An indoor pool in a residence: why comfort is decided by the building services

In brief

Whether an indoor pool is pleasant to live with day to day is decided by the building services, not by the basin. Once the structure holds the water, the job that goes on around the clock begins: managing the air around the pool. Warm water evaporates day and night, and how you handle that air decides whether the glass stays dry and whether the hall is still free of corrosion years later. The climate of the hall – the moisture balance, ventilation with dehumidification, keeping the glazing above the dew point, and heat recovery – is designed together with the architecture, because it sets the duct sizes, the storey height and the space for the plant room. I consider a pool inside the house itself a good choice, but only if that system has been worked out in full; worked out after the fact, it turns the hall into a nuisance instead of a pleasure.

The basin is the simplest part

The structural side of a pool – groundwater, uplift and the watertightness of the basin itself – I described separately, in the text on a pool in high groundwater. That is hard work, but once it is done well, it no longer needs daily attention. With the climate of the hall it is the other way round: keeping it right goes on all the time. The surface of warm water gives off moisture without a pause, and the air has to have somewhere to take it. Whether it is pleasant inside is settled not in the basin but around it, and on paper, before the architect fixes the heights. An indoor pool is not simply a pool with a roof over it; it is a separate room with a microclimate of its own, and it has to be designed for that microclimate from the first line.

Why a pool inside the house and not in the garden

Before we get to the air calculations, there is an earlier question: where the pool is to stand. One side of that decision I know from my own experience. The first time we set up a pool in our garden, it turned out that over the whole summer I had barely been in it twice, so I decided that the next year I would either fit a heating system or we would not put the pool up at all. The difference in comfort was enormous, and since then I cannot imagine swimming in a pool that is not heated. I still have the pool in the garden, heated now.

But heating extends the season; it does not change the weather. In summer it looks fine – 26–27°C and sunshine – but there is a wind, and beside an open-air pool it is not pleasant, because you feel cold; in a hall you can sit in the water even with the window open, and you use it all year round, winter included. That is why I am in favour of a pool inside the building. For most people an outdoor pool is a poor choice, because the weather in Poland leaves little time when you can enjoy one.

That does not mean the hall has to be a closed box. It is great if you can fit large windows and open the hall up to the garden in summer, or give the pool a wing of the house on the ground floor. Then you have a pool all year and the weather is no concern of yours. You only have to tell the ventilation designer about it at the start, not after the fact: a glazed wall that opens, at that size, is also the hardest part of the indoor climate.

There is one condition you cannot get round. It really has to be planned and designed all the way through, with the services built to match, so that the pool does not become a burden on living in the house: the humidity must not run too high, and neither must the bills.

The moisture balance everything depends on

Everything in the hall starts from one figure: how much water evaporates from the surface in an hour. The warmer the water and the drier the air, the more evaporates – and not only from the surface of the pool, but also from the wet surrounds, the splashed floor and the skin of people climbing out of the water. That flow has to be calculated, because the duct sizes and the dehumidification capacity depend on it. Ducts need room under the ceiling, and if nobody calculated them before the heights were fixed, there is nowhere to find that room later.

I do not calculate the balance myself; that is the job of a pool ventilation designer – a specialisation of its own, not something an installer does on the side. My part is different: to give him real data – the water temperature, the area of the surface, the cover, the glazing and how the pool will be used – to leave room in the design for the ducts and the plant room, and not to let anyone water down his design assumptions along the way. I do not do that calculation, but I have to know what goes into it and what is missing from it, because the hall as built is my responsibility.

Water temperature and air temperature

Most depends on the water temperature. It decides the comfort, and along with it the size of everything else: every extra degree of water temperature means more evaporation, more dehumidification and a higher bill. The air in the hall is kept 2–4°C warmer than the water, and that is not a whim of the designer. Wet skin coming out of the pool loses heat through the film of water evaporating from it, so warmer air stops you feeling cold, and at the same time draws a little less moisture off the surface.

That leads to the thing that surprises people most. The heating will run practically all year, because we have to keep the air there warmer than the water. There is no heating season and no summer break; there is one state that simply goes on, and with it the power reserved in the grid connection. How much electricity a house with a pool, a heat pump and a sauna needs at once I worked out in the text on a residence’s grid connection capacity.

The water temperature and the pool cover are your decisions, and I bring you tested options to choose from well in advance. When I hear that the water should be as warm as possible, I answer that this means larger equipment and a higher running cost all year round.

