ᐅ Specialized ventilation systems needed for older buildings

Hello,

I have an old building and will be replacing old windows (almost 50% are 68 years old) and possibly upgrading the very poor insulation. In my view, this means making my house more airtight. Inside, there has been noticeable drafts during strong winds even with closed windows and slightly open doors. Since this natural air exchange will be reduced, I am considering a controlled ventilation system with heat recovery. Ventilation in every room is not an option due to the extensive installation work required. The interior of the house has already been fully renovated. A central ventilation system is likely not feasible for the same reasons. Now I am trying to find out if I can manage with a few small, decentralized units. Certain features would be important since I would like to operate a single system across multiple rooms (cost/space/effort).

Kitchen/Bathroom 1: Both are next to each other and important for ventilation. Here I would prefer a system with a humidity sensor that serves both rooms. The tricky part is that it would need to switch to, or I would need to be able to switch to, the room that is currently more humid. If I simply connect both rooms in parallel through ducting, the airflow in the damp room is reduced by half and the supply and exhaust air is not evenly balanced in each room. This means one room is exhausted more and the other receives more air supply. This creates positive pressure and pushes air from one room into the house. If this is the damp room, moisture and possibly odors are pushed into the house.

Living room/bedroom or living room/basement: Here we have a cold room combined with a very warm room. I would need a 2-room ventilation system with separate heat exchangers or one that can heat the air to different degrees. So, a larger device. Especially for the basement, we want a ventilation system because we plan to use it occasionally as living space and in winter for drying clothes, etc. However, it is poorly insulated and therefore quite cold. The bedroom situation is similar but less extreme. We could combine two rooms here, but I don’t want to cool the living room with air from the cold bedroom or fail to heat the bedroom properly.

Smart ventilation with heat recovery: Temperature is an important issue. In winter, it’s about warmth and heating costs. Unfortunately, our installer severely undersized the heating system. I have requested larger radiators, but it still isn’t quite enough. Summer temperatures get very high. Are there ventilation systems that can be set for a preference of cooler or warmer air and then operate accordingly? Of course, heat recovery always helps. What I mean is that during hot summer days there is less heat exchange, and when the unit detects cooler outdoor temperatures, it works harder to cool the room.

Decentralized ventilation with heat recovery and a small air conditioning unit: This would be the next step. Do such combinations exist?

I hope you can help me and that I won’t get answers like “google it” or “use the search function”… ;-) Of course, I’ve already researched, but I haven’t found these specific things. I just thought of the air conditioning aspect now and haven’t searched for that yet.

Thanks in advance,
Best regards
derMartin

New windows definitely make the house more airtight. But why do you believe you still can’t do without various ventilation systems? Have you already had problems with damp spots or mold?

What renovations have you done so far?

I would first insulate all easily accessible and cost-effective areas as much as possible: the top attic ceiling/slopes and the basement ceiling. This will raise the wall temperature in critical corners.

What is undersized: the heating system (which is already a challenge to get right in a single-family house today) or the radiators? Additional radiators can be easily installed later if you know where the pipes run.

What kind of exterior wall construction do you have?

In most cases, a mechanical ventilation system is unnecessary. Proper ventilation habits should actually keep the house free of mold and damp spots. Using 1–3 hygrometers helps, too. This way, you can always monitor the indoor humidity and take appropriate action.

Installing a ventilation system in old basements can make sense, but it must be controlled by both indoor and outdoor temperature and humidity; otherwise, you will just bring in warm, moist air again. An alternative and cheaper solution is to run a dehumidifier in the basement during summer when needed.

derMartin schrieb:
Installing ventilation in every room is out of the question because it would require extensive work. The interior of the house has already been fully renovated.

Ideally, renovations should be done from the outside in, and window replacement also creates dust and dirt.

Hello dertill,

dertill schrieb:
But why do you now think you can’t do without various ventilation systems? Have you already had bad experiences with damp spots or mold?

I have had a few minor mold spots. Nothing serious. But until now, the house has been far from warm and airtight.

dertill schrieb:
What exactly have you renovated recently?

Floors, walls, furniture, heating, bathrooms, kitchen. Why? What does that have to do with the question?

dertill schrieb:
I would first insulate all easily accessible / inexpensive areas as much as possible: top attic ceiling/slopes, basement ceiling. That already raises the wall temperature in the critical corners.

Will do.

dertill schrieb:
What is undersized: the heating system (which is already tricky to get right in a single-family home today) or the radiators? Additional radiators are easy to retrofit if you know where the pipes run.

The radiators are too small. Yes, they can be retrofitted. On one hand, I have had bad experiences with heating system installations, and on the other hand, that is not the route I want to take. I waste too much energy. Bigger radiators might improve that somewhat, but I want to reduce energy loss. Currently, my heating demand indicator (HWB) is 180.

dertill schrieb:
What is the construction of your exterior walls?

I can’t say yet.

dertill schrieb:
In most cases, a mechanical ventilation system is not necessary; good ventilation habits alone should keep the house free of mold and damp spots. Also useful are 1–3 hygrometers. That way, you always keep an eye on the indoor humidity and can react accordingly.

Unless I’m not there. That happens quite often, especially in winter, sometimes for up to a month. Also, my wife finds it hard to be convinced to ventilate due to heat and the baby.

dertill schrieb:
Installing a residential ventilation system in old basements can make sense, but it must be controlled by both indoor and outdoor temperature and humidity, otherwise, you only bring humid and warm air back inside. Alternatively—and more economically—you can run a dehumidifier in the basement during summer if needed.

Yes, a dehumidifier is an option. I even have one. With 0.5 kW though, it’s not exactly energy-efficient.

dertill schrieb:
Ideally, you renovate from the outside in, and window replacement also creates mess.

