# How many BTU required to heat air

Good Day folks,  this is my first posting to HEPF although I used to regularly participated in the LinkedIN group.  I am approaching year 6 on my build of a single family home.  Been quite the journey.

Anyway, I need some advise regarding the best approach to offset the cooling created by a Heat Pump Water heater.  I requested and received some data from RHEEM re their 80 Gal Rheem Hybrid DHW tanks.  The unit pumps out 4000 BTUH of cooling into a room.  If the exhaust was ducted to the outdoors, the flow would be 150 CFM.  And they do not recommend ducting the intake to the outdoors where the outdoor ambient temp can drop below 50F (so that option is not on the table for me as we get down below 14F in winter)

I will be placing this unit in a 3312 cuft (414 ft2) below grade room that is well insulated on all 6 sides and separated from the main dwellings building envelope.

I need to see what makes the most sense, not ducting the exhaust and just heating the room an additional 4000 BTUH   OR   Ducting the exhaust and heating the required makeup air from outside.  I just need to understand how to do the calculation.

I used this site https://www.simplex.ca/en-CA/btu-calculator and entered a room size of 1x1x150 to simulate the makeup air volume.  It uses ºC so I used -5 and 20.  This gave me a needed BTU of 210 which would be per Minute as that is the air flow scale.  So this would be 12,600 BTUH.  Right??

In this case it is obviously cheaper to not duct the unit and just heat the room the extra 4000 BTUH.

If this calculation is correct, there would not really be a time when it made sense to duct the unit. The only time that the makeup air heating requirement was equal to the 66 BTUH (4000 BTUH / 60 minutes), was when the outside air was within 1 degree of the inside set point. And when the outside air is warmer, then obviously the cooling of the heat-pump is a beneficial reduction on the cooling load.

Are these calculations correct?? Any insight would be appreciated.

Tags: BTU, Calcs, Hot, Hybrid, Tank, Water, air, heating, make, up

Views: 498

### Replies to This Discussion

also think about how long the hot water heater will last?    How hot of water is needed?  120' 130' 140'  the hotter the water the more lime ( mix of hardness) will set on bottom or sides.   Also think about the sacrificial Anode Rod?   Once the sacrificial anode is gone the tank will rot out unless poly or stainless.   I have taken out a 8 yr old water heater 40 gal water heater that were 60% full - so just 20 gal of use were there.

Thanks Eric, this is good advise regardless of the HWH type.  Fortunately our water is pretty soft (comes from above ground).  Also, I have a reoccurring reminder in my Outlook calendar to change out the anode rod on my rental's HWT and also did so in my previous home.  Our old HWT just started to leak the month before we tore down and was over 17 years old.  I had replaced the rods twice in that life time (5 yr cycles) and each time there was nothing left.

If your room is air sealed and insulated on all 6 sides, where is the thermal energy going to come from? If you duct air from outside, the air is to cold in the winter for the heater to operate in heat pump mode (and COP falls off below around 65 degF). If you duct air to/from the house, where is the heat in the house coming from? A HPWH only makes sense if you are taking wasted heat or heat from the earth or from warm outside air or from a room heated with wood heat. This is only a heat pump, and the colder the air, the more electrical energy needed to pump the thermal energy.

The 4,000 BTU/Hr quoted is for running time. It does not relate to gallons of water used, temperature rise of water to be heated, and temperature (and RH) of ambient air at HPWH.

Thanks for reply Brad - I am aware of the constraints you have stated.  The unit would be drawing heat from within the room during the 'cold' season as discussed above. The room will be heated with a Air Source to Water Heat Pump providing the hydronic space conditioning for the dwelling (ThermAtlantic DX2W).

What I don't agree with, unless it can be proven, is "A HPWH only makes sense if you are taking wasted heat or heat from the earth or from warm outside air or from a room heated with wood heat."  A hybrid heat pump can make sense anywhere that the room heat being extracted by the hybrid, is created in a method that is more efficient than the option of a 100% resistance HWT.

I believe that the two heat pumps in unison still end up being way more efficient and cheaper than going with a 100% electric resistance heater when taking into account operating costs. Do you not agree?

And as you say, this extra penalty of 4000 BTUH is only while DHWHP is operating and only during the heating season. During the 'hot' season, there will be no penalty and in fact it will reduce the cooling load of the room (and potentially the house if I create a ducted spill gate between that room and the rest of the house - as this would help greatly in reducing the latent cooling load).

The only question is what will be the cost for the extra 4000K of heating needed if I go with a Hybrid, and how does that effect the payback period.   Not sure that data for this exists easily.

Would depend on outdoor temps and operation time of the Hybrid which I do not know how you could estimate.

One final comment - this has been done (combining two heat pumps) and the results measured by Alex at GreenBuildingAdvisor (https://www.buildinggreen.com/blog/heat-pump-water-heaters-cold-cli...).  Yes the performance drops during the heating season, but it is STILL saving money.