The dew point, or why the glass is the weakest spot

In a warm, humid room every surface colder than a certain threshold collects water. That threshold is the dew point, and the coldest surface in a pool hall is, as a rule, the glass together with its frames. You know this from old windows: in winter water runs down a single pane while the wall right next to it stays dry. The pane is cold, so that is where the air gives up its moisture, and in a pool hall exactly the same happens, only with far more moisture. If a surface regularly drops below the dew point, water will condense on it, often for hours on end, and damp, warmth and chlorine are the conditions in which fittings corrode and the insulation inside the wall gets wet.

The defence is to let no surface get cold, which is why warm supply air is run along the glazing. A stream sent over the water itself would only speed up evaporation, while one sent up the glass forms a warm curtain against it and helps keep its surface above the dew point, provided the details have been worked out properly. Supply air alone is not enough if the pane is cold and the frame forms a thermal bridge. The most common mistake I see is a simple one: glass down to the floor, beautiful, and nowhere beside it to run the supply air. And here the hall you want to open in summer comes back: a large wall of opening glazing is exactly the element that works hardest in winter. That is not a reason to give up such a wall; it is a reason to tell the ventilation designer about it before the architect finalises the elevation.

The wall itself is designed for this one microclimate, with a vapour barrier sealed on the warm side so that moisture does not get into it. I know this physics from ordinary house ventilation: when warm, humid air passes through a cold zone, it can start to condense inside the duct. Better glass does not settle the matter on its own, because even good glazing can drop close to the dew point on the coldest nights; the point is for it to keep a margin above that line, with the supply air helping to hold it. That is why the glass and the air are chosen together.

The energy that escapes as vapour

Here is something few people expect. In an ordinary house the largest item on the bill is heating the rooms; in a pool hall, heating the volume of air is usually not. Most of the energy escapes through evaporation: every kilogram of water that turns into vapour takes heat with it and leaves the water in the basin a little colder, and the heating has to make that up without a break. That loss goes on around the clock, at night too, when nobody is swimming.

Picture a pot on the stove. Without a lid it steams away endlessly and you have to keep heating it to hold the temperature, yet put the lid on and it settles down. An uncovered pool surface behaves in just the same way, except that it is being heated non-stop.

That is why good systems do not simply throw the warm, humid air outside. A pool air-handling unit with a heat pump condenses the vapour out of it, recovers the heat and returns it to the water or to the supply air. A heat pump works on the same principle as an ordinary household fridge, and that heat does not appear by magic; physics simply moves it from one place to another – from air that has to leave the hall anyway to water that has to be heated anyway. Not for free and not in full, but without that recovery, running a year-round hall makes no sense.

The real running cost comes down to five things, and each of them is a decision, not an accident:

Four of the five are settled on paper, because the equipment, the ducts and the walls depend on them. The fifth, the pool cover, is cheap and simple to use, and it makes a surprisingly large difference to the bill: a closed roller cover cuts evaporation sharply and lets the system run at lower speed and more quietly. That is why I ask about it early. Will the pool be covered when nobody is swimming? The answer changes the choice of equipment and what the system costs to run.

Dehumidification, fresh air and the plant room

The conditions in the hall are maintained by equipment that runs without a break, and the solutions fall into two families. The simpler family consists of dehumidifiers that work on the hall’s own air in a closed loop; they are cheaper and enough for a small pool. The more elaborate ones draw in a little outside air and recover some of the heat, though they do not fully solve the question of fresh air. The second family is a full pool air-handling unit, which dehumidifies, replaces the air with fresh air and recovers heat. In a residence, with a hall used all year and a large area of glass, it is the second that makes sense. You will notice the fresh air with your nose, by the way, because it is what carries away the smell of chlorine. The smell itself is usually chloramines – the result of the water chemistry and too little ventilation at once – and ventilation does not replace good water treatment.

The equipment has to be built for this environment: higher corrosion resistance and sealed electrics, because damp, chlorinated air wears out standard equipment within a few years. The system as a whole adjusts to the rhythm of the house: it runs flat out when the pool is in use, is turned down at night and runs almost at idle when the pool is covered. And if a sauna and a steam room are to stand next to the hall, those are three different climates under one roof, each with its own system, and at most they share a plant room; I went through that in the text on a sauna, a hammam and a spa at home.

That leaves the place where all of this stands. The plant room needs an access route for servicing and careful thought about what lies next to it, and there is one more thing you do not see on the floor plan. This equipment runs for long hours, at night too, and the room where someone sleeps has a habit of landing right above the plant room. More than once I have come across people (not my clients, fortunately) who switched their heat-recovery ventilation off altogether because the noise was too much to bear. With a pool unit it is worse, because you cannot switch it off: a hall without dehumidification will tell you through its windows in the first season. That is why the plant room gets acoustic insulation and the equipment sits on anti-vibration mounts. The plant room is also built with room to spare, because the air-handling unit will not last the whole life of the house, and replacing it must not mean breaking open walls.

