FAQ'S Pool Contractors

Recotherm have been building ventilation systems for indoor pools for 35 years and have refined there system so to reduce the running cost to as little as possible.

A simple off the shelf ventilation unit would not be able to offer the energy saving features of the Recotherm unit and would not be able to with stand the corrosive effects of the swimming pool atmosphere. Recotherm have units in operation that have been running for 30 years.

No a heat pump dehumidifier is just a different type of unit it works by recirculating the air and removing moisture in a direct expansion evaporator the heat from this process is then use to heat the air and water.

Unfortunately the dehumidification process runs on electricity which is expensive and by recirculating the air without removing the chlorine and body odour smells it give a poor atmosphere in the pool hall. The Recotherm unit will give you a better atmosphere and cost less to operate.

No this is a myth. Condensation occurs when air is in contact with a surface that is colder than the dew point of the air. The dew point of the air is a function of the temperature and humidity of the air. You can have the air temperature 5 °C below the pool temperature as long as the dew point of the air is below the temperature of the internal surfaces.

The Recotherm unit controls the internal humidity and temperature and thus controls the dew point. With an external temperature of -1 °C and a room temperature of 30 °C a humidity of 50% will prevent condensation on a double glazed panel.

If the air temperature was 25°C then a humidity of 55% would prevent condensation on a double glazed panel.

No Recotherm have single phase units up to and including 1m3/s which is large enough to accommodate a 100 m2 domestic pool.

Yes the units can be weather proof for external use but the display panel must be sited internally or protected in a weatherproof box.

Yes the display panel can be sited up to 800m from the unit and is connected by a core telephone style cable.

Recotherms expertise lies in swimming pools, so any comments we make are specifically related to swimming pools. There is no specific information from the Government with regards to pools, but we can apply the government guidance which has been produced, in conjunction with the CIBSE (Chartered Institute of Building Services Engineers) the current advice is:-

“It is recommended that any ventilation or air conditioning system that normally runs with a recirculation mode should now be set up to run on full outside air where this is possible. 

In buildings with mechanical ventilation systems extended operation times are recommended. In demand control systems CO2 set points should be set to 400ppm to increase the delivery of outside air. Ventilation should be kept on for longer, with lower ventilation rates when people are absent. It is not recommended to switch ventilation systems off in any buildings, even those temporarily vacated, but to operate them continuously at reduced speeds.

Recirculation of air between spaces, rooms or zones occupied by different people should be avoided. However, in the case of any systems serving a single space, partial recirculation of air within that space, such as through a local fan coil unit, is less of a concern. The reason is that the primary objective is to maximise the air exchange rate with outside air and to minimize the risk of any pockets of stagnant air.”

Normally in a swimming pool we are serving a single area so some recirculation is accepted. There are a lot of systems on the market that use mechanical dehumidifiers and only introduce a small amount of fresh air indeed there are some that don’t introduce fresh air, at all. These systems will find it difficult to comply with the latest government recommendations without fundamentally redesigning their units. That is because the fresh air they have to introduce, because of COVID, will eliminate the need for a dehumidifier.  Most of the units on the market that use mechanical dehumidification do not have any other form of heat recovery on the fresh air, so they become total lost and you end up heating the local county.  

At Recotherm we have always championed the use of fresh air as the best way to dehumidify the pool hall, so every unit we sell can bring in large amounts of fresh air.  Indeed, wherever possible, we have designed our units to be able to run on full fresh air, even in the middle of the winter when the ambient temperature is at its lowest.

But there is a downside to switching to full fresh air, in the Winter the ambient air contains very little moisture so only a small amount, say 10-20%, is required to control the humidity.  Under these circumstance the Recotherm units would normally reduce the amount of fresh air introduced, but if we are to stipulate that they must operate a higher fresh air rates, then the internal humidity will drop increasing the evaporation of the pool and thus increasing running costs – but also it will make the wet occupants feel colder as the water on their bodies evaporates quicker. 

So, I feel we need to be flexible.  We need to increase the level of fresh air, but not to a point where we are deterring people from coming to the pool because they feel cold.  It may well be that in 5 years’ time, when we have had time to analyse the data, that we find out that COVID cannot live in a swimming pool environment. But for the moment we have to err on the side of caution and increase the amount of fresh air to a level that is sustainable without affecting the daily running of the pool. 

A pool cover would reduce the evaporation from the pool. We would highly recommend a pool cover for domestic application where the pool is only in use for 1 to 2 hours a day. For commercial use the advantages are less clear you may wish to have the pool uncovered for aesthetic reasons or the hours of usage may be so long that the pool is only covered for a short period and getting your money back within the life of the cover may not be possible

Yes because the pooled the evaporation is low and therefore reducing the air temperature will have only a minor effect on the rate of evaporation. This can be linked into the Recotherm unit so that the temperature is automatically reduced when the pool cover is on.

