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HVAC options

ConnecticutTony | Posted in Energy, Heating & Insulation on December 11, 2020 12:34am

I understand there are 100 ways to skin a cat so to speak. 

We are in the process of remodeling (not full gut) a 1978 log home. Think lincohln logs. No chinking. We have a central AC (4 ton) unit with a single zone. The house is 3500 square feet. The heat is a small part infloor radiant and the rest tube and fin hydronic. The heat and hot water are from an oil boiler. This is a 2006 Burnham unit. 

There are significant hot and cold parts of the house. There is no AC in the basement.

I’ve had 2 contractors out. So far, one wants to put in a several minisplit units in the poorly served areas (mater BR, entry, and basement). This will also handle heat in this area as well. The second contractor said to place zoning units due to 14″ spiral ducting trunk lines and make the house 4 zones. Replace fin tube in the mBR. Add minisplit in the basement. The only similar line item was minisplit units in the basement. 

Both quotes have been ~20-25k. I’m going to have another contractor out to get another opinion.

This may be difficult to visualize, but hope to get some general feedback.

Thanks!

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  1. catmandeux | Dec 11, 2020 08:42am | #1

    If you don't have one already, I suggest you start with a HVAC load calculation.
    That will tell you how much heating / cooling you need, and indicate on how to balance the A/C system.
    If balancing does not work due to varying heat gain in different parts of the house, then look at zoning.
    For the heating, how many zones are there now?

    1. User avater
      tjh1 | Dec 11, 2020 12:32pm | #2

      Just went through this process myself. Although every authority insists that a manual J calculation should be made by any proposing HVAC contractor, good luck finding one that will. They all love rule of thumb and what do you have and did it work adequately.

      I searched in vain for someone to do the manual J calc and did find some "energy auditors" that would do so......for almost a grand! And, they all seem to be participating in various energy improvement programs and want to push all electric, net zero goals with multiple mini-splits installed around the perimeter of the house. Which I didn't want because there was no easy way to get power to all the locations for the outside units without serious electrical service upgrades and I really didn't want conduit running across all the walls on the outside of my home like some industrial building. To say nothing of the interior wall units in each room looking like some cheap motel.

      I did find a very highly rated and very busy energy consulting contractor that I actually booked a consult with and then cancelled when a principle told me in a pre-consult phone conversation that they only looked at what the building science indicated and that cost was not a consideration. They do all sorts of split A/C installs, crawl space sealing and insulation upgrades. Money doesn't matter is a sure way to defeat any home improvement project.

      After reviewing what my neighbors had and how it worked as far as HVAC and solar PV systems, and taking a hard look at natural gas prices per therm vs electric equivalent (@39.3 kWh/therm) and the price of electricity here in PG&E land, I realized that there would be no payback in my lifetime for using the all electric, net zero approach. Even at the highest rates I pay (we're tiered) gas is less than 1/4 the cost of electricity in therms equivalent and, even a 10 kW, expensive and expansive solar PV system on my roof with a 20 year life wasn't cost effective.

      One contractor quoted me $93,000 for such a system. Incorrectly "allowing" for tax and incentive credits to bring the total price down to "only" $59,000 for two high efficiency heat pumps, two 80% AFUE hybrid heaters, a 10kW solar PV system, a new heat pump water heater, all new ductwork in crawl space and and electric panel upgrade.

      My next door neighbor has the two heat pump hybrid central HVAC system and a 9kW solar array and says he still has to pay about $1,800/year to PG&E for true up each year because he doesn't generate enough electricity to offset his use. And, that's switching the to natural gas heat in winter because the heat pumps drive his electricity use through the roof when used for heat. And, that is in our very mild climate in Vallejo, CA.

      I realized that the 4 ton and 3.5 ton central air units with 80% AFUE natural gas furnaces we inherited with the house, had proved adequate for the house for 30 years and went with virtually the same only higher SEER A/C for $35,000 and no tax credits or rebates. It is being installed in early January. I may also go with a more reasonable, 5 kW solar PV system that should provide enough electricity annually to offset the A/C compressor use and provide a 5 year payback.

