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Community and Q&A

Balanced ventilation for a retrofit

GBA Editor | Posted in Energy Efficiency and Durability on


I am looking for a whole house ventilation system for an existing house in the UK (so relatively cold).

Heat recovery ventilation sounds good but the conventional installation is impractical because of the need to install ducts to supply air to all habitable rooms. The house is 3 storeys and we currently have one central column which houses all the waste & ventilation pipes. We don’t have room to add new supply pipes in addition to the exhaust pipes and even if we could, this would mean that the new air coming in is virtually next to the extracted air so would lead to very little air flow around much of the house.

My husband has come up with the idea of using this space to install supply ducts to habitable rooms. However, to extract the damp air generated in the bathrooms (and create a balanced flow) he wondered about putting a heat recovery box in the loft with a vent in the loft ceiling above the central stairway. (This is at the opposite side of the house from the existing exhaust ducts so air would flow across the house). The warmed air from this unit could then be pushed down the supply ducts using another fan

Does this sound like a viable solution?
Can anyone think of any potential problems?

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  1. GBA Editor
    Martin Holladay | | #1

    Because your terminology is a little different, I'm not exactly sure what you are asking. But it sounds as if you are asking, "Can I install a heat-recovery ventilator in an attic?" If that's your question, the answer is yes.

    It's important to remember, however, that HRVs include filters that must be cleaned or replaced on a regular basis (every 6 months or so). An HRV should be installed in an attic only if access is easy (via stairs, not a ladder and a hatch). Otherwise, the filter will be neglected.

    In theory, you can run ducts from your HRV to the opposite side of the house. The longer the duct runs, however, and the more elbows installed, the greater the static pressure that the unit's fans must overcome. Be sure your ducts are adequately sized, airtight, and installed with as few elbows as possible.

  2. Philippa Richard | | #2

    Thanks for the reply Martin.
    I don't know about in the US but in the UK the ventilator is often in the attic so I assumed this would be fine, but thankyou for the comments about the filters.

    The part I am less sure about is whether you have to have ducts for both extract and supply or whether you can have just one set of ducts.

    Option 1:
    Having ducts to pump the fresh air into rooms and then have just a ceiling vent at the top of the house (with a fan attached) that would suck up the stale air.

    Option 2: Have ducts to the wetrooms to suck up stale air then have one vent pumping fresh air out into the landing.

  3. GBA Editor
    Martin Holladay | | #3

    Now I understand your question. As you probably realize, the best HRV system will have dedicated ventilation ducts that exhaust stale air from bathrooms and laundry rooms and deliver fresh air to bedrooms and the living room.

    Anything less is a compromise. But systems with incomplete ducting will still work. As you suggest, you can exhaust stale air from a central hallway and deliver it to your bedrooms and living room (option 1) or you can exhaust stale air from your bathrooms and deliver fresh air to your central hallway. Either option will work, but the performance of the system will not be as good as a fully ducted system. If you choose either of these options, be sure to balance the airflow (cubic feet per minute) so that the supply airflow is close to the exhaust airflow.

    Have you considered another option: running new ductwork along your ceilings, from room to room, and then boxing the ductwork in — that is, covering the ducts with a plaster-faced box so that the ducts are not visible?

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