Depends. For example, if you want to move in, it’s good to have at least one bathroom. Sure, you’re right. But there are practical reasons that sometimes prevent this.

Best regards
derMartin

Hello,

I have now answered these questions. Can someone please answer my questions regarding (decentralized) controlled residential ventilation with heat recovery?

- Are there systems with separate “heat circuits”? That is, a unit that can ventilate a cold (basement) room and a warm room without pumping heat from the living area down to the basement? Otherwise, it seems the alternative is to have two separate units.

- Is it possible to switch the air outlet? For example, to connect two rooms and direct the air to one room or the other as needed. Ideally, the switchable exhaust air would be controlled by a humidity sensor. But maybe that is too complex.

- How “intelligently” can these systems operate? For example, in summer, when you want to keep it cool, to ventilate less, and as soon as the outdoor temperature is lower than indoor, to ventilate as much as possible to cool the living area.

- Are such units available in combination with air conditioning?

Thanks in advance,
derMartin

derMartin schrieb:
- Are there systems with separate "heat circuits"? Meaning a system that can ventilate a cold (basement) room and a warm one without transferring heat from the living area to the basement? The alternative is obviously having two systems.

Two systems, anything else is nonsense.

derMartin schrieb:
- Is there a possibility to switch the air outlet? So that two rooms can be connected and, depending on the need, air is directed into one room. Ideally, with a switchable exhaust air controlled by a humidity sensor. But that might be too much.

Two systems, anything else is nonsense.

derMartin schrieb:
- How well can these systems operate "intelligently"? For example, in summer, when you want to keep it cool, ventilate less, and as soon as the outside temperature is lower than inside, ventilate as much as possible to cool the living area.

Basically not at all. Typical decentralized systems are designed for ventilation and lack intelligence. Additional intelligence is not free, and since decentralized systems are usually designed to be as affordable as possible, you can imagine the rest.

derMartin schrieb:
- Are these devices available combined with air conditioning?

Well, you can combine anything. But then it still lacks intelligence.

That’s why there are central systems which leave nothing to be desired (with appropriate equipment), offering VRV, air conditioning, humidification, and so on.

derMartin schrieb:
Floors, walls, furniture, heating, bathrooms, kitchen. Why? What does that have to do with the question?

It is sustainably easier and more cost-effective to achieve energy savings and prevent mold growth through constructive measures without relying on additional technology, and only when that is (practically) not possible should you consider permanently running technical systems. A mechanical ventilation system should not be the first choice for solving problems, but rather an add-on or additional comfort feature.

derMartin schrieb:
I waste too much energy. Bigger radiators might help a bit, but I want to reduce energy loss.

A ventilation system only reduces energy loss caused by active airing through open windows. Especially in older buildings, this share is very small compared to the total heat losses. If you really want to save heating energy, you should not start with the ventilation system.

derMartin schrieb:
I can’t say yet.

If you want to reduce the energy loss in your house, you should find out about the construction details. What does your facade look like (brick, render, wood)? Wall thickness? Solid construction? Building age? Any later additions or conversions? — This information is sufficient for an initial assessment.

derMartin schrieb:
Unless I’m not there. That happens quite often. Especially in winter and sometimes for up to a month.

If nobody is in the house during winter, and the building is continuously heated, and you do not have water damage, mold will not grow there—whether you have a ventilation system or not—because no one is introducing moisture into the indoor air by their presence or activities (showers, cooking).

Mold needs four things to grow: nutrients (wallpaper, dust, organic dirt), surface humidity of 70% or higher, surface temperatures below 17°C (63°F), and a pH value below 9.

The colder the surface/air, the higher the relative humidity at the same absolute water content. Warm air can hold more water vapor than cold air. For example, air at 22°C (72°F) and 50% relative humidity contains about 8 grams of water per kilogram of air. If this air cools down to 15°C (59°F) in a room corner, it still contains 8 grams of water, but now the relative humidity is around 75%. If it cools further, those 8 grams per kilogram correspond to 100% or more relative humidity, and condensation will occur—forming fog or droplets.

Even if you have a ventilation system and keep your humidity consistently at 45–50%, you can still get mold in room corners if they are too cold and the relative humidity there is 70% or higher.

The goal should be to increase the surface temperature of the most critical spots, such as room corners or penetrations in the exterior wall. This is easiest to achieve by insulating the roof slopes and the top floor ceiling (i.e., between the attic and the top floor) as well as installing insulation on the underside of the basement ceiling. This is affordable, easy to do yourself, and effective. It saves a lot of heating energy and reduces mold growth in room corners. The insulation above/below the corners raises the temperature there, lowers the relative humidity, and removes the conditions required for mold growth.

Replacing windows is also a good measure, but don’t forget the reveals. Usually, when replacing windows, replastering is necessary anyway, so you can also apply calcium silicate insulation boards there or at least use lime plaster and silicate paint (pH value >10) in those areas.

With better windows and insulation of the top floor ceiling/basement ceiling, you will need less energy for space heating and won’t require larger radiators. Insulating roof slopes and the top floor ceiling also reduces heat entry from outside during summer, keeping the house cooler in summer.

If you have done all this and still face mold issues in several places, then you should consider a mechanical ventilation system.

EDIT: I overlooked that you want to make the basement livable. In this case, insulating the basement ceiling is pointless since the basement won’t be cold.

BUT:

Making a non-insulated basement from 1960 livable and heated without insulating it is not feasible and will have serious consequences, with or without a ventilation system. The surface temperatures of the floor and walls are constantly around 10°C (50°F). Sealing the certainly existing outside air openings and heating the space, combined with additional moisture from laundry and occupants, would lead to condensation in the walls and floor. For living purposes, exterior insulation of the basement walls is necessary, and even then, the floor must be insulated very carefully and with expert knowledge only.