Conflicting Mechanical Design
I’ve engaged a mechanical contractor to design a ducted heat pump system and an energy consultant to model the energy usage and provide advice on design options.
They are both providing me with some conflicting information. I’m interested in what others think. The house is approximately 3,400 square feet over 2 levels (walk out basement included). We are in Atlantic Canada (Zone 6). Will also have an ERV, high levels of insulation and triple glazed windows.
The mechanical contractor recommended a 4ton LG hyper heat central air source heat pump which had a minimum heating capacity of 19,000btu, rated at 50,000btu and max of 60,000 btu.
The energy consultant modeled our house requiring 30,155 Btu/hr design heat loss and suggested that the unit provided was overkill and we should try and get a unit that had a lower minimum rated heating capacity so in shoulder seasons we weren’t using as much electricity. They suggested something with a minimum of around 10,000btu/hr.
The mechanical contractor then came back with a 2ton LG unit which has a minimum heating capacity of 10,800btu, rated at 27,000btu and max of 30,000 btu. Which puts us under the energy consultants model. Energy consultant seems to think this is fine but the mechanical contractor doesn’t think it’s good enough for the house.
We won’t have any other supplemental heating in the house and I’m concerned about putting the wrong unit in now and we have to change it later.
Cooling capacity figures are essentially the same as the heating numbers, but lots of houses around here don’t have any cooling and it generally isn’t a problem.
Let me know your thoughts.
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Your designer is most likely correct. Energy models tend to be conservative, so slightly under sizing with backup strip heat to make up for the shortfall is definitely an option. In most cases the backup heater never needs to run or if it does it is only for a couple of hours per year.
Take a look at here:
In the top corner there is a box "Advanced Data - Sizing for Heating".
If you click on that it lets you enter your design details and show you how the system will operate including how much heat a backup strip would need to deliver. You can also re-run that for the 3 ton model and see how it compares.
Thanks, this is very helpful. They did propose a 5kw backup heat strip which from what I can tell should be sufficient.
Your energy designer has calculations to backup their opinion. Does your contractor have specific experience where an energy designer's recommendations fell short? Have they done their own calculations, or is their opinion just what they are used to? Also can you contact an LG rep to get their input, or find an LG certified mechanical contractor to give you a second bid/opinion?
All valid questions, thanks for your input. We are bit limited in terms of vendors in my area (small community). Of those I've contacted they all have their own different units. The other quotes were crazy so I discounted them pretty quickly.
The mechanical contractor hasn't done any calculations yet. They said they would do that once I committed the project to them. Unfortunately not many people around here have experience with energy efficient housing, so that's probably why the mechanical contractor has such a conservative viewpoint.
With fuel-burning equipment there was very little penalty for oversizing. A radically oversized forced air system might short-cycle a little but that's about it. The penalty for undersizing is you have to tear it out and replace it. So there tends to be a bias toward oversizing, particularly among people who have been in the business for a while.
I also like the idea of under sizing the heat pump and making up any short fall with an element large enough to carry the house should the heat pump fail.
The first advantage is a smaller HP will have a lower minimum output allowing it to run without cycling at warmer outdoor temps that you are likely to see many hours of each year. Second is a smaller HP will need to operate at its max speed more hours per year making its warmest air it can more often. Like all machines one day it will fail and when the HP goes down most of the time the resistance will still work making the would-be crises in to a minor expense. The reality is the resistance will come on a few hours a year on the coldest nights when the HP is at its least efficient temps. A side benefit is the installers will stop fighting for a larger HP because they feel safe that the resistance will keep the house warm so you will not be asking them to upgrade an under sized unit for free.
What is the design temp used in the manual J calculations?
Are the BTU output numbers you are quoting the number of BTUs you can expect at your design temp?
Thsi phenomenon is experienced everywhere .Are these units modulating full,inverter variable speed blowers? If so your risks are lowered. Do you have 70ndhere days with y0 per cent humidity in spring and summer? If not, short cycling probably not so likely . Is this new construction and tight?
2 tons with backup is superior for cost, comfort, and efficiency. If residential units were made >5 tons, I shudder to think what contractors would try to install.
as long as the manual-j was fed the right info, trust the energy consultant.
what were your blower-door test results?
i have a similar 2-story + basement project with a bit more sf and
the cool-calc.com manual-j site is showing about 3-tons,
but i have some two-story space and high window area.
just out of curiosity, can u share info on wall-assembly, windows, erv and r-values?
So you're faced with two options. One is to trust the guy who calculated the load, and whose job it is to do that. The other is to trust the guy who just pulled the number out of his ass. Unfortunately, it's pretty standard for HVAC contractors to know nothing about energy modelling, and just use the method of pick a big number, then add 50% to it, then round up to the next biggest size unit.