Sean wrote: "The only question is what will be the cost for the extra 4000K of heating needed if I go with a Hybrid... Not sure that data for this exists easily. Would depend on outdoor temps and operation time of the Hybrid which I do not know how you could estimate."

Several studies have touched on this (e.g., NEEA, Heat Pump Water Heater Validation Study, Ecotope, 2015), but they most point to the need for further research (although I may have missed something more recent).

Fortunately, it's easy enough to do a thumbnail estimate of the reheat energy required if we have the following data:

• HPWH output capacity (BTUH, specified at a given water & ambient air temp)
• two COP rating points for air-to water spacing heating system
• Vancouver Harbour BIN temperature data (I have this)
• gallons per day assumptions (we can put upper/lower bounds on this)

Most HPWH mfrs don't publish BTHU capacity, at least for the heat pump alone, but if it wasn't included with the other data you received, just ask.

Are you meaning the space conditioning heat pump or domestic hot water on your bullet 1?  How is this relevant for room heating. Regardless, I would not be able to get this info,  the 4000 btuh of cooling introduced into the room came from a ex-employee who was part of design team. But I doubt I could bug him further.

The gals per day I also do not think effect the calcs as much as knowing the cycling of the hybrid to meet that demand. I do not believe this data would be available.

I probably CAN get an estimate from ThermAtlantic on the costs related to just an extra 4000 BTUH heating load on a 100% bases (design load) and will try to get, this coming week.

The abbreviation HPWH refers to a domestic water heater, not an air-to-water space conditioning heat pump (the latter is also a chiller).

I need to know the HPWH capacity (BTUH) to heat the water via the heat pump alone so we can estimate the run time. Some manufacturers publish this number on the spec. If not, you'll need to get it from Rheem. If you can't track it down, post the model number. I've never had a problem getting that number when needed.

The 4,000 BTUH number you were given for 'cooling' is a corollary to the heat pump's output to the water tank, which is the number I need.  I can't use cooling capacity to estimate run time because there's no way to know if the 4,000 represents sensible cooling only or total cooling (including latent) or if it includes the heat created by the compressor motor.

-----

Typical or avg gallons per day is exactly what we need to determine the expected run time of the Rheem and thus the amount of heat . It doesn't matter how often it cycles - whether it's 20 cycles of 40 minutes or 5 cycles of 160 minutes, it takes approx the same amount of reheat. We must assume the heat pump alone can handle the daily load.

-----

Don't bother asking ThermAtlantic about the incremental cost of producing 4,000 BTU. They can't begin to answer that question without your temperature BIN data and making assumptions about the load profile. But this stuff is vanilla for me. You just need to find out the COP at two cold-weather rating points, which should be on the spec or in the AHRI database (typically @ 45F & 17F).  Again, if you can't find these numbers, post the model number and I'll grab them.

Thanks David - I knew HPWH definition, just was not sure why you wanted (and still don't) its specs to find out the costs of heating the room the extra 4KBTUH the hybrid would cool the room by.

The setpoint temp for the room this potential hybrid would be installed into would be 68º-70ºF and be maintained at that temp by the ASHP conditioning system.  So the HPWH would always have that air temp to work with.

I am guessing what you are doing is estimating the run time of Hybrid, so we can determine the number of hours of the 4000BTUH of cooling being dumped into the room??

I have spent a few hours and have been unable to find the info asked for in the first two bullets on any documentation.  The only thing I found was that the 'Compressor BTUH' spec is 4200 (with no water or ambient air temp provided).   The model is not listed on the AHRI database (only the 15A version -PROPH80 T2 RH350 D15 - is which I assume has much smaller resistance heating elements, and I do not see the info you are asking for listed for it either)

To fill in a little more on the 4000BTUH number provided by ex employee I have copied some of the email in case it helps.

1)            What would be the BTU of cooling introduced into this room (I need to calculate the added heating load, introduced by this unit) - 4000 BTUH

2)            If ducting the unit's exhaust to exterior, what is the cfm being exhausted? (if only ducting exhaust, I need to determine amount of make up air is needed for the room) - 150 CFM

3)            What will be the temp and moisture content of this exhausted air (concern re condensation and ice forming on termination cap in winter) - Moisture is not a concern. It will be dehumidified air going out of the unit.

The 30A model # is PROPH80 T2 RH350 DCB.  I would be happy to follow up with Rheem on Monday if you tell me how to approach.  The '1-800' number was of no use when I called last week to try and get the cooling effect of room.  After being passed around from department to department for 1.5 hours while generally on hold, I was cut off.

Re ThermAtlantic - they have the weather data for Vancouver and did give me a pretty good estimate for the main house conditioning which I have attached.  So I am confident that they could provide the costs for 4000BTUH as long as I was able to give them the duration that the load would be imparted on the room.