How to tell the hall’s ventilation is not coping

When the climate in a hall is wrong, you can see and smell it before anyone measures anything: glass that will not dry completely, a smell of chlorine right at the door, rusting fittings and stains under the ceiling. The worst symptom is the one you cannot see: moisture gets into the wall through a leaky vapour barrier or a badly resolved detail and condenses there, corroding steel and soaking the insulation for years before anything shows on the surface.

Almost every time the source is the same: treating the pool hall like the rest of the house, only bigger. Household heat-recovery ventilation is not an active dehumidifier and is not built for constant moisture or chlorine; connect the hall to it and the windows will tell you so in the first season.

When it comes to fungus and mould, nothing is worse than humid air standing still indoors. All the more so when it is warm. I have unfortunately seen the effect of missing ventilation many times, though fortunately not on my own builds: walls covered in mould. In a pool hall the air must not stand still even for a moment, and it has to be carried away through ducts sized for this one purpose. Every one of these symptoms comes from the same cause: the climate was added on at the end instead of being part of the design from the first line.

What I do not promise with a pool hall

There are a few things I do not promise with a hall like this. It will not be cheap to run, because physics does not allow it; if someone promises otherwise, ask what they are leaving out of the sums. Nor will it look after itself, because even good automation needs servicing and someone keeping an eye on it; the day-to-day running after handover is something we can take on under a separate contract. I will not give figures for the running costs until the design tells me the water temperature, the glazing and whether there is a cover.

Nor do I promise that a pool hall can be added to a finished design without changes to the design itself. The climate of the hall can be designed only as long as the heights, the glazing and the space for the plant room are still open. When the design is finished and the pool is to be added to it, there is no longer any way to fit ducts under the ceiling or to find room for a plant room. Rescuing the climate with equipment alone would mean that equipment running louder and more expensively than it should, so in that case we go back to the design and, together with the designer, look for room in it for what the hall needs.

And one last thing: the hall costs money even in a week when nobody sets foot in it, because the equipment keeps the conditions steady whether or not anyone swims. If the pool is meant for a few swims a year, that cost is spread over those few swims, and that conversation is better had before the design than after the first season. It is not an argument against building; it is a question of whether you really intend to use the pool.

What I ask before I say yes

If you are considering an indoor pool, these questions will come up anyway. Better that they come up before the design:

1. Is the hall’s climate designed together with the architecture, or fitted afterwards to finished interiors?
2. What is the water temperature meant to be, and is it understood that it sets the size and the running cost of everything else?
3. Have the glazing and the details been designed to stay above the dew point: supply air along the glass, a vapour barrier on the warm side?
4. Do you want to open the hall in summer, and if so, does the ventilation designer already know?
5. Does the system have heat recovery and fresh-air exchange, or is it a dehumidifier working on a closed loop?
6. Will the pool be covered when nobody is swimming?
7. Where is the plant room in relation to the bedrooms, how is it reached for servicing, and is there room to spare for replacing the equipment?

I go through this list myself on every project like this. If any question has no answer, the design is not ready, however good it looks in the visualisations. And questions about working with me, from the budget to how payment works, I answer in the Questions section.

Frequently asked questions

Does an indoor pool at home need its own ventilation?

Yes. The pool hall gets its own ventilation system with dehumidification, separate from the household heat-recovery ventilation, because that is not an active dehumidifier and is not built for constant moisture or chlorine. Without a separate system, the moisture will find the coldest surface in the house and condense on it, quietly and for years.

Why do the windows in a pool hall mist up, and how do you prevent it?

Because the glass is the coldest surface in the room, and air gives up its moisture most easily against a cold surface. You prevent it by running warm supply air along the glass and by designing the glazing so that its surface stays comfortably above the dew point. The glass and the ventilation are chosen together, not separately.

How much does it cost to run an indoor pool?

I will not give a figure before the design, because several things set the cost at once: the water temperature, the humidity of the air, the pool cover and the efficiency of the heat recovery. Most of the energy escapes through evaporation from the surface, which is why heat recovery and covering the pool when nobody is swimming make the biggest difference to the bill.

A pool in the house or in the garden?

A garden pool costs less and stands unused for most of the year, because the weather in Poland gives you a few weeks of pleasant swimming. A pool inside the house needs a climate that has to be maintained, but you use it whatever the weather. And if the two can be combined, with a hall whose large windows open in summer, then in my view that is the happy medium: expensive, but much better than a pool outdoors.

If you are thinking about a pool inside the house, bring the subject to me before the architect fixes the heights and the glazing; at that point the hall’s climate can still be designed rather than rescued later. Let’s talk about your project.