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]

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.

 

[LittleInch]

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.

 

[Alistair_Heaton]

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.

 

[LittleInch]

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.

 

[Alistair_Heaton]

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.

 

[LittleInch]

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.

 

[Alistair_Heaton]

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 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.

 

[Alistair_Heaton]

https://www.pv-magazine.com/2023/03/17/field-data-on-heat-pump-efficiency-cold-climate-performance/

Just for interest.

 

[LittleInch]

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.

 

[Alistair_Heaton]

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.

 

[LittleInch]

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.

All IMHO.

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

 

[Alistair_Heaton]

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.