps will not turn on because it's too cold. Yikes.

 

[AdrienneGC]

Your original post discussed electric reheat - I assume you mean supplemental heat. I design mostly in Florida, and we always provide supplemental heat for heat pumps.

Adrienne Gould-Choquette, P.E.

http://www.adicotengineering.com

 

[dbill74]

Providing supplemental electric heat on a heat pump really depends on the anticipated heating load and geographic location plays a major role in this decision.
Where I am in southeast USA, south Georgia/north Florida specifically, using heat pumps with electric heat is common since the heating capacity of the heat pump is not enough for winter temperatures (near freezing). Go to south Florida, heat pump is more than capable of proivding enough heat. Go to north Georgia and even with electric heat, heat pumps in general are less ideal.

You "can" use a heat pump without electric heat, BUT the cooling capacity of the heat pump will most certainly be more than you need resulting in short cycling of the unit and inefficient operation.

Your OP seems to imply this is a retrofit job. What type of system is being replaced?

 

[trashcanman]

Little Inch is right on the money. Regardless of what the catalog says, provide supplemental heat for the units. This is so that if the compressor fails, the air handler still has adequate heat for the conditioned space. Use ASHRAE for the design temperature in Winter and add 10& safety factor. You DO NOT WANT TO BE INVOLVED IN A CLASS ACTION LAWSUIT.

 

[Alistair_Heaton]

To note all temps used are in Deg C

The design codes that I know of in Europe and UK don't have a requirement for emergency backup alternatives.

That said some installations have it but it depends what you install.

A2A refrigerant nothing.

Air to water

If you have a monoblock external its an additional module installed internal to building on the feedin line. But its optional most don't go for it unless they are trying to reduce the glycol loop.

If its an internal building compressor unit with outdoor heat exchange unit then they tend to have an in line immersion.

Personally I have a ground heat pump and it has a 9 kW backup heater which is used for sterilisation runs on the hot water system as well. You can limit it down to 3.5kW if you have supply capacity issues.

I could have set it up with a hybrid system with a 50kg bottle of propane and burner off the buffer tank which would be triggered by the heatpump controller but as we have wood burner fires in the house and a lack of space in the cellar and having to get the place gas certified I didn't

To be honest the heatpump scene is a mess this side of the pond with the push away from hydrocarbons both for transport and domestic heating.

The electrical capacity of buildings is an issue here as well in low temps. EV's charging over night and requiring batteries heated are just getting chucked in and nothing to cut them, in the event of the heating going into emergency mode.

There are also issues with people running the buildings at too low a temp for the defrost cycle to clear the outside unit in low temps.

Here is my local temps this year in Jan.

I had to rescue some one on the 7th who's unit snowballed and didn't have enough energy building internally to complete the defrost cycle. The local solution when it happens is to take a propane burner outside to the heat exchanger and melt the ice off. I got him to light all the wood fires in the house and get the temp up to 20 degs in all the rooms. Turn on all the multi indoor units and give the defrost cycle enough energy to work with. Took the house temp down to 15 degrees doing it,

But engineering philosophy wise. Why do heatpumps require more backup than conventional heating systems? Never seen a hydrocarbon burning heating system with electrical backup. The back up was always mobile electrical room heaters. And they all go when there is an electrical power cut.

 

[Alistair_Heaton]

if the heatpump is the problem have a back up heat pump....

Panasonic range has a that option which I saw while sorting out my workshop heatpump Paci NX.

 

[LittleInch]

Why do heat pumps require more back up?

Because like you saw, at some point the CoP gets close to or below one.

At that point there is simply no point trying to extract energy from the air when it's taking you more power to do so than the heat produced, so you might as well simply use that electrical energy to heat the house....

Now to stop people having two systems the option is to replace the heat coming from the heat pump side and just connect in a heating element.

So e.g. if your system uses underfloor heating then the electric element just heats the water, or does the same if you're using an AHU

plus if your heat pump system fails or the external unit freezes over then you're equally stuffed if you don't have a bunch of fires or a propane burner - probably difficult if it's a block of flats....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Alistair_Heaton]

When I was doing the specifying for my place the 10kw unit had an output at the min specification temp at -21 of 7.3 kW with a current pull of 3 kW at 20 degs internal temp and 7.8 kW and pulling 2.84 kW at 16 internal. Which is more than 2 COP.

