Fireplace wetback -> radiator heater in HRV/DVS system

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MarkM536

309 posts

Ultimate Geek

#319867 9-Jun-2025 19:53

Seeking advice or experience on air ventilation systems for heat transfer, particularly systems that use a hot water loop for heating. Does it exist as I think? No air ventilation system is installed yet.

My lounge has a wood burner fireplace. At the other end of my house is 3 bedrooms.

The fireplace has an option for a 'wet-back' hot water loop for radiators (add-on). Which got my curious on heaters for air ventilation systems that can use a hot water loop.

It turns out there are existing systems...? E.g. The Ruck FTW200/250/315: https://www.moisturemaster.co.nz/products/air-heaters but I am unsure if these are suitable to use with a wood burner (vs an electric hot water heater).

This is my idea of the system:

Water loop from the fire's wet-back -> radiator in the ducting -> back to the fireplace (that seems to be a no-no) or to a tank/pressure relief -> pump -> returned to fire wet-back.

Either moving air from another room or sourcing air from outside for the ventilation system. I would like this system to also bring in air from outside during summer.

Ideally NZ certified and installed system.

tweake

2391 posts

Uber Geek

#3382185 9-Jun-2025 20:13

ventilation system as a heating system, no. use a heat transfer system. the difference between them is the amount of air flow, ventilation systems don't flow enough air. most ventilation systems (positive pressure systems) will also slow right down due to the cold outside air.

a combo transfer/ventilation system in transfer mode will put all the ventilation air into the lounge and then the heat transfer side will suck it out of the lounge and pump it to the bedrooms, where most of the air will return under the doors etc back to the lounge.

using a water coil to heat the air transfer air? i know of a company pushing that sort of product. however one big consideration is what happens if it leaks and how do you stop the water from boiling ? with a wetback hot water tank, its a lot of mass to start with, and if it boils it overflows outside and refills with some cold water.

i would just stick with a heat transfer system. no water to ruin a house with, a huge amount cheaper and easier to control. a lot of them now have controllers so they only turn on when the lounge gets to hot (or whatever temp you set it to).

wolf84

2 posts

Wannabe Geek

#3387894 29-Jun-2025 07:50

Hi, just looking at this myself. I have radiators installed already however so not such a leap for me vs you starting from scratch.

An uncontrolled heat source (wetback from a fireplace) will required an open vent pipe off the "loop", going to atmosphere should the system overheat, preventing dangerous pressure buildup, ie allowing the system to boil.

To be of use to you the system would need storage volume (a cylinder) to enable heat to be stored and thus supply heat when the fire isn't actively going (night etc). Radiators use alot of heat. To put in perspective I have a wood fired boiler (marshall heater) hooked up to another storage cylinder (total ~500L). Once heated to maybe 80oC this is enough to heat radiators on a timer throughout the night and morning, radiators are luke warm the following afternoon.

I think a ventilation system that draws it's own air from your roof space (vs heat transfer system) would be more effective as it positively pressures your internal house (vs atmosphere) pushing air out through cracks etc, vs sucking air in potentially with heat transfer.

Hopefully this helps.

tweake

2391 posts

Uber Geek

#3387901 29-Jun-2025 10:07

wolf84:

I think a ventilation system that draws it's own air from your roof space (vs heat transfer system) would be more effective as it positively pressures your internal house (vs atmosphere) pushing air out through cracks etc, vs sucking air in potentially with heat transfer.

the big problem with using ventilation system is it sucks in cold air and you push warm air outside. imagen if the radiators worked the same way by, heating up cold water from the tap and letting warm water drain out the radiator at the end. it would perform terribly. we want the air to do what water does in a normal radiator system, return back to get reheated.

then there is the other issue of ventilation airflow being very small, compared to normal hvac airflow thats required.

if done correctly, you would actually have both a ventilation system and heat transfer system. you can buy systems that do that. its normal to have a ventilation and hvac system, typically joined together to use the same ductwork.

bit of a side story, but heat transfer systems can suck in outdoor air in the living room (including via the fireplace chimney) and this has caused houses to fill with smoke and not be far off killing people. this is caused by not having an air return from the supplied rooms (ie bedrooms). i have only seen one manufacture mention the need to install returns and it seams to be a common short cut to do cheaper installs.

MarkM536

309 posts

Ultimate Geek

#3387939 29-Jun-2025 12:29

tweake:

i would just stick with a heat transfer system. no water to ruin a house with, a huge amount cheaper and easier to control. a lot of them now have controllers so they only turn on when the lounge gets to hot (or whatever temp you set it to).

That's where I'm at.

The idea of hot water radiator in ventilation is nice, but a lot of money/hassle.

The only downside I can see with air transfer from my lounge is blowing any smoke/ash smell to bedrooms. I think this could be mitigated with some sensors in the duct (custom controller board and I'll make it Home Assistant compatible).

MarkM536

309 posts

Ultimate Geek

#3387940 29-Jun-2025 12:38

wolf84:

An uncontrolled heat source (wetback from a fireplace) will required an open vent pipe off the "loop", going to atmosphere should the system overheat, preventing dangerous pressure buildup, ie allowing the system to boil.

To be of use to you the system would need storage volume (a cylinder) to enable heat to be stored and thus supply heat when the fire isn't actively going (night etc). Radiators use alot of heat. To put in perspective I have a wood fired boiler (marshall heater) hooked up to another storage cylinder (total ~500L). Once heated to maybe 80oC this is enough to heat radiators on a timer throughout the night and morning, radiators are luke warm the following afternoon.

Yes, over-boil and pressure relief valve is needed.

Holy smokes batman for your water storage tank to be 500L..... there is no way I could fit a tank anywhere in my house that big. I was thinking a small tank around 80L would be enough.

I really like your system having enough thermal mass (the 500L tank) to keep radiators hot-warm all night long.

wolf84

2 posts

Wannabe Geek

#3387941 29-Jun-2025 12:55

To be honest,

It's probably most practical/easy to just have a fireplace (with a wetback to heat your domestic hot water or not), a heat pump system for rooms (in roof setup to do whole house or separate units) or hrv. Radiators use a lot of wood... Id probably not go through all the fuss of installing them again. I'm considering solar panels for heating domestic hot water/radiator boiler also but it all costs $$.