Home hydronic system considering new controller and perhaps buffer tank (opinions, please!)

dstapper · February 2021

My 3 story home (new to me since a year ago) is about 2500 sq ft, no garage, cement slab in basement. In Quebec Canada, so I need to design heating system for -15C (5 f). Hydronic system is presently powered by single electric boiler @ 61K BTU ("Thermo2000" unit is about 15 years old). System is seriously burning through electricity.. (I believe due to poor system design). Here are some of the current parameters. (1.) It is running through 2 old-ish watts stainless manifolds that are attached in parallel; (2.) currently 11 loops operating; two into cement slab in basement; three go to various cast iron radiators in the house; the rest go to infloor (1/2 inch pex in-joist but not a great install; no heat trasfer plates..) (3.) house is 13 years old and is not insulated that well and has LOTS of windows. I did a heat loss for -15C (5 f) and got 120K BTU (around 35-40BTU/sq ft). (4.) Main problem is that entire house is setup as a single zone with a single thermostat on for air temp on the main floor. So lots of lag in the system. I also have wood fireplace but only use it from time to time and its on main floor so only confuses the thermostat, and gives opportunity for thermal loss from the slab in basement while it cools off..

So I have been doing research and have some ideas, but looking for advice /sound-boarding here. I wonder if:

Option A: perhaps, is refining system by splitting into zones, so that the slab (especially) is treated separately from the rest. I understand that slab ideally is heated by lower temp water, faster flow; while other zone(s) (low mass loops!) need hotter water and slower flow. So Q: how to do this most efficiently? Is this as simple as buying a controller and some actuators + thermostat and installing these onto 'slab loops' on my existing manifold ? I suspect best will be to buy and plumb a separate manifold for the slab, and this should be quite simple to accomodate. It will have its own pump.

Option B, perhaps (as my heat loss seems to indicate) I need more BTUs and I should use current boiler to run only infloor and perhaps the radiators; but get a second appropriately sized (not oversized) boiler to run the basement slab. This will address the difference (gap) between my 61K BTU boiler size and the fact that my heat loss calcs seems to suggest I need closer to double this..

Option C. Q: will installing a buffer tank help me in Option A or Option B ?

Mosherd1 · February 2021

Is your home currently able to stay comfortable with the boiler you have now, or when it gets down to design temp does it run non-stop and the house still continues to get cooler? If your house is able to maintain temp at design temperature now your boiler is at least big enough.

SuperTech · February 2021

A buffer tank is usually only necessary when the boiler is oversized. This helps eliminate issues related to short cycling.
My initial thought is that an electric boiler seems crazy given what I pay for electricity compared to fuel oil, but I understand pricing varies by region.
The best thing you can do is tighten up the building envelope. Add insulation and the transfer plates if possible.

dstapper · February 2021

@Mosherd1 it stays on basically full time and struggles to keep warm.. but hard to know how much is due to boiler being undersize vs terrible system design, which need reworking.

dstapper · February 2021

@SuperTech here in Quebec electricity is among (or is) cheapest rates in North America..

Jon_blaney · February 2021

It is hard to understand you home lay out. How many floors? What is the livable space? Some outside pictures may help.

dstapper · February 2021

@Jon_blaney 32 x 32 ft footprint, above ground basement has 11 ft ceiling; cement slab; is basically built right into bed rock on upslope side. Mainfloor has bath, major large kitchen living room open space, w/21 ft catherdral ceiling; 2 bedrooms, huge windows; Upper floor has bath, master bedroom 12 x 32 or so.

Zman · February 2021

When you run high temp water into a big slab, it sucks up the BTU's and pulls all of your water temps down. This starves the upper floor and radiators of the higher temps they need in order for the system to be balanced.

Your boiler is probably large enough. Is it multi stage or on/off?

You need to start by performing a heat loss calc on the whole house and an "as built" of the current piping.

Hiring someone to design this for you might not be a bad idea.

dstapper · February 2021

thanks @Zman. My boiler is a T2 Thermo2000 electric boiler, currently running in 'fix set point' mode but it passes through 4 stages. I have already done the heat loss calcs. For me, priority is determining how to split the slab off as a separate zone. (hard to believe its been run for more than 10 years like this!). Some options: (a) add zone values on existing manifold; (b) get a separate small manifold to plumb in, and add separate circulator; (c) simply run the boiler in mode 2 ("primary/secondary") and re-plumb to accommodate (I think this option would also be best served with adding a second circulator. Any thoughts?

Zman · February 2021

I would suggest creating a separate loop and zone for the high mass slab. This loop would have it's own circ and a Taco I-series mixing valve. If you utilize the outdoor reset and anti-condensate features on the I series, you will raise the slab temp slowly and in sync with the actual heat loss. This should save energy by only heating the slab to the temp needed to offset the heat loss. The anti-condensate feature will prevent the slab zone from drawing down the boiler temp and starving your high temp zones.

Home hydronic system considering new controller and perhaps buffer tank (opinions, please!)

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