daikin heat pump - supplemental heat needed?

[TheShanMan]

I'm getting a bid from an HVAC company that wants to put a Daikin RX/FTXS system in my outbuilding (600+ sq ft downstairs, 400+ sq ft upstairs). Would this system be able to maintain 70* in the Portland, OR area where lows in the winter are generally low 30's (highs in the 40's) but will occasionally dip into the 20's and on rare occasions the teens?

The outbuilding does have insulated walls but it has a concrete floor (which will eventually be carpeted).

He maintains that it will have no problem but I'd like a 2nd opinion. I'd hate to install a system only to find out it isn't adequate!

Secondarily, is the efficiency going to be so poor in those conditions that I'll wish I had a gas furnace?

[Rhizzlebop]

Sir, you have asked an impossibly difficult question.

First off, as others will say here, the only way to know the heat loss of your space, is to have the contractor do a calc called manual J to determine that BTU loss at the design winter conditions. Design conditions is set by ASHRAE or the IBC, I forget which, and is usually a pretty cold winter day, but not the coldest winter day. Here in SC, we see a few nights a winter in the teens usually, but our design day (i'm a consulting engineer) is 21F. So WHEN you get your guy to show you those calcs, ask him what that design temp is at. That is the ONLY way you're gonna get a decent answer and better to get it BEFORE you spend money.

With that said, I have a question. You say this outbuilding has an upstairs and downstairs but the unit you describe wanting is a unitary, non ducted minisplit. So, its either going upstairs, or down, but not both. No way it'll keep it all at a good temp. It's impossible.

With THAT said, the FTXS and the RX heatpump series function down to 0F. It depends on which size you get as to how much cooling and heating you'll get from it. Luckily, you CAN "oversize" these units for your space since its an inverter driven compressor and will modulate back to match your room space load.

I JUST last week had the Daiken quaternity series installed in my bonus room. Its main difference from the one you're looking at is that it has hot gas reheat so it can actively dehumidify, and it can heat down to 4 below zero. Otherwise, very similar I believe. I went with it for the great brand name, great warranty, and awesome feature list. It is also SUPER quiet. A few decibels quieter than the one you're looking at, but you can get larger capacity sizes in the one you're looking at.

Even at the lower range of temps you describe the efficiency of these things is still pretty good. I know my unit produces about 50% of its heat capacity down at like 17F. Which is also pretty good.

AS you know, I assume, these units don't have any backup electric heat like a typical split system heatpump unit.
WORST case, if there proves to not be enough heat, you can add a couple baseboard heaters on a t stat JUST for when the heatpump can't quite get it done.

Now, last thing in my longest post ever. Considering you have an upstairs and down stairs, you may want to consider a ducted model that you can run justa little duct off of and get some throw split to the upper and lower parts, OR you may want to consider one of their multi type units. It has one outdoor compressor part, and then you can hook 2 or more indoor parts to it and they work semi independently. Shouldn't be THAT much more than what you're already looking at since you're basically already talking about paying for one outdoor and indoor unit. All you'd really be buying extra is another indoor unit and a slightly larger outdoor unit.

Google up daikens website and check out all the stuff they have.

[TheShanMan]

He said there would be 2 indoor units (multi) - one upstairs and one downstairs.

So are you saying doing one of these "manual J" calculations is kind of "standard procedure"? I walked around with him while he looked the place over and he certainly didn't do anything that a layman like me would recognize as any kind of measurements upon which to make such a calculation. If this is something he should have done I'll definitely ask about it. Could it be that by observation he determined that the system would produce so much more output than necessary that the manual J calculation was simply not needed?

Appreciate the input!

[Rhizzlebop]

No way he did it that way (by eye). I personally am an engineer so I "run loads" on buildings we do using very sophisticated software that account for the materials that makeup each wall, roof, door etc and include each window and its specific properties including the glass U value, shade coefficient, even the frame material and whether there is an outside overhang, or internal shading and especially the direction each wall faces, N, E S, or west. All this stuff makes a difference. To say the walls are insulated is even vague. Is it bat or board ? What is its R value? Etc etc, you get my point I take it?

