Oversize HVAC
Hi all,
I am building a home to be as energy efficient as my budget allows, so it will be air tight with higher insulation etc. The home is a total of about 6000sf and is a ranch with basement in the midwest. I am now looking at my HVAC bids but noticed that some of the bids have over sized HVAC systems. The bids include a 5 ton ac 20seer unit, a 110k BTU 98% furnace, ERV fresh air system, and humidifier/dehumidifier.
I’ve read concerns around oversized systems with short cycling but would that really be an issue since the ac unit is 2 speed and the system includes an ERV which will dehumidify the home even if the AC unit doesn’t run long enough? If that is the case than comfort won’t be an issue with an oversize system correct?
I did get a bid from a green hvac vendor but the cost was 5k higher for a system that does not seem as high end (spec sheet) than the non-green bids. I do realize that the green hvac vendor is going to actually do a better manual j/p/d calculation and probably rightsize the system and ducts but the higher cost is harder to justify.
我还有一个问题是，超大系统实际上会为异常使用模式提供更好的补偿吗？我们通常在夏季有很多聚会，因此，正确的尺寸系统将能够与其他人以及门打开和关门快速冷静下来。
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Replies
D.L.,
这里有几个问题。
首先，您写道，您“注意到一些投标具有超大HVAC系统”。您可能是对的，但是您的评论提出了一个问题：“您怎么知道？”您是否委托并为能源评估者或能源顾问付费，以执行准确的加热负荷和冷却负荷计算（使用手动J）？
If you have, good for you.
如果没有，也许您告诉我们的是，一些承包商估计您的房屋的供暖或冷却负荷比其他承包商更高。
如果这些估计值有所不同，通常可以安全地假设估计高加热或冷却负荷的承包商正在超大设备，并且估计较低加热或冷却负荷的承包商更接近真相。因此，从理论上讲，您可以将较低的估计值用作设计指南。但这是设计加热和冷却系统的一种非常粗糙的方法。即使是估计较低的承包商也可以超大设备。
So get a Manual J done by someone you trust -- and tell the consultant that you are planning to build a house that has a very low rate of air leakage and that will have above-code levels of insulation (if that's the case). Whoever does the Manual J calculation has to measure your windows and has to know your window specs and window orientations for each room, and has to know your actual insulation specs. You can't guess.
第二：您是对的，两速设备比单速设备更宽容了尺寸错误。
第三：您的假设之一是错误的。您写道，您的“ ERV ...也会使房屋脱落，即使AC单元运行不够长。”实际上，当天气潮湿时，运行ERV会为您的房屋增添湿度。ERV不是除湿机。运行ERV的时间越长，您将添加到家中的湿度就越多。关于ERV，您可以说的最好的事情是，它不会像HRV那样快地增加湿度。有关此问题的更多信息，请参阅HRV or ERV?
Hi Martin,
1.出于某种原因，我以为我在某个地方读到能量之星房屋的经验法则为1500-2000SF/吨，但是当我回去查找经验法则时，它是1000SF/吨，我猜是5吨的单元不像我想的那么大。
3. I made a mistake in my posting, I meant to say the system includes an ERV and stand alone Dehumidifier that should dehumidify the home even if the AC unit doesn't run long enough. I am kind of new to all of this so I appreciate the hrv or erv link as it helped me understand how those two systems would add to the humidity when there is hot and humid weather.
I guess what I am trying to understand is what I am losing by going with the non-green hvac vendors vs the green hvac vendor. From what I see the green hvac vendor is going to include the proper calculations and proper sizing of hvac and ducting system, does that typically translate to a higher cost? I could definitely use this money somewhere else in the home but this is the first energy efficient home I am building so I didn't want to make a mistake and regret it later.
Do a quick-and-dirty calculation (15 mins) and find out for yourself:http://www.loadcalc.net
如果您获得的数字类似于“非绿色”研究员提供的数字，那么这将有助于您的决策。但是，如果非绿色的家伙会在阁楼上放置管道，那就是一个不开始的人。不要犯这个错误。
D.L.,
在设计供暖和冷却系统时，我不确定什么是“绿色”承包商或“非绿色”承包商。
承包商犯的最大错误通常是无法进行准确的房间加热和冷却负载计算。在这方面，好承包商和坏承包商之间的区别与“绿色”无关。您想要的是一个具有计算器和计算机和锋利铅笔的聪明人。极客很好 - 您不需要环保主义者，只有受过教育和准确的人。
HVAC承包商犯的第二个最常见的错误是设计一个不良的管道系统 - 要么将其定位在错误的位置（建筑物信封外），使管道散布，无法指定使用乳胶的使用来密封管道接缝，或无法将管道设计基于手动D方法（这取决于手动J进行的准确的房间计算）。“绿色”承包商（无论那意味着什么）可能比该部门的非绿色承包商更好 - 但是，您再次希望一个聪明的极客而不是浪漫的环保主义者来执行计算和设计。
一位从幻灯片规则开始的老式工程师，并且对拯救地球没有兴趣，这可能是您最好的选择。