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Is hydronic heat worth installing in an old house?

I have a client who is convinced that replacing their 80% gas-forced-air furnace with a hydronic system will improve their comfort. The plan is to install a gas-fired boiler to serve both the heat and domestic hot water needs, and staple PEX below the floor in the joist bays. We've discussed the options of a more efficient furnace, or a duct-less mini-split, but byronic is still their preference.
There is not, unfortunately, any enthusiasm for insulating or air-sealing this old, drafty house. Can their goal - increased comfort - be achieved by changing to a hydronic system, without any improvements to the envelope, or is this a waste of money?

Asked by Lydia Marshall
Posted Wed, 02/19/2014 - 11:15


7 Answers

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Comfort is a fairly subjective category.

Obviously, a well designed, well installed forced air system will be more comfortable than a poorly designed, poorly installed hydronic system.

Either system can be comfortable.

If the hydronic system will cost more than a forced-air system, I would strongly urge the client to invest the money in air sealing efforts and improved insulation.

By the way, I like your typo. We are all in search of a Byronic system -- romantic, dashing, poetic, and willing to swim the Hellespont.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 02/19/2014 - 12:23
Edited Wed, 02/19/2014 - 12:25.

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Thank you…and let's pretend the typo was intentional.

Answered by Lydia Marshall
Posted Wed, 02/19/2014 - 13:05

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Radiant floors ARE more comfortable than ducted hot air, but in a not-so insulated house in a cold climate a cheap staple-up requires water temperatures too high to take advantage of a condensing boiler, and "cheap" is only in comparison to extruded aluminum heat spreaders for the radiant.

An intermediate expense choice that would still be a big comfort uptick is to go with low-temp hydronic panel radiators (eg: ), which, if sized correctly WOULD be able to deliver condensing efficiency, spending the difference in cost between that and a radiant floor on air-sealing & insulation.

All optimal heating designs start with a room-by-room heat load calculation. It's possible, (even likely) that staple-up PEX would be unable to meet the heat load at the 99% outside design temperature even at the rated 180F max operating temp of the PEX. If one designs the radiation (floor, or panels) to meet the 99% load with 140F average water temps (or lower) it's possible to hit the mid-90s on efficiency, using an optimally sized modulating-condensing boiler (or even a condensing hot water heater.) It's also possible to set it up as a 2-stage system with both low-temp panels as the second stage, and staple-up PEX as the primary stage and still come in at less cash outlay than an extruded heat-spreader solution (that also might not make it on design day.)

There are many ways to screw it up though- micro-zoning with low mass radiation can lead to serious short-cycling of the boiler, cutting into both efficiency & longevity.

As a retrofit it's pretty easy to run north of $30K for radiant floors that really deliver, but in an "old drafty house" spending the first $10K on fixing the air leakage and spot insulation will result in less stringent specifications on the radiation, and a smaller boiler. It may be "worth it" to install radiant floor in the few rooms that they really care about, and low temp panel radiators sized to run at the same water temp as the floors for the rest. But any way you cut it it's probably not going to come in under $20K without a lot of DIY labor, compared to $4-5K for a right-sized condensing furnace, with maybe another $1-2K in duct sealing & duct insulation, assuming the duct design isn't a total disaster that needs to be ripped out.

Answered by Dana Dorsett
Posted Wed, 02/19/2014 - 17:07

Helpful? 0

Air-sealing question: Assuming that forced-air delivery creates positive pressure within the envelope, we'd be pushing the conditioned air through a leaky envelope. If the space conditioning is performed hydronically, i.e. without creating the positive pressure, is the envelope's integrity as much of a concern? I can picture stack effect creating a negative internal pressure, but the ceiling plane is tight, so I can't picture how we'd be bringing unconditioned outside air into the house.

Answered by Lydia Marshall
Posted Thu, 02/20/2014 - 15:50

Helpful? 0

A well designed forced air system with sealed ductwork will not create positive pressure in your home with respect to the outdoors.

A forced air system is a closed loop. The furnace pulls indoor air from the return-air grilles (perhaps in your living room and hallway), heats the air, and then delivers it to the supply air registers in your living room, kitchen, and bedrooms. Installed correctly, it will neither pressurize nor depressurize your home.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 02/20/2014 - 16:00

Helpful? 0

Forced hot air inevitably creates some room-to-room pressure differentials, but that can be mitigated substantially by air-sealing the ducts (both supplies & returns), taping the seams of the air handler, and providing sufficient return paths from rooms with supplies that are potentially doored-off from a dedicated return duct. This can be achieved with jump ducts, sometimes with door-cuts, etc. Any impedances in the return paths can create an infiltration drive resulting in air-handler driven house leakage. But even the best balanced and perfectly sealed duct system will result in some infiltration drive.

Air sealing both the ducts AND the house can reduce that infiltration factor by more than an order of magnitude, do where it becomes a "don't care" situation.

Right-sizing the furnace & air handler (or using variable speed air handlers) has the potential reduce that infiltration drive, due to the lower induces pressures. But whether the longer duty-cycles of the lower-volume airhandler adds up to the same, less, or more whole house air-infiltration are system-specific. (In most it's a net-benefit, lowering the infiltration volumes.)

Answered by Dana Dorsett
Posted Fri, 02/21/2014 - 12:51

Helpful? 0

Thank you everyone for your responses. We will recommend that the HVAC contractor do heat loss calculations for both no airsealing and airsealing. This will put the issue before all of us clearly. The hydronic system that is proposed costs $14K-$15K, so we want it to be comfortable.


Answered by Lydia Marshall
Posted Mon, 02/24/2014 - 14:08

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