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Electric Reheat for Heat Pumps

Electric Reheat for Heat Pumps

Electric Reheat for Heat Pumps

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?


RE: Electric Reheat for Heat Pumps

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

Reheat or supplemental heat?

RE: Electric Reheat for Heat Pumps

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.

RE: Electric Reheat for Heat Pumps

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.

RE: Electric Reheat for Heat Pumps

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.

RE: Electric Reheat for Heat Pumps

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

RE: Electric Reheat for Heat Pumps

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...

RE: Electric Reheat for Heat Pumps

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.

RE: Electric Reheat for Heat Pumps

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.

RE: Electric Reheat for Heat Pumps

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?

RE: Electric Reheat for Heat Pumps

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.

RE: Electric Reheat for Heat Pumps

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.

RE: Electric Reheat for Heat Pumps

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.

RE: Electric Reheat for Heat Pumps

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.

RE: Electric Reheat for Heat Pumps

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.

RE: Electric Reheat for Heat Pumps

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.

RE: Electric Reheat for Heat Pumps

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.

RE: Electric Reheat for Heat Pumps

"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.

RE: Electric Reheat for Heat Pumps

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.

RE: Electric Reheat for Heat Pumps

Quote (lttleinch )

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.

RE: Electric Reheat for Heat Pumps

Quote (Alistair)

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

You really should also look at how often and how long in any year the thing needs to operate at the min design outside temp vs. the more typical temps.

Selecting for min can lead to running the thing at the lower limit of its turn-down for much of the year, where it's likely not as efficient as advertised.

And if the lower limit of turn-down is too much heat then you get on/off cycling which can be annoying at least.

RE: Electric Reheat for Heat Pumps

Exactly right.

Alistair could do that as an individual but most developers and people just want the cheapest they can get away with.

So as ever it's a balance between performance and cost. Hence why supplemental heat is often used to cover those really cold days but allow a smaller unit to cope with 90%+ of the heating duty.

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

RE: Electric Reheat for Heat Pumps

yep agreed,

I installed an wx station last summer and that plot above is from it so its local data.

This year has been remarkably mild.

We get -20ish 2-3 times a month between Nov and end of this month when we get a northly from the Siberian high. They just recorded the years low of -27 last week.

Most of the time it sits around -5 to -15 during the night. Which will mean a -15 degree unit will be hitting its limit pretty much every month about 15% of the time. But they don't shut down when they hit it they just fail to defrost. And the multi splits have no protections against indoor units being shut down for energy reclaim.

The enough energy to defrost seems to be a major missing link with installers where I am especially with multi splits. To get the -15 as far as I can tell the owner needs to have all the indoor units turned on with at least 16 deg room temp around each unit. Having a couple of rooms out of 4 indoor units heated to 18 when its -10 or below just turns them into a snowball.

Maybe UL approved units are different to the stuff they sell over here.

I did look at the min compressor output, which at -10 is 3kW at 500W power pull. Which for a 90m2 1.5 story building with 26 delta T I reckon I will need just over 4kW. I reckon at -3 it will hit min output. BTW I don't believe the COP of 6.

Also one of the selling points of going for this systems for me was it has an occupancy eco mode with sensor which alters the hysteresis to 2 degrees and adjusts the temp ramp up when it does run to the optimum efficiency power usage.

But this would not be an acceptable solution for domestic situation. But stopping my workshop fluids from freezing it ticked a load of boxes for me. but even if I run it at 20 deg at -20 I still get a COP of 2 which is better than resistive heating.

And you lot are lucky having modern updated codes. When I went through my local codes ex soviet state they seem to have not been updated since the collapse when they copied them from Finland. So no mention of heatpumps and a lovely little gem that got me laughing with the chimney requirements for turbo wood burners. If you have flu gas from one burner at 500 degs and another with 600 degs gas same volume. You have to design your exhaust stack for 1100 degs C not 600 degs.

The OP's situation is not unique but the risks have to be mitigated by design for requiring resistive heating backup. And I suspect it will be a ducted unit which is easier to ensure it has enough energy for defrost. Group heating is easier as you can mitigate by having cascade outdoor units with powered defrost as they swap over and a much lower minimum output as one unit can stand down after defrost. And both can be fired up in -20 conditions. And compressor failure is also mitigated against for a period.

BTW I did see in my research that indoor units here they have this year added heat mass included indoor units, but I can't see the installers promoting it as an option as they are 60 kg and twice the price of a normal one.

But the heatpump market in the UK also seems to be a mess and it never goes below -5 degs there apart Cairngoms Scotland and Pennies. They have the added bonus that they don't want them to cool in the summer so are restricted to air water for planning compliance.

But upgrading the site electrical capacity to use resistive heat is not a cost effective solution.

RE: Electric Reheat for Heat Pumps

The UK systems do seem to be more about heating than cooling and hence a lot of new builds are under floor heating so can use the 35C heat output a lot better.