My understanding is when contractors do this similar evaluation in the field they use some forms that they might carry on a clipboard and they fill out square footages and material properties and usuallly go back to ther office anddo all the calcs and it'll result in a design heat loss or heat gain. You gain heat in the summer and you lose heat in winter.

Sometimes contractors wil try to make ball park or rule of thumb estimates, and he may be doing so here. The problem is, you're looking at a special kind of unit, a good one at that for sure.

Example, in columbia SC, a ball park guess would say if I were putting a standard heatpump on a residence, I couldsize for about 700 sqft per ton of cooling in the unit. That can change based on exposure and if a home has a lot of windows ( basically all the things i mentioned above) so you run calcs before actually picking equipment.

Anyway, moving on. Your winters sound similar to ours here oddly. I would have thought colder but I guess you ge warm pacific breezes up there.

So, if you were going on a similar estimate of 700 sqft per ton, that says a 1.5 ton unit should do, or 2 3/4 ton units.

There are however a couple issues with that. Your upstairs might need a bigger unit than the downstairs for summer cooling, and your downstairs will need more heat in the winter cause some of what you poduce down there will float upstairs pretty quickly.

The biggest issue is these little inverter units dont have any backup heat coils so as you know when it gets too cold, youll have no heat. In my rule of thumb sizing of 700 sqft per ton, its assumed that during a winter here, those aux electric coils will run some on the colder days and such.

You an find the spec sheets for your unit online and you shoyld see heat pump rating points. I'm guessing at around 20F, a 24,000 btu outdoor heatpump will give you about 10,000 btus of heat. I might be off on that but I feel pretty good that its between 8,000 and 11,000 at that temp.

Well, I'm guessing your 1000 sqf space prob is losing heat at 20F at around 12,000 btuh. Point being without the cals, we don;t know, but its very close. Too close for him to call it by some rule of thumb. so, if he didnt do any calcs, he needs to.

Some here would say, if the contractor doesnt do the calcs, move on to another contractor cause hed never do a good job if hes already half as$in it from the start but you can choose for yourself whether to be that harsh.

Like I said earlier, these little units work great, are super quiet and efficient and I'm really liking this quaternity one, so I wouldn't necessarily back off of it either way. But you might wanna do a couple baseboard heaters, down on the wall by the from door, and maybe another near the biggest window wall upstairs or something and put them on a t stat and set it a couple degrees cooler than your setting for your split. Since those are electric, they are power hogs compared to a heatpump, but they'd only run at those coldest times and you'd be good to go then.

BTW, I am guessing hes prob suggesting a 2 ton (24,000 btu) outdoor unit and 2 12,000 btu indoor units. Sounds alright, but you need some kind of calc to seee how tight you are on heat and if you'll need a backup source.

[TheShanMan]

I totally get what you're saying about all the factors that go into how well insulated it is. I very much appreciate your input. I hate being uneducated when spending significant money on something so what you've said is very helpful to me.

It's not absolutely critical that the building stay 70* 365 days a year - if there's a day or 2 that it's not enough (i.e. very rarely and hopefully for most years, not at all), that's fine since it's not "living quarters". But I certainly would be disappointed if we get a cold spell and it's like 55* in there for like a week straight.

Thanks a lot!

[Rhizzlebop]

You're very welcome! Let me add another thing before turning in for the night. I get what you're saying about it being ok if a couple days its not 70 in there. The thing is, after someone does the manual J they should be able to tell you the balance point for the building with the equipment you're looking at.

What I mean is, if say for example, when the inside temp is set to 70, and the outside is 20F, maybe the calculation says your room will be losing heat at a rate of 12,000btus per hour. Well at that same outdoor temp, the equipment can produce (forexample) say 12,000 btus. So, that would be your balance point, though you normally wanna have a little margin for error.

