Replace one of two furnaces with radiant heating - ventilation??

DevinCortno

Hey guys, brand new here - I tried a lot of searches and can't find anything that seems to cover what exactly I'm wondering about ... so here goes.

short version: if I replace one of two furnaces with in-floor radiant and remove all of its ductwork, will the other furnace be able to handle air replacement & ventilation for the whole home

long version: I've got two furnaces in my house, Canada climate zone 7A - very cold in winter, pretty hot in summer, and extremely dry. These are both HE furnaces. House is sealed up pretty good, excellent for its age.

Basement is currently undeveloped but we're in the early stages of getting it done. With the two furnaces, it has a TON of ductwork in the basement, almost all of which is running perpendicular to the joists and hanging down quite a ways. Headroom throughout half of the basement, once soffits are built and drywalled, will be below code for livable space.

Since the furnaces are old and the ductwork is a problem, it has me thinking of replacing the larger of the two furnaces, which services the basement and main floors (approx. 2,200 square feet combined), with a boiler and doing radiant in-floor heating for the main floor and basement. Staple-up retrofit on the main, something like Heat-Sheet R4 panels for the basement... and I'd rip out all of its associated ductwork to get our headroom back.

This leaves me with the issue of how to get fresh air into those basement spaces. Been reading up on ERVs and the like, but since I will still have one furnace in the home (servicing upper level with heat in winter, AC in summer), would that not be enough on its own? Would just passively venting the basement to the main floor, which has a large great room with vaulted ceilings open to the top floor, be enough to change the air down there, or will I have to force some air into it?

I am not opposed to the idea of having active intake/exhaust on each floor but all ductwork for it would have to run with the joist instead of perpendicular so it doesn't impinge on our headroom.

Floorplans for both the basement and the main are very open - only enclosed spaces in the basement would be an office, a bedroom, and a bathroom which will have its own fan. Only enclosed spaces on the main are a bedroom and a bathroom. Basement is below grade.

Jamie Hall

Let's assume at least for the moment that you have determined that the heating capacity of the remaining furnace is adequate for the spaces which it is intended to heat. That's sort of step one here.

If so, then it is quite likely that yes, it will have enough air flow capacity. There are two factors involved here. One is to get adequate fresh air in from outside. The tighter the house is, the harder it is to do -- and the more important. One to two air changes per hour is a common recommendation for a residence. The best way to do that is with a heat recovery ventilator -- and HRV. Energy recovery ventilators (ERV) do recover more of your heat -- but they also recover a substantial fraction of indoor air contaminants. Maybe not quite what you want.

The second factor is getting that ir to flow more or less evenly in the building. Oddly the problems with evenness almost always come from inadequate return air register and duct capacity, rather than supply air register and duct capacity. In your situation, I'd at least consider some supply air in the basement, but most of it upstairs -- but generous return air capacity in the basement. The HRV could be coupled in in he basement.

DevinCortno

Jamie Hall said:

Let's assume at least for the moment that you have determined that the heating capacity of the remaining furnace is adequate for the spaces which it is intended to heat. That's sort of step one here.

If so, then it is quite likely that yes, it will have enough air flow capacity. There are two factors involved here. One is to get adequate fresh air in from outside. The tighter the house is, the harder it is to do -- and the more important. One to two air changes per hour is a common recommendation for a residence. The best way to do that is with a heat recovery ventilator -- and HRV. Energy recovery ventilators (ERV) do recover more of your heat -- but they also recover a substantial fraction of indoor air contaminants. Maybe not quite what you want.

The second factor is getting that ir to flow more or less evenly in the building. Oddly the problems with evenness almost always come from inadequate return air register and duct capacity, rather than supply air register and duct capacity. In your situation, I'd at least consider some supply air in the basement, but most of it upstairs -- but generous return air capacity in the basement. The HRV could be coupled in in he basement.

Yes, the existing furnace handles the upstairs very well. We had zero comfort issues up there last winter through a -30 F cold snap that stretched on for a few weeks. Excellent room to room balance, too, really happy with it compared our last house.

I did finally dig up the relevant codes for ventilation in my area. Per code I only need 90cfm per 1,000 square feet. A single ERV per level, with the existing furnace taking care of the upstairs (heat/cool/humidify in winter). Seems fairly straightforward.

Mad Dog_2

You're making  a very wise decision.  I've lived in homes with furnaces, hot water hydronics, Radiant and Steam (Currently).
Nolo Contendre! Jamie knows what he speaks of.  Mad Dog 🐕 

DevinCortno

I did a ton more reading, found all kinds of fun retrofit panels, none of which are available in Canada. I would have to DIY the system instead but doesn't seem too complicated. Staple up on main floor, well understood and straightforward.

Basement I would lay down R4 rigid insulation, cut 3/4 plywood into sleepers, and lay aluminum heat transfer plates into the gaps between them. I could cut the rounds on the end with a circle jig & router for the end loops. My cost per sqft would be about $4.15 CAD for material, the in-floor system only, assuming my labour is worthless.

Jury is still out on boiler, manifold, control, etc. my jurisdiction requires engineering stamps so I will have to get somebody to design that anyway.

By code my ventilation requirement is actually a lot lower than I thought, only 100cfm total for the two floors... very easy to handle that. The upstairs furnace obviously moves far more air than that so I would have more than enough ventilation to change the air in the house many times over.

hot_rod

Will you fasten the foam and plywood down  to the concrete? Adhesive and fasteners into the concrete.

DevinCortno

Yup, of course. I did finally find a place to get Roth panels from.. the convenience of the install with those would be nice and they have the full aluminum panel on top. We'll see what the cost looks like

DevinCortno

I knew having CAD drawings of the house would come in handy

plan had to change, it's simply not feasible in this climate to do only radiant heating on the main floor. So, this is with a radiant-only basement with 50cfm ERV, with staple up radiant supplementing the great room with its big vaulted ceiling which is not adequately heated by the existing setup. One furnace will go to the scrapheap along with its plenums, then the remaining furnace will be tasked with forced air heat for the main floor & upstairs. That could be nice because in the future, we may want to cool that zone.

This is with R 1.4 flooring (light carpet & pad) in the basement. I do have a heating shortfall there no matter how high I set water temp but I think I actually prefer it that way. Computer & home theater setup will more than make up the difference.

With such a low radiant heat load and water temps in the 110-120 degree range, I think I'm well outside the range where I would need a boiler, and could run this off a hot water tank? It would be a separate tank dedicated to the system. As I understand it, this would also simplify my control system quite a bit and save quite a bit of cost there too.

I have seen some "combi-boiler" hot-water-on-demand systems that apparently are built for things just like this but I wasn't at all a fan of the Rinnai tankless system in my last house, took forever to get hot water and the noise was obnoxious.

I am at the point where I have to hand this off to professionals (need stamped drawings etc.) and am just educating myself to handle that negotiation and push back on exorbitant equipment specs, pricing etc.