I don't dive into numbers as well as David does because if you do analyze a situation only by numbers, you need to be very careful to include ALL parameters that might have an effect on the solution. When using assumptions, such as delta-Ts or gallons-per-day, you have to factor in some sort of error range when looking at the data, and the more parameters that are assumed, the more room for error from synergism. With the wide temperature swings we have been seeing lately, HDDs is becoming a less useful (but still very important) parameter. I love math and science, but I don't do that sort of analysis on a regular basis so I leave that to others. I am fairly good at looking at the whole picture and thinking about all of the parameters that could have an effect. Alex's blog post that you referenced made some reasonable assumptions but did not take all parameters into account, starting with the floor. He estimated the R-value of the floor and a delta-T between the house and basement but did not include air movement between them. Having done many blower door tests in houses, I find that rarely is the basement not reasonably well connected to the house for air movement. It is also interesting that he uses a Geospring HPWH, which has had a poor track record. GE stopped making them and Bradford-White has taken over warranty issues.

Unless there savings in your situation are huge, a numerical analysis must be done with great care. Unless the savings calculated from measuring an existing situation are huge, you cannot compare the results unless you carefully compare all of the parameters, which would be many. I am looking forward to more studies and careful analyses of these solutions for heating our homes and hot water.

You can rest assured on one point - if I go ahead with hybrid, I will definitely be reporting on its performance and electrical consumption.  I may even put a flow meter on the hot exit to have accurate water consumption data.

I want to Thank David for a marathon call we had today.

We went through a lot of assumptions and data points. The end result was that I found an additional error in my spreadsheet that makes payback even less than what I reported (I was dividing by the cost to operate the HPWH instead of the savings between the two methods of heating).

With the annual hot water gallons tweaked down to 30,700, and resistance heat cost increased to \$.0147 to represent the fact that if I am doing resistance heat, I will for sure be in Tier 2 billing, the payback (not counting the cooling load imparted on dwelling) would be 2.75 years to offset the \$1850 premium this equipment represents.

I was then able to get the ThermAtlantic spreadsheet, later in the day, used to create the graphic I posted and modify it as needed. Assuming that the HPWH ran 24/7 for the 246 days of my heating season (which is obviously way more that it will actually run), and assuming that the HPWH would never be exhausted to the exterior during the heating season, the cost to supplement the 4KBTUH would be \$191 annually using my planned DX2W and a 3 Ton Goodman Heat Pump. This would only drop payback to 3.84 years.

SO – it is quite obvious that the Hybrid HPWH makes total sense for this dwelling and our use. Our hot water use would have to drop almost 50% to below 17,150, before the payback exceeded even a most pessimistic 10 year life span expectance of the appliance. Even if I gave up 100% of the baths, this would never be achievable.

I am thankful for the education this post has brought me, and the need to ‘defend’ my plan to utilize this technology. Now I can confidently proceed knowing it is the right decision.

Yes, quite the phone session that was! Having spent time with you at the site early on, it's amazing and exciting to see it all finally come together!

After slashing those sink estimates by 2/3rds, your new total seems realistic. But as 30k+ gal/yr is still well beyond what the 2-person household would consume (for good reason), this is really the ideal project for a HPWH, especially given your steeply tiered electric rate.

One concern remains... By my math, assuming 6,000 BTUH capacity (which we don't know), your heater would need to operate 8 to 9 hours a day in winter to fully satisfy your average load. That's probably at or near the limit of this heater in terms of daily load without using the COP-robbing electric element. In fact, I think most folks would consider your load impossible to support without supplemental heat. But I KNOW you'll take that on as a challenge, enforced by disabling the electric element! It will be very interesting to see how it works out. If you end up needing to use the backup element, I hope you'll set up a circuit-level energy monitor to track supplemental kWhs.

Regarding your original question... You'll be pleased to know your \$191 estimate for reheating the exhaust air is probably high by about a factor of 5 (with the caveat that I'm relying on your spreadsheet calcs). You already knew it was worst case. Here's the rest of the story:

The 246 heating "days" from the table (presumably derived from TMY3 '5F BIN hours' converted to '1C BIN DAYS') cut off at 15C (59F). We estimated the HPWH's 'leaving air temperature' at 45F, above which you'd want to exhaust to the outside until there's a cooling load. As it turns out, only about half the hours (days) on the table are below 45F, thus cutting your estimate by roughly 40% (not 50% since space heat COP goes down with ODT).

Next, you assumed 24/7 operation for simplicity to put an upper bound on reheat cost. IF the HPWH is able to satisfy your load without supplemental heat, the compressor will run about a third of the time as noted above, so that drops you to about 20% of the \$191 estimate, or \$35 to \$40 a year to offset exhaust air cooling.

BTW, I estimate the interlocked temperature-controlled exhaust and make-up air system will save what would be an additional \$20 to \$25/yr in reheat costs if you don't do outside exhaust. Something you want to keep in mind as you journey down that path :-)

As an aside, I think most folks with access to natural gas would go for a gas water heater without a second thought. Depending on your local rates, it might save as much as 40% vs the HPWH, or about \$10/mo, but the pesky monthly (fixed) service charge would more than wipe out those savings!

I very much look forward to visiting your new digs once you're past the crunch. Likewise, when you're ready to take a few badly needed days off, you guys are always welcome here where it's (usually) warm and sunny. Sorry, no sandy beaches.

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