      Bottom line: You probably won't be able to find an HVAC contractor who will do the Manual J calcs. Don't even bother with Manual D for ductwork sizing as most HVAC contractors I've encountered are just going to go with whatever ducts they can fit in an existing built situation.

      It's wise to look at what your neighbors in your climate zone are using, how it is working out for them and what their electric and/or gas consumption looks like as a reference point. A good HVAC contractor can figure out a system that will make up for your systems current shortcomings without breaking the bank.

      Building science and going green is wonderful. But, when it comes to existing buildings and replacements and retrofitting, many of the modern "solutions' are just too damn expensive. To the point of unreasonableness. It makes more sense to go with what works and is cost effective without major rework or expense. Eventually, the building and it's various systems will be come to the end of their life and it will be more cost effective to replace with the latest and greatest mechanicals, insulation, windows, etc.

      1. catmandeux | Dec 11, 2020 04:29pm | #3

        Without a detailed HVAC load calculation, everything is just a guess.

        Room by room calculations are required to determine interior ducting / balancing information for forced air systems, and radiator / radiant panel sizes for hydronic systems.

        The calculations are easy enough to do, but can be tedious to take all the measurements, determine the characteristics of an existing building and enter the data into programs or worksheets. Hence the high cost to get one done by a consultant.

        An interested homeowner should be able to do one using an online calculator, and get usable results. Having a heat loss/gain value for every room will also show where any renovation budget can be best used.

        Manual J online calculator:
        https://www.loadcalc.net/

        Slant Fin heat only calculator: http://www.hydronicexplorer.com/HeatLossCalculator

    2. ConnecticutTony | Dec 11, 2020 07:54pm | #4

      Thanks for the replies!

      I have no calculations. Just the rough estimates. (back of napkin--I know not ideal).

      I would like to use existing if able, and one contractor says yes and other no. Currently, AC has 1 zone. Main level thermostat. Freezing cold in upstairs rooms, warm main level, and hot at the far ends of the house. No AC in the finished partially below grade walkoutbasement. Frequent short cycling. Heat is 4 zones, upper level, main level, and basement all fin tube base from boiler. Kitchen/mudroom in floor radiant on own stat.

      The house is located in PA with newer double pane andersons, and moderately poor insulation throughout house.

      Only way to improve insulation is with giant buckets of money.

  2. catmandeux | Dec 12, 2020 11:26am | #5

    Suggest you have the HVAC load calculations done. There is a lot of glass in that one wall, and that will have a major influence in heating / cooling. The calculations will let you know if the existing system is sized correctly and distribution needs to be fixed, or if additional capacity is required.

    The AC has a serious distribution / balancing problem. I suspect the short cycling is due to the thermostat location, probably being cooled by a draft from the colder zone.
    As a starting point, the system needs to be balanced by adjusting the dampers.

    If you are up for balancing the system yourself:
    Fully open the dampers in the ducts to the hotter zones, and start closing down the dampers to the cold bedrooms. Measure the flow rate using a draft hood and velometer.
    Ideally you would balance the flows for each room in proportion to heat gain values from the HVAC calculations. If that is not available, the surface area of the outside walls/ceiling is a good starting point.
    You can make your own draft hood from cardboard, and any velometer will do.
    A guide to trouble shooting: https://www.achrnews.com/articles/85446-troubleshooting-ductwork-is-easier-than-you-think

    For the fin tube base, measure the length of the finned section in each room, and compare that to the heat loss in that room. That ratio should be similar in each room. For single tubes, heat output is about 600 btu/h per foot of fin tube. If any room is short of heat, install additional fin tube. If any room is excessively long, blank off the excess length with a sheet metal cap over the fins.

    What is the construction of the radiant floor? If slab, not much can be done. If PEX between joists, the distribution can be adjusted by additional PEX loops / heat plates in trouble spots. Adding some fin tube near the door is usually an easy fix if that is the problem area.

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