For complexes requiring back up eg computer centres, nursing homes etc they have cascade systems with backup external units And alternate working cycles and powered defrost.

I agree with you the 410A refrigerant heatpumps are as you say but the R32 ones have much better low performance. Panasonic also have Nordic editions which are setup for marine environment with blue fin anti corrosion and humidity sensor factoring defrost cycles.

 

[LittleInch]

As you can see though you're getting half the heat power compared to 6C, but losing double the heat. Result- the building will get colder.

Now a workshop it might not bother, but if it's your apartment then you need to supplement the heat input just as the heat loses go up.

Also would be good to know if those figures allow for the reheat cycle at lower temperatures? So 7Kw, but only for 70% of the time??

That's why you need additional heat.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Alistair_Heaton]

You just need to spec it for you min output at your min design outside temp. Which is what I did.

And the recycle temp is specified for outside humidity zone. Which is why the Nordic ones have a dynamic recycle defrost addition.

The defrost cycle from what I can tell around me, most problems are linked to the internal energy and user issues.

When it triggers with enough internal energy its done in under 5 mins at -21. And locally to me it triggers hourly with 40% humidity. When its warmer in the -5 range it actually does it more often as the humidity is 80%.

Like or not the management of heatpump systems is way way more involved than the old hydrocarbon powered heat systems. Its taken me 2 heating seasons to figure out the best way to run my ground heat pump and it doesn't have the variation of air temp to factor in. And I have 5 tons of scree UFH to help as a thermal store.

Even with cascade backup it will be cheaper than upgrading the local grid connection. And operating costs half that of resistive heating.

 

[LittleInch]

"at your min design outside temp."

That's where the issue lies. You've thought like an engineer. SO by the sound of it yo chose -8C as your Design outside temp?
Plus if you've got a variable speed compressor then it will just power down if it gets to 0 or +5 I think so your system doesn't start short cycling

Most people will think like a parsimonious home owner and use 0C.

I have no issues at all with heat pumps being better environmentally than burning natural gas, but people just need to understand their limitations and what you do about it when you reach them and vendors need to be a bit more proactive and not so defensive.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Alistair_Heaton]

No I have taken my required thermal requirement at -21 outside with an inside temp of 16 which requires 6.4 kw according to my heat loss calcs for the building so could manage about 21 internal if I want. The pump will put out 7.8 kW at that differential with a pull of 2.84 kW. And yes its DC inverter so variable output not on off. MIn output 3 kW with a pull of 0.5 kW at -10. I will more than likely have 1 start cycle between end of Nov and start of April.

The unit with that delta can defrost with 200m3 of air to use as a heat sink. I have 16 tons of aerated concrete ceiling plate to act as a thermal mass in case of power cut. Which will last more than 24h.

 

[Alistair_Heaton]

but people just need to understand their limitations and what you do about it when you reach them and vendors need to be a bit more proactive and not so defensive.

Your spot on with this. And this side of the pond the installers range from spanner chimps throwing them in, to proper building services designers specifying. But most domestic solutions a bunch of bananas need to be on standby.

In relation to the code above, there needs to be more info of if the apartments are going to be solo with installation or if its going to be a block heating plan with apartment billing of heat energy.

Either way you can cover the reserve for point failure by design, solo installation will require more hardware and be less tolerant for defrosts. And the end user needs to be blocked from letting the internal energy store from being depleted below defrost requirements.

Air to water gives you more scope for back up and less issues with defrost.

But I would design and argue that as long as the heat requirement is covered off a risk assessment of failure then resistive heating is not required. If you get hit with an area upgrade of grid capacity the starting point is 100k$ where I am.

But I was surprised how cheap that PACi NX was as a solution I am under 5000$

There is also a hot water solution to heat a tank.

It took me 3 months to research my setup and it started with being told I couldn't have 3 phase 400V and had to have 3 1 phase 230V units at more than double the price and extremely limited control capabilities rated at -15 deg C. If I had gone for 10 kW single phase unit then it would have pulled 12 amps and that would have put into danger territory with my 32 amp supply with the ground heatpump pulling 24 amps in rush on start up. Now its going to be max 4 amps. Which is another reason why the inverter heatpumps are the way forward as they have very little start up inrush or power factor issues.