What that then means is, if say its 15F outside, your space might be losing 14,000 btu per hour and your equipment might then be producing 10,000 of heat. Obviously your room will get colder. Well, as your room gets colder, the difference between the indoor and outdoor temp decreases so the rate of heat moving from inside to outside decreases so say the room falls to 65F, then the rate of heat loss might be 10,000 btus then. Well, if your unit is making 10,000 at 15F, then 65 is where you'll be at in the space with the unit running wide open at 15F.

Point being, it might be more than a couple days in the middle of the night and it might not. It might instead be anytime its under 30F and thats prob more of an issue. If you have a guy run the calcs and you say you want to jkeep it at 70, he should be ableto say,"this equipment should be able to handle that as olong as the outside temp is xF or higher, and then you can make the call to add a baseboard heater or not.

To quickly tout......Dsaiken is a great brand and I think their R&D is second to non. I am loving the unit I bought and when the outdoor temp gets under 15F it prob wont stay quite to my warmest liking, but that'll come in the middle of the night, maybe 4 times all winter so its no big deal to me. Just look at their warranty vs others. 7 yrs on the compressor, 5 on parts? Hard to beat, very hard to beat. In my opinion you can feel the quality when you pick one up, and its a cadillac of minisplits in my book.

Last thing, the fact that you're on a concrete slab actually helps you due to the inherent mass in that slab. It takes heat to warm that slab but once its heated up, that slab gives off heat to the room slowly too. It might not be a big difference, but if in the middle f the night the room drops a few degrees, that slab will start giving heat back to the room a little and possibly help you get through those coldest few hours of the night better than without it. I know slab mass is counted in our heat calc software but I'm not sure if the manual j counts that or not. If you've ever been in a concrete block building on a cool night after a sunny day ( like at the beach sometimes), its warm in there because that block (heavy mass) absorbed heat from the sun for hours and it takes hours to give it back off to the space so theair in there stays warm a while whereas a framed wall with studs has very little mass so it cools down quickly and doesnt retain a lot of heat to begin with.

goodnight and good luck.

[mchild]

The only way to do it right is to do the actual load calc. Everything else is just guessing. Download the program form this site and do your own. It is well worth it. Size the system to the higher of the two loads - cooling or heating but suspect it will be your heating load. While oversizing is not as much of a negative issue with these systems I would be careful to not go too far.

If you are going to have an indoor unit for each floor it may be worth considering the 2:1 setup within the same family of Daikin systems. These have two indoor units connected to a single outdoor unit. Seems redundant to have two outdoor units if it isn't necessary.

The heating performance of these system provide about 60% of the heat output at 14*F.

[TheShanMan]

I'll look for that program and see if I can't figure out the load on my own. And the proposed system is a 2:1.

Thanks!

[mchild]

I'll look for that program and see if I can't figure out the load on my own. And the proposed system is a 2:1.

Thanks!

The 18,000 BTU system?

[TheShanMan]

I want to say 24,000, but I'm not positive on that. I should be getting the bid any day in the mail so I'm sure I'll know more specifically then.

Should I not be able to see posts in the Software And Calculations forums?

[mchild]

I want to say 24,000, but I'm not positive on that. I should be getting the bid any day in the mail so I'm sure I'll know more specifically then.

Should I not be able to see posts in the Software And Calculations forums?

If you are looking for the load calculation software it is located in the bar to the right of the Search button. It is called HVAC Calc.

[flange]

I believe that particular system comes as two 18000 btu indoor units. it should work well. I also believe that daikin offers an expansion board to control a stage of electric backup heat if required, they do on most of their products. one of the problems is that they stop rating their equipment at certain temps, but it actually works better than they advertise. if you use their specs, you will be fine, and probably not even need the electric backup, but only a load calc can tell the tale. due to the inverter technology, there are published tables(at least there used o be) that gave you a generic rule of thumb by zone in the us and tonnage required per a given room size. this generally would give you an oversized unit in many cases, but the inverter and electronics made it ok. he may have used that chart.

[Rhizzlebop]

Hey OP, are you still around?

What did you end up doing with this building?