The retrofits have trouble as you either need bigger radiators or much better insulation as at best I think they run at about 50C.

We're not used to having AHUs in most domestic properties so the cooling / heating aspect is not as prevalent. They are also quieter...

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

RE: Electric Reheat for Heat Pumps

Supplemental heating is not an issue but by the sounds of it the Op needs to cover the complete heating requirement not just any short term projected shortfall.

And they will more than likely be full of induction hobs and the like.

RE: Electric Reheat for Heat Pumps

The OP seems to have given up on us as he or she hasn't been seen since 3rd March....

However another issue, especially if this is "wet" system is the heat up time / boost for when a system is turned back on.

Most gas boilers / furnaces will be oversized for the steady state heat loss so that they can warm a place up from a cold state. Many of these heat pumps systems don't have that extra boost to do that hence people are finding they need to leave them on for long periods or you need something extra to heat a place up before reverting to the steady state heat loss.

We don't know anything about the OPS system so difficult to say.

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

RE: Electric Reheat for Heat Pumps

the oversizing of gas condensing boilers in the UK is also a problem. they never get up to condensing temps and are inefficient as hell.

I had issues with my wet ground heat pump installation when they specified 200 ltr buffer tank I now have 40ltrs. I have zero chance of anything rapid happening when my primary radiant surface is UFH is 75m2 and 5 tons of scree :D I usually work on 1 deg per hour temp change. If its below -5 outside and I run it with a set temp of 19 during the day after 6am and then up to 22 when cheap electricity starts at 23:00 Then its one cycle per day and 14 kWh of electricity which is under 1.50 euro. If it is left at a constant 22 then it will cycle twice an hour which I am happy enough with.

RE: Electric Reheat for Heat Pumps

The biggest issue is that the headline quote of "three times more efficient than gas boilers" somewhat misses the point that electricity in the UK at the moment is more than three times more expensive than gas. Their median CoP was actually 2.8 and 2.4 on "cold" days.

It also doesn't address the fact that you need to run the heat pumps for longer periods to avoid the longer warm-up time you get from heat pumps.

It's also rather sparse on data about costs and for those relatively cold days in the UK what was the worst CoP at 5 or 6am as it usually does warm up during the day so the median number isn't always that useful.

I think heat pumps in new builds are a great plan, but retrofit comes with a set of issues which need to be accepted by those doing it. Trying to avoid the issue won't work if there is enough anecdotal data saying my electricity bill is now more than I was paying before with Gas and electric and also when it gets really cold, the house doesn't warm up the same.

I've downloaded the report itself and will see what is says on more detail.

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

RE: Electric Reheat for Heat Pumps

I agree, the reaction times are linked to how your getting the energy into the house. UFH has the reaction time of a tortoise and fancoils of a Hare.

More by luck than knowledgeable judgement I seem to have chosen the right options for my ground Valiant heatpump and air to air with the R32 Panasonic.

And I also agree retro fit is an utter nightmare.

I am quite interested to hear what your views on the report data are. I have downloaded it but it will sit until a long haul cruise gets rostered to look at.

RE: Electric Reheat for Heat Pumps

The report I downloaded didn't really add much tbh.

There is a lot of scatter though in the results they do show so it would good if they actually said that to get in 90% window that e.g. the performance of the heat pumps in cold weather could vary between 1.5 and 3.5.

They also need to do an overall energy sum for the year which is what people really need to know. There's not much point in having a system with a CoP of 2.5 to 3 if you need to run it 1.5 as long as you did before.

The issue is that I think people tend to be either 100% for or 100% against this technology. It probably is a good way to go if you can get electricity prices below 2 x gas prices.

And for those pushing the systems to acknowledge the issues and where heat pumps are not the same as what everyone is used to.

Interestingly the heat pump flow temperature in the 90# spread goes from about 32 to about 45C. So to get hot water at 60 C they must be adding heat somewhere. There was also mention of auxiliary heating in the circuits presumably to go from 40 to 45C up to 60 or 65.

So if your alternative was direct electric heating, this is a no brainer. But at current price differentials, going form gas fired to electric heat pumps, you're at best about the same and cold be a lot colder or poorer. A lot depends in terms of the planet on where your electricity comes from.


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

RE: Electric Reheat for Heat Pumps

it would be a miracle if there wasn't scatter.

90% issue seems to be the specifiers and the thermo box they are dealing with. TBH its just a normal heat source to me... nothing special. The economics are extremely flexible economically. But there seems to be no engineering risk assessment.

And the technical level of the installers and specifiers doesn't seem to be in the mix for the individual site. Then the post install support seems to be completely none existent.

Yes i have my set up but its been a complete fight with the installers and specifiers. As far as i can tell they are all plumbers. And the basic rules of that are shite goes down hill and don't lick your fingers.

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