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Heating system advice for zone 7 new build

Toyoland66 | Posted in Mechanicals on

I am building a home in Grand County Colorado, climate zone 7 that will be used as a vacation/ second home for my family and as a rental property when we are not there. It is still in design but the plan is around 3,000 SF on 2 stories plus around 1,500 SF basement, R35 walls/ R75 roof/ triple glazed windows. The property does not have natural gas service but does have electrical service and we plan put in a propane tank.

I am researching different options for heating and DHW, cooling will not be required.

I want to have some heat in the attached garage to keep things from freezing in the winter, there will also be a space over the garage for a future in law suite. I also want to have underfloor heating in the bathrooms.

One option I have looked at is a propane multi stage forced air furnace and using electric radiant floors for the bathrooms, figure out another way to heat the garage and a solution for DHW. Another option would be to use the forced air furnace with either a hydronic coil or a burner, and have a boiler of some kind to heat the garage, basement, and bathroom floors. This seems like it could be costly as it would require more equipment.

For these reasons I have been leaning toward a hydronic heating system, specifically underfloor radiant in the framed areas of the house, and in slab radiant in the basement and garage. The system that really has me interested is the Rotex (Daikin) HPU hybrid heat pump boiler, it’s a boiler that uses an air source heat pump down to a certain temp, then goes into hybrid mode supplementing with gas until going into gas only mode in really low temps. It can also provide DHW with a sidearm type water to water coil. One concern I have with using a heat pump is how loud it may be.

I know a lot of people on this site recommend mini splits, I don’t know how viable an option these are for this location based on how cold it gets and for how long. I have also read the articles warning against in floor heating in super insulated houses, but zoned radiant heat seems to me like an efficient way of delivering heat to where it needs to be with minimal losses.

What am I missing?

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  1. Toyoland66 | | #1

    I forgot to add that it will be air tight construction (<1.5 ach) and there will be a ERV installed.

  2. GBA Editor
    Martin Holladay | | #2

    There are a lot of ways to heat a home, and you seem to already have your preferences. Lots of different approaches can work.

    My own advice is to invest heavily in the envelope -- it sounds like you will be doing a pretty good job there -- and to keep the heating equipment as simple as possible. Heat pump plus boiler plus hydronic distribution sounds unnecessarily complicated to me -- but it's your house.

  3. BillDietze | | #3

    As far a mini-splits go, the altitude is important. Say you're in Winter Park, Colorado. The elevation is 9,000' and the air is 30% less dense - so you need to oversize the equipment by 30% just for that. The Winter Park design temperature is -16F so you'll be within the cold-climate mini split operating range most of the time. Is that enough safety margin? I'd be curious to find out...

  4. Toyoland66 | | #4

    It is in Grand Lake, 8,600' according to google earth.


    I am under the impression that the hybrid boiler is a single indoor unit, and a outdoor unit. However, it may be unnecessary complication, I could also look at a standard LPG boiler and forgo the heat pump aspect.

  5. Expert Member
    Dana Dorsett | | #5

    Can you even GET the Rotex units in the US?

    If yes, how far is it from Grand Lake, Colorado to the nearest Rotex distributor or other technical support?

    Before becoming enamored of any heating solution, it's important to carefully calculate the room-by-room heat loads of the place, preferably at the 99th percentile temperature bin, not colder. (In a super insulated house the 95th percentile bin might be a better choice.) Only then are you likely to find the most viable & reasonable solutions.

    Cold-climate mini-splits with electric boiler backup is probably going to be both cheaper & greener than than a Rotex hybrid, but without knowing the actual layout & loads that's just a WAG. Even the smallest propane furnaces & boilers are likely to be oversized for this house, if those R-value numbers are "whole assembly" numbers with thermal bridging factored in.

    The Daikin Altherma hydronic air source heat pump has a rated output down to -20C/-4F, but even with low-temp radiant floors it's capacity is falling off fast at that temp- faster than a Mitsubishi FH or Fujitsu RLS3H wall-blob type mini-split. It may or may not be a suitable solution here, but at least it's available and supported in the US. It's a lot more expensive than cold climate mini-splits too.

  6. user-2890856 | | #6

    Chris ,
    You could very well use fan assisted hydronic fan coils . These could easily be supplied with heated water from the water heater . Some are fitted with very low CFM fans .

    Radiant ceilings work very well in Colorado , The RPAs Executive director has 2 homes in Co , one at elevation also . Has radiant ceilings and the floors stay a toasty 69* average .

    I Would be interested in seeing some of the room by room numbers as would Dana , and ACH . Some more precise recommendations could then be made .

    The Rotex looks impressive , i will look into it further as an alternative to ugly boxes on walls and outdoor units that may leave many wishing they had chosen something else . thing is now though that you don't really need an expensive boiler that short cycles and is expensive with complicated controls . The stored mass of a water heater has just been sitting there all this time begging to be put to use . Building requirements dependent you could heat a home where you live for 3 hours at design before firing the unit and using any fuel . Standby losses are now beneficial also with the homes being built the way they are when folks like yourself come here and learn how .

    How sunny is it in your location and specifically at your home location and will the home be situated to take advantage of the solar gain ? Zoned properly as you mentioned also is a game changer . you can also make sure you have the power to heat the place on those 99th percentile days without oversizing anything , what you don't need does not get used and with outdoor reset you will only use the fuel you need , never more .

    Having enough DHW at all times is also a pleasure . You are on the right track . Just make sure you find a competent designer , one who can vet your local talent also for the ability to install it properly . Try the RPA website or to locate these folks . You will not be disappointed .

  7. Toyoland66 | | #7

    Very good points, we are still too early in design to perform that analysis but we will be doing energy modeling to determine what is required. I don't know what the availability of the Rotex/ Daikin equipment is in Co so it may be a non option. Those are assembly r values, using cavity and outstation in the walls and thick ceilings.

    I like the idea of using a tanked water heater, I know the efficiency goes up when there is a bigger delta between supply and return temps so running the water in circulation until it hits a lower temp sounds like a good idea. I wouldn't want to mix the hydronic water with the domestic water so I would need to find a heater that keeps the two separate.

    It is sunny on our lot, the plan is to orient the building with the long side facing south-east which should allow good morning solar gain while reducing afternoon gain. This is also the direction of the lake view so we probably won't deviate from this alignment.

    I need to research fuel and utility rates as well, I am hesitant to use anything electric resistance for heating because of how inefficient it is, but I know that propane is also an expensive fuel source.

  8. Reid Baldwin | | #8

    The main issue I have heard regarding radiant floor heating in highly insulated homes is that you don't get one of the main features that make people like radiant floors - that toasty floor feeling. The floor doesn't need to be very warm to radiate enough heat. You could remedy this particular complaint by installing it in only a few strategic rooms, such as bathrooms with tile floors where you most value the warm floor feeling. Let the heat get to the other rooms by air circulation. Temperature distribution is much less of an issue in a well insulated home because the heat is not escaping nearly as fast at the perimeter.

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