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Piecemeal Hydronic Heating Retrofit

Ian Watson | Posted in Mechanicals on

I’m going through the process of upgrading the energy performance of my home and I’m starting to think about my heating system. There’s obviously lots of planning and calculations to run before I take any concrete action but I would appreciate any thoughts/analysis before I go deeper down this path.

Context
Zone 6. I’m currently renovating on a room-by-room basis because I’m living in the house, I’m DIYing much of the renovation work (with assistance from my renovation contractor father), and I don’t have one big bucket of money to drop all at once. As I go I’m taking walls from ~R4 to ~R28 (centre of bay) and putting in a proper air barrier. The roof is uninsulated and will eventually be a big OUTsulation retrofit once the current shingles need replacing (probably 5+ years).

Current System:
– Oil-fired hydronic heating (cast-iron radiators)
– No AC
– One zone

Goals:
– Zoned (probably 6 zones)
– AC, at least in the bedrooms
– Get away from fossil fuels in the next few years

Challenges
– The phased approach to renovating my home necessitates a phased approach to the heating system. For example, many of my current radiators are in the way of my INsulation, but I don’t want to move/replace them until I’m working on that particular room. This obviously creates a problem where the design load of the system will change over time as I renovate each room, and will also necessitate rebalancing of the system as new zones get added, etc.
– The layout of my house is absolutely terrible for mini-split heat pumps.

Other Considerations
I like the cast iron radiators, and would ideally keep them in rooms where I don’t need cooling. The oil furnace is also not that old and is running very well.

Plans
– Install a buffer tank heated by my current oil furnace.
– Install a manifold.
– Every time I renovate a room, swap the room onto its own zone on the manifold.
– In rooms where I don’t need AC, reuse the cast-iron radiator if demand calcs show they can still provide enough heat with low source temperature (accounting for the fact that room insulation and air sealing is going way up). If demand calcs show they won’t provide enough heat at low source temps, switch to new panel radiators.
– In rooms where I do need AC, replace the cast-iron radiator with a hydronic fan coil unit.
– When my oil furnace gets old, swap it out with an air-to-water heat pump and rebalance everything for low source temperature.

Questions
– Am I crazy?
– Would this approach have sufficient flexibility to allow a gradual shift of my system?
– Any specific pitfalls, especially in terms of balancing, design load, etc.?

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Replies

  1. William Hullsiek | | #1

    I am in zone 6 ( Minnesota) and shifted to low temperature water back in 2016, i heat my system through my water heater using heat exchanger, but my system is ready for a air to water heat pump. For transition purposes you may to put an injection mixer before the buffer tank to bring the temp below 120F.

  2. Mike Ferro | | #2

    Building out your chilled water hydronic system piecemeal can work as long as plan properly for it. Some thoughts and caveats for you:

    You'll find that that iron radiators and boilers tend to leach that iron and crud into your pipes. For boilers that have fairly open iron heat exchangers this crud doesn't cause much of an issue. However, when you swap to an Air-to-Water heat pump that use a plate or concentric heat exchanger where the orifices are much smaller there is a chance that the crud will plug the openings. You can flush systems with powerful cleaners (fernox is a great option), but nothing beats a system that is void of iron components. It may be worth completely separating your new buffer tank and hydronic system if you can. If that's not an option, you might think about installing a magnetic separator. They're cheap insurance.

    The second issue to consider is that hydronic design is fairly nuanced, particularly when you're micro-zoning. Careful planning of your controls and pumps will be critical to getting the right flow at each emitter. Consider a constant pressure pump that will modulate flow based on how many zones are calling.

    The third issue worth considering is that if you're planning to ever run chilled water through your pipes you'll need to make sure you are able to well insulate ever inch of them (this includes pumps, zone valves, and manifolds). Any uninsulated pipe will sweat. Chilled water fan coils require similar piping and electrics as a mini-split (insulated supply/return, drain pipe, 120v power) and I'm wondering if you can't accommodate mini splits how you'll manage to work these fan coils into your design? Perhaps just the workability of pex versus refrigerant line sets will help you here.

    1. Ian Watson | | #4

      With a last name like that I half think you're in the business of selling magnetic separators ;)

      In seriousness though, all very good points! Thank you. Yeah, the iron will be an issue. I hope to eventually replace all of the piping with PEX, but I'd still have iron rads and also of course there would be a transition period with iron pipes. I will look into separators.

      The issue with mini-splits in my home is less to do with piping runs, and more the sheer number of heads you would need to evenly condition the space. But yeah, I can see insulation being a task and a half.

  3. Charlie Sullivan | | #3

    I think this is a great idea!

    Mike Ferro's suggestions of a magnetic separator and a insulating all the pipes well are great points. A dirt separator is good to have anyway and the magnetic ones aren't much more expensive. Cleaners for the hydronic loop are also a good idea, followed by a conditioner that inhibits scale and corrosion. On insulation, in theory you could insulate only the ones you are planning to run chilled water in, but I would insulate all of them so as to allow the flexibility of using different ones for chilling in the future, and also get better control over zoning--delivering heat just at the emitter not along the whole run.

    On the PEX runs, I recommend avoiding elbows and just bending the PEX in swooping large-radius curves. Some of the runs in my house were installed by a plumber who really liked to make things behind the walls look tidy with lots of right-angle elbows and straight PEX runs, and those runs require higher pump pressure than the rest. Just because of those few runs, I need more pumping power for my whole system, with the other runs throttled back.

    1. Ian Watson | | #5

      Good suggestion on the insulation all the whole system. I guess if I'm going to do it I might as well DO it. Especially because, as you suggest, flexibility is part of the big goal in this overhaul.

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