How much are mini-splits really affected by altitude?
你好 -
I’m sizing a heat pump mini-split for my 20×20 garage, which has been pretty well air sealed and insulated with closed cell foam. I’ve had a series of installers out and the recommendations were all over the place, ranging from 12k to 18k BTU, and from $3k to $12k.
Two of the companies actually bothered to do a load calculation, and I tried it online as well – but those numbers had a broad range, too.
把它们整合在一起，我猜我需要大约6k的Btuh冷却和22k的加热（尽管，但其中一个结果在4K处冷却，另一个在49K处加热。我正在使用这些数字，因为真正花时间进入负载计算的安装人员产生了这些数字，这在另外两个结果之间落下。
Now, I can look at different models (like the Mitsubishi & Fujitsu) and choose a size. One thing I don’t understand, however, is how altitude plays into this. I know that mini-splits have reduced capacity at altitude, but how much would one typically deduct from the ‘perfect world’ capacity of a minisplit to account for altitude?
Is there a formula for this? I’m at around 5350 feet. thank you!
GBA详细资料库
由气候和房屋部件组织的一千个建筑细节的集合
Replies
Dave,
我要邀请GBA读者对您的问题发表评论。与此同时，这里有两个相关资源的链接。
选择加热，用于高空房屋的冷却装置。
高空HVAC（此PowerPoint演示文稿包括解额定值表。）
The short answer is that it varies, and you would have to consult with the manuturer. The derating will not be the same for each model or vendor.
A good rule of thumb is to decrease the mini split output in proportion to the absolute air pressure.
Fujitsu responded right away with a very clear document. I have attached it.
我还问了三菱进行信息。
Things are getting clearer.
似乎最值得信赖并花费最多时间分析负载的安装人员正在富士通18RLB推荐。在该单位的规格中，它说'最小/最大加热BTUH 7,000〜20,000'。如果我弥补高度，大致弥补了海拔，那就是我所在地点的大约16,600个实际加热。
同样的HVAC Pro估计我的空间20,503 btuh的最大负载。这不是意味着他提出的单位在加热方面有点动力吗？
I also noticed that for the construction quality, he entered 'semi-loose'. This is a tough call because I sprayfoamed and airsealed the garage like crazy, filled all the bays with closed cell, and installed the windows extremely tightly. There is a garage door in there, but I've put in a new vinyl seal on the exterior and a big brush seal inside, plus upgraded to a well insulted/sealing door. I have a feeling this would be considered slightly more on the tight construction side then semi-loose, but I'm not really sure.
Each of the HVAC pros who saw the garage all said it was the tightest garage they'd ever seen. I sort of get a kick out of the airsealing. It's like a game so I'm spend tons of time caukling and sealing. But, there is a lot of thermal bridging since I wasn't able to mitigate that with my skill level.
Obviously there are a lot of moving parts to this - but with so many professionals giving different information, it's tough to know what is 'best'.
- Dave
Does this altitude de-rating reduce the efficiency, or does the electrical consumption drop commensurately?
I'm not certain nor do I completely understand all of this, but reading the text on the PDF they sent it seems like the it's the 'capacity' that is derated, so I think it's derating the BtuH of cooling/heating that the unit can produce.
Dave, none of these calculations are going to be able to reflect your actual air leakage, and without a lot of detailed calculations, they aren't going to reflect your actual thermal bridging. So there's some risk that you'll end up with the sizing off somewhat. The traditional solution is to oversize it to be sure. But you could also consider:
1) You can probably tolerate having the garage temperature a little lower than you'd want your living room to be in the middle of the night on a cold winter night. That means you can get away with less heat being supplied, and the heat pump will actually be able to put out a little more heat at 60 F than it does at 72 F.
2）如果您想要更确定，您可以使用电气空间加热器加热一个月左右，测量它们的电力消耗，并与温度记录进行比较，以便在温度和加热负荷之间返回精确的关系，并使用为了计算您需要的东西，即使您的实验不包括1月份。
高空通过改变热交换器的有效性来影响热泵性能。热泵的内部结构不知道大气压是什么，但它们在任一端看到的温度更差（冷侧降低，热侧更高），因此它产生较少的热量并效率较低。
Attached is some 2012 Mitsubishi data on high altitude performance.
Why spend 3k to 12k on a mini split in garage? This isn't living space. A gas garage heater would cost 10 times to 40 times less and work a lot a better. Even a electric version. Mini splits aren't made for on demand heating that a garage environment demands. Infrared is perfect for a garage. Heat the vehicle right before you take it out or work on it. Shut off the rest of the day. So not only is the initial cost many times cheaper, but the cost of running the equipment is cheaper (fuel & maintenance) when put on a timing system. Want AC put a $100 window AC and be done with it. Sometimes the easy solutions are the best.
嗯，你提供了一些漂亮的教条信息，而不知道我打算在改造的车库里做什么。
你怎么知道这不是生活空间？我正在改造车库将它转换为办公室/工作室，并将运行一个包含视频工作室的业务，温度控制和噪音水平非常重要。我们需要能够做长视频镜头，并以低噪音水平保持稳定，一年。
Your $100 window unit suggestion is a no go but thanks..
You said a garage which are used for vehicles. If you mentioned that is going to be an office I would recommend a different solution. Don't be offended. Sometimes clarifying yourself helps.
一定是分享这个空间的预期用途和占用模式的分享。
是一回事支付高端热泵heating & cooling a recording studio, another for heating & cooling a bedroom or living room, and still another for intermittent use as a pottery studio or glass blowing shop, and still another if it's going to be for cultivating cannabis (legal under some circumstances in some high altitude US locations.) The "one size fits all" or "one type fits all" approach to HVAC equipment selection isn't always going to end up with optimal solutions. A mini-split solution might be a reasonable choice, but it also might not be the most optimal.
A blower door test might help in zooming in a bit closer on the infiltration contributions to the loads.
这听起来我的评论是关于它的评论，如果热量没有在寒冷的夜晚继续保持寒冷，那么现在我知道空间用于什么。您仍然可能决定接受那里的某些风险，但它需要一些时间来赶上，所以早上也很冷。
If the garage door is not in regular use, there could be ways to temporarily seal it for the winter, making it a lot more air tight and perhaps also better insulated, while still allowing it to be used if and when needed.
If noise levels are a concern, it might be that an oversized system running at part capacity would be quieter than a right-sized system running at full capacity. It would also catch up faster if you had to turn off the heat during a recording session.
嗯，在这种背景下，噪音很容易处理。一种高度的'Whoosh'声音像MiniSPLIT一样，如果您有适当的麦克风等，那么可以很容易地处理。它是那么低，隆隆声，那种来自墙上的单位真正变得棘手的声音。像那样的“哼哼”很容易出现，但值得避免，所以迷你分裂是胜利者。
In regard to sizing, the garage door (which has been pretty well sealed and is insulated) will very rarely be opened. And the unit can run continuously while recording. In fact, it's almost better if the cycles are long, so that there is less 'on and on' noise in the background if any of that noise manages to leak through.
在我看来，这里的风险是，如果我选择那个特定的单位，在某些情况下有可能让房间完全加热。然而，这可能是一个非常罕见的事件（即每年可能是几天，而且可能只在晚上）。如果这是正确的，那么我可以从房子里带来电气散热器并不是一项大事。