Heat pump only, or combo?

nicholas bonham-carter

I'm wondering if a 20,000 squ ft newly built, well insulated, building can be heated most economically with only a new air source heat pump (new generation), or if we should have some gas backup, such as a boiler, and radiant.

Our design temperature is 0 degrees, gas cost is $.78 per therm, and electricity is $.048/ kW.

I know that the new generation of heat pumps are more efficient than the old, but am worried about the diminishing amount of heat in air below freezing.

Not a huge amount of information to go on, but any advice would be helpful-thanks--NBC

heatpro02920

You need to find a chart

that shows your systems efficiency and the outdoor temp then see where the heat pump starts to use more energy than gas would...



I think the math will show running the HP at 0 degrees isn't going to be as efficient as running gas furnaces... you need the fan assemblies anyway, I would personally push the dual fuel system....



I know you stated 20K sq, is this separate units or one space? Will this be roof tops, conventionals, or???



I would imagine you are thinking rooftops, look at the Daiken Rebel units, pretty nice unit, gas aux, heat pumps are good down to 0, economizer system is really nice, connectivity is nice, hinged doors, just a real nice unit IMO... I have done a few they are expensive but worth every penny..



20K sq ft is big, sizing is the most important factor, it gets stressfull at them sizes... I did a a storage facility that was 16K sq ft a while back, if I remember correctly it was 2-12.5 ton units, on the other hand I did a Hibachi {just opened this year, its called Nami on fed hill in prov, for the local guys} that was only around 6000 sq ft and needed 2-10 ton roof top units.. so depending on the use you could need as much as 40 tons or as little as 15, depending on a bunch of variables...



Also I am guessing this needs ac too?

Jamie Hall

I'd be worried

about the low temperature end of things, too.  But I'm not familiar with the new generation heat pumps... but that is something I'd want to check out very thoroughly indeed.

Harvey Ramer

What is the building for?

Is comfort a big factor in your considerations? Or just installation cost and running efficiency? There are other factors as well, longevity, maintenance and replacement costs to name a few.



Hydronic is the big winner in most of these categories. One of the many beauties of hydronic distribution is that you can stage your heat plant to perfection and still deliver heating/cooling to every corner of the building using only 6% of the energy that would be required to move the amount of air required to deliver the BTU's.



Harvey

nicholas bonham-carter

Funeral home

Most like a church, with a staff of 10 during normal business hours, with a couple of funerals a week attended by up to 200 people.comfort is a big issue, but I thought during the colder months the gas supplemental heat could be more economical than simply an air-source heat pump.

I know these calculations of energy cost have been covered before here in regard to heating system choices (NGO, LPG, electric etc);but I was interested in the gut reaction of experts here.--NBC

Harvey Ramer

Quick system response.

In scenarios like the one you are describing, you need a system with the fastest response time possible. You'll probably be best served with fan-coil terminal units or a ducted air system.



As far as natural gas versus heat pump. If you choose a high efficiency invertor drive such as the Daikin Rebel, the heat pump will run cheaper than natural gas down into the teens. That is of course, at the prices you mentioned. The heat pump will probably reach an equilibrium of heat output versus load, somewhere in the high 20's or low 30's. It will vary from unit to unit.



Ultimately, with a heat pump of this type, you would want to supplement with gas at low ambient.



By the way, $.048 KWH is really cheap for electricity!



Harvey  

heatpro02920

A church

I would definitely look into the rebel units, did you do the math yet to see what kind of energy you need?

Spence

Electric

Your electric costs are 4 times the national average? Something doesn't seem right.

Mark Eatherton

If it were my building...

I'd go with RFH, tubes in cement. Forget about set backs because funerals don't take vacations, and high mass buildings are not conducive to set back or set up.



I'd also add mass to the system design using one of the new bio engineered Phase Change Materials. This will lower the loads overall and add to the flywheel mass effect.



Due to the nature of the transient cooling loads, you might want to consider a DOAS (direct out side air system) cooling system. Having the mass is to your advantage. Considering that you will have a need for DHW, then it might be a good idea to set a hybrid system whereby when you approach design considerations, that you can blend a gas fired modcon into and out of the picture as needed. Remember, each person coming to the funeral is going to be brining around 50 btuH with them… This modcon will be primarily responsible for the DHW, but can lend its capacity to the space heating needs of the building as well. The majority of the sensible load can be handled by the radiant cooling system, but depending upon your location, you might have to maintain a large differential between the dew point and chilled water supply temperatures,



Consider also the use of polished stained concrete instead of carpet an pad for finish. The less R value placed over the finished floors, the lower the operating temperatures needed at design conditions. BTW, these manufacturers are claiming COP's above 1.5 to 1 even at ZERO degrees F outside temperatures.



As for the cost of electricity, .045 is CHEAP. I pay .10/kwh, and that is considered fairly inexpensive.



Calculating cost is simple math.



100,000 divided by 3413 = 29.29 KWH per therm.



$.045 X 29.29 = $1.31 per therm (raw electrical cost at 100% efficiency).



If the heat pump has a seasonal coefficient of performance of around 2.5:1. then $1.31 divided by 2.5 = $.54 per therm. Much cheaper than the gas costs.



Friends don't let friends pour concrete without having plastic tubing in it first.



Friends don't let friends build massless metal buildings without having PCM's incorporated into its construction. http://www.phasechange.com/index.php/en/



Friends don't let friends build funeral homes that are not radiantly comfortable.



I consider you a Wall friend Nick.:-)



ME

SWEI

Thats $0.048

not $0.48

heatpro02920

I thought it was cheap

I pay double that...

ChicagoCooperator

Funeral Home, eh?

I'll offer my two cents worth from the Architects perspective. My firm designed a funeral home recently, which was intended to be heated with geothermal heat pumps, however the client value-engineered that out in favor of package rooftop units (gas fired) too late in the game to re-engineer the structure, etc (Village approval on appearance had already been granted), to accommodate ductwork instead of the planned piping to fan-coil units. This led to all sorts of ductwork acrobatics to make the system work - it did, luckily, thanks to a well insulated envelope, but we had to add joists and lower ceiling heights in a few locations to accommodate the larger ductwork. 



I think air source will be easier to deal with of course and I'm assuming the ductwork will be planned out in advance . One thing we didn't get to take advantage of was the cooler (I don't know if this is standard in all funeral homes, but they only rarely embalm here, so the cooler was needed) which could have been tapped for it's exhaust or wasted heat (rooftop condenser).



So my advice is get that all worked out with the client before signing off, i.e. what they expect and how they plan the operate their system. I think Harvey's advice - they will be running setbacks on the unused chapel space (depending upon how busy they are - our client ultimately was planning upon multiple services a day and may not bother with setbacks because of the high volume of services)- is sound.

nicholas bonham-carter

Corrected rates

Our electricity is $.1025 per kilowatt.

Gas is $.55 per therm--NBC

Harvey Ramer

Oy!

That makes a big difference!



I'm on Long Island right now awaiting some brain food to be dished out by the HVAC Prof. I don't have my calculators with me and further more, I'm drinking beer. Some wallie will run the numbers.



Harvey

Mark Eatherton

Math

29.29 times .1025 = $3.00 per therm. Economics don't look so good now, but electricity has typically been much more stable than NG and other fuels, even though NG is used in its production…



Talk to the utility purveyor about off peak electrical rates that may match your original estimate. Then the PCM's start looking REAL interesting.



ME