Hydronic system controls

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New Member
Oct 21, 2022
GNU England
I'm planning on revamping my hydronic system, while adding a wood boiler (Heatmaster G7000). Right now it's just a single zone of finned baseboards in two parallel loops, ~20 year old oil burner, bang bang control. Judging by the duty cycle of the burner in the winter, heat loss could be around ~50-70 kBTU/hr (small ranch but bad insulation). I know I'd get a lot out of addressing the insulation, but the subsidies on wood boilers end this year, so I put that first.

Down the line I'm eyeing some additions - zoning the existing baseboards, light storage to reduce oil burner cycling, outdoor temperature "reset", DHW, a heat pump instead of oil burner for secondary heat, adding some radiant zones, significant addition to the house, part-time heated garage, a greenhouse, maybe even another outbuilding. I'm trying to plan what the overall system should look like eventually, so I can head towards that. I've seen a few system designs here, have read much of Siegenthaler's "Modern Hydronic Heating", understand primary-secondary loops, etc.

But there seems to be a gap between the innovative designs using things like variable speed ECM circulators for injection, electronically controlled three way mixing valves, and the lack of components in the market to actually implement that design at the scale of a home.

I was an embedded engineer in a past life, and I admit I'm focused on the control systems. I'm looking for components that will integrate with some sort of central controller where the logic can be changed later, rather than siloed controls like a thermostatic mixing valve or delta-T circulator with their own sensors that won't talk to anything else.

For circulators -

I've found only two that take 2-10VDC control signal - Grundfos 26-99 Alpha1, and Resideo PCVF-ECM2020. The Grundfos seems hard to get and twice as expensive as comparable circulators, and I'm wary of the Resideo because it's not a standard circulator brand. I know Resideo is ultimately owned by Honeywell, but my point remains.

There's also the Taco Viridian 2218, which has thermistor inputs for delta-T so it seems like I could either reverse engineer its input stage or just use a digital potentiometer to supply my own signal.

There's Nofossil's 3-speed-with-relays hack, but I've read 3-speeds are bad with power consumption and its not continuously variable. There's taking a 3-phase circulator or adding a 3-phase motor to a traditional dry rotor circulator, and adding a VFD, but that seems like bespoke overkill and not something I'd want to do for multiple circulators. There's controlling a traditional PSC circulator with a TRIAC chopper, which seems ugly and inefficient. Theoretically one could control a PSC motor with a 2-phase VFD instead of the capacitor, but I've yet to see one off the shelf.

I'm not in love with any of these circulator options! And yes, I know that I ultimately need to choose circulators based on pressure/flow calculations, but that seems difficult when there are so few options!

Also I haven't come across any of these extremely low power low flow "micro" circulators I've heard mentioned for things like zoning with circulators.

For valves -

I've found the Taco EBM for zone valves. Kind of expensive, and non-modulatable. The other zone valves that use power just to stay open seem ridiculous (this isn't the 1980's). Ultimately I'd like to be able to power my system off storage batteries or a small inverter generator, so energy consumption matters.

Mixing valves, I've found the Tekmar line. Those seem like they're upwards of $600 a pop? $200 for the valve and $400 for the actuator? Seriously?

Meanwhile I see generic motorized ball valves (one brand: "US Solid"), which are a basic DC motor heavily geared down. The standard config takes one polarity for one setting, opposite polarity for other setting. Limit switches stop the movement (there's a 5-wire version which reports the limits). It seems like these should be able to modulate by powering them for a partial cycle.
Perhaps not super repeatable (due to variable backpressure on the ball?), but it might not matter as part of a closed loop feedback system. Stainless steel ones are ~$40 or so, in whatever size I want. Longevity / cycle count is unknown, but at that price, I can stock spares, and the electronic feedback means I can get alerted to a valve being broken rather than having to figure it out.

For controls -

I haven't looked too hard, but I've seen the Tekmar line. They all seem so narrowly specific. Where is the general hydronic controller you can hook valves and sensors up to, and then fill in the logic later as your needs adapt?

Alternatively, generic PLCs are available for several hundred at Automation Direct, have built-in Ethernet/MQTT functionality (to tie into further home automation/monitoring), and are very flexible for whatever changes I might need down the line.

But as much as I have the skills to create a bespoke control system from the ground up, that still doesn't mean I want to do so! Especially with future maintenance, if I don't want to have to personally work on it, someone else is going to inherit it some day, etc. But it's hard to ignore that the hydronic market seems quite non-competitive. If my system needs a controller, mixer, two circs, and 6 zone valves, that's like $6000 for this dedicated hydronic gear I've found, or $1200 for the generic controls. I'm not trying to just "cheap out", but I also don't enjoy being taken for a ride!

What systems are being used by everyone here to control their fancy primary/secondary loop plus storage, well-zoned systems? Are there any component options I've missed?
Nice write up ....
Being in this business for quite a while, I can tell you that you are 10-15 years ahead of the curve.
In my opnion the "mismatch" is using an outdoor boiler for this.
The G7000 is a 130,000 BTU/hr boiler with 234 gal of water.
Do a heat load calculation first: based on oil or gas consumption and local Heating Degree Days for the last 3 years.
While I probably understand why you are going the outdoor boiler route, I'm still advising to hook it up to a large thermal storage tank (1,000 gal) inside the house, other wise all the nice stuff you have in mind realy goes out of the window; your boiler sits outside
Or are you putting the wood boiler inside ?
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Well maybe by the time I actually finish all this, the curve will have caught up.

I know the G7000 is very oversized for my current needs. I want to grow into it, and it also has double the water storage of the G4000. I figure I won't load it full, and should be able to reduce the heat output with the damper settings. And if this winter turns out to be a terrible smoldering mess, then I'll just have to build that greenhouse sooner.

I understand the benefits of more storage (130kBTU/hr into 234 gallons is only 35 minutes for a 40 degree rise), and may add it down the line. I'd need to clean out the basement before I could build down there, and I don't have a shed for the boiler yet. I'd probably lean towards keeping the storage outside as well, so that if there were a containment problem the water would be less likely to end up in the basement. Although I still have to figure out the best way of keeping the underground loop from betraying me and draining all the water into the basement anyway.

What does the wood boiler being inside vs outside have to do with anything, apart from piping and exposure? I've seen a few posts on here with a bit of a bias against being outside, but it feels like a matter of keeping the right expectations rather than the actual placement.
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Tremendous heat loss - Pure physics
Assume boiler water is at 180F
Outside is at 32F or 0F
Delta T is 150F or 180F
Waste of wood, waste of labor to cut that wood, ....