Historic ductwork system in the slab - how to decommission permanently?

[Avemo]

I don't think the channels cause any harm just by being there. If there's a musty smell, I believe the solution is more to prevent air exchange with the room than to arrange forced ventilation.

There are hygrometers with logging functions. If you place one in the channels, you can see how the humidity there varies over time. In house foundations, you typically want to keep it under 70% RH to prevent the wood from molding. This problem should be less of an issue here since there's nothing that can mold.

I am also not at all convinced that there will be any major energy losses. Until someone conducts a scientific empirical study that clearly shows how significant the losses are compared to other options, I will maintain that the losses are moderate. Then one has to weigh any additional costs against the advantages the system has.

It is quite well-established that underfloor heating causes higher energy consumption than radiators. Especially with poorly insulated slabs, which one can assume a house built in the 1950s has. I recall that the difference in one study was 12% with 50 mm insulation in the slab. Then, of course, one can always discuss what constitutes significant energy losses.

 

[Leif i Skåne]

The best would most likely be to use the system as it was once intended.
Then you should have a warm and dry foundation, warm and cozy floors, and good ventilation.
I am also not at all convinced that there will be any significant energy losses. Until someone conducts a scientific empirical study that unequivocally shows how large the losses are in comparison to other alternatives, I will insist that the losses are moderate. Then one has to weigh any additional costs against the advantages that the system has at least a little (air).

I encountered the system when we were looking for a new home in 1982. It was a widow who owned the house we were looking at, and she had lived there since 1951 when the house was built. She was completely enthusiastic about the heating system!!
With 3 children long since moved out, she said they never had to clean since the air from the baseboard carried the dust to the exhaust vent in each room and then down to the basement where the large low-speed radial fan sucked the air through a filter cassette which one could easily rinse backwards with a garden hose once a week.
Everyone has probably seen how much dust floats in the air when the sun sends a ray through the room.
From this perspective, the system was more superior than people understand today.
Imagine never having to dust and vacuum.

 

[BigR]

I don't think the channels do any harm just by being there. If it smells musty, I think the solution is more to prevent air exchange with the room than to arrange forced ventilation.

There are hygrometers with logging functions. If you insert one of these into the channels, you can see how the humidity varies there over time. In house foundations, one usually wants to keep it below 70% RH to prevent the wood from molding. That problem should be smaller here since there is nothing that can mold.

It's fairly well-established that underfloor heating causes higher energy consumption than radiators. Especially with poorly insulated slabs that one can assume a house built in the '50s has. I seem to recall that the difference in some study was 12% with 50 mm of insulation in the slab. Then one can always discuss what constitutes large energy losses.

So you're advocating for preventing air exchange, i.e., plugging all holes to the system? It's the simplest measure, at least.

I would guess that the main channel and pipes together constitute 1-2 cubic meters of space with air in the slab in total. That would mean no air exchange at all. Consequential problems?

And as I said about underfloor heating, there won't be any in this house.

 

[BigR]

I came across the system when we were looking for a new home in 1982. It was a widow who owned the house we were looking at, and she had lived there since 1951 when the house was built. She was absolutely ecstatic about the heating system!!
With 3 children long since moved out, she explained that you never needed to clean because the air from the skirting board carried dust to the exhaust vent in each room, then down to the basement where the large low-speed radial fan sucked the air through a filter cassette, which you simply rinsed backward with a garden hose once a week.
We've all seen how much dust floats in the air when the sun sends a beam through the room.
The system from this perspective was more superior than people understand today.
Imagine never having to dust and vacuum.

Vacuum cleaner and heating in one! Yes, that's a point. Well, it's still not enough for me to convert back.

That house seemed to have a few more vents. Here there was just one big fellow.

 

[Magnus Larsson]

Connect the system to a low-speed evacuation fan to maintain a slight negative pressure in the system and prevent any odor from entering the house. It should also keep the ducts dry and be almost maintenance-free. I think, along with most others, seem to agree that you should avoid sealing it up and forgetting about it.

 

[Avemo]

So you're advocating for preventing air exchange, i.e., sealing all the holes in the system? That's definitely the simplest measure.

I would guess that the main channel and pipes together make up 1-2 cubic meters of space with air in the slab in total. It would then be completely without air exchange. Follow-up issues?

Seal it in a way that keeps out musty smells but is easy enough to reopen for another measure if it turns out there are follow-up issues. My suggestion is, therefore, to seal it without forgetting about it.

 

[BigR]

Seal again in a way that keeps out stale odors but is easy enough to reopen for another action if it turns out there are follow-up problems. My suggestion is to seal up without forgetting about it in the meantime.

If you go down this path, the sealing of the channels at the outer wall becomes permanent. You can fairly easily fill them, but not access and remove them. So, it won't be reasonable to open them again. For the main channel that runs centrally in the slab, you can probably put a hatch to have a moisture meter in.

If you (later) want to ventilate as Magnus Larsson suggests above, maybe two holes in the main air channel for ventilation would suffice? This wouldn't, however, ventilate the smaller channels particularly well.