The amount of evaporation from the pool surface is proportional to the difference in the vapour pressures of the air and the water it is also affected by the air movement above the pool and by the amount of activity in the pool i.e. A pool with high occupancy will have a higher evaporation rate than an unoccupied pool.

The heating load for a system with a standard heating boiler is calculated for a worst-case scenario of full fresh-air requirement and full heating requirement for pool room with the pool cover off in mid-winter. Standard calculation is for a regular gas or oil boiler with min flow and return temperatures of 65/45C.

The cost of fitting a larger boiler is relatively low but this isn’t the case with the heat pump systems. So, we have to compromise and we do this by:

1. We lose the ability of the unit to operate on full fresh air in the winter (the fresh air in the winter has a much lower moisture content so less is required to do the same dehumidification duty).
2. As Heat pump systems have a much lower flow and return temps there has to be compromises and some changes. Often the unit changes to a larger one because we can’t get the air off temperature we require, even after fitting a larger heat exchanger inside the unit, as we are limited by unit size, air flow resistance and fan duty.
3. You should expect the heat up time to be extended and accept that in the depth of winter the required pool hall temperature may not be reached because we are working on closer design margins.

All of the above is done to reduce the heat pump duty, and thus the cost of the equipment, in order to enable that solution to be cost-effective which is only achievable by applying the above points.

If clients or heat pump providers want guarantees or for Recotherm to take responsibility for performance of low temperature solutions in high temperature environments they may find their duties and costs increasing massively. We try to keep a sensible balance for both customer and energy provider as keeping a room a room at 30C with a 45C coil temp at a reasonable capital cost is not easy.

Recotherm design and build high end swimming pool ventilation units, we do not supply pool water heat exchangers. But we deal with a lot of pool contractors and they are increasingly coming across homes that will be heated by air or ground source heat pumps, indeed after 2025 it will become illegal to fit a fossil fuelled boiler in a new home, so this is only going to increase.  

The issue with the current catalogues from the suppliers of heat exchangers is in the sizing charts, as they show a quantity of water and then a kW rating for the unit, based on three different primary flow temperatures. They do not ask how quickly the water needs to be heated.

The heat required to keep the pool up to temperature is a fraction of the heat that is required to raise the temperature from cold fill to working temperature, so the heat exchanger is always sized on the initial heat up load. Pool contractors will tell you that they want to heat the pool at a maximum of 0.5 ^oC per hour, so if you are going from 10 ^oC to 30 ^oC, that would mean 40 hours (some manufacturers charts seem to be based on 46 hours).

When you are doing this via a standard boiler it doesn’t matter if the heat exchanger is a little larger than it needs to be, because the cost of a slightly larger boiler is relatively cheap. But when we are dealing with heat pumps the difference in the size of the heat pump could be thousands of pounds. It’s therefore imperative that you take into account the time scale when sizing the heat exchanger. If instead of a 46 hour heat up period, we had a 96 hour (4 day) heat up period the heat exchanger would be 48 % of the size, and the associated heat pump could be sized closer to its actual running condition. 

The reason we raise this issue is that we are increasingly getting involved in sizing pool water heat exchangers, and calculating the size of the heat pumps, because pool contractors are sizing the heat exchanger from manufacturers charts. We appreciate that they are trying to make it easy to do the selection but  feel they should first establish how much heat is actually required, before moving on to which heat exchanger is required based on the flow temperature of the primary water.  

Yes we have add capacity on the internal controller to monitor and control the temperature of the pool you will require a temperature sensor and control valve which are both available from Recotherm Ltd.

Yes.

No the ventilation unit can usually be serve off the boiler that heats the pool water.

This boiler should be sized to heat the water from cold to 30 °C in either 48 or 72 hours once it has got the pool water up to temperature there is usually spare capacity to supply the ventilation unit. You may require extra capacity on a commercial project depending on the rate of back wash.

The air should be distributed in either metal or builders work ductwork and blown over the cold surfaces in the pool hall, this should mean the windows if you have any surfaces colder than the window then you should rectify the problem.

The return air can be from a single point on one of the walls. (Please note the position of the return air grille has no effect on the distribution of the air) You should avoid blowing the air directly at the pool water surface as this promotes evaporation and will make the pool occupant feel cold.

Yes a spa pool is at a high temperature than the pool and when in use is highly active you should let who ever is selecting the unit know that you have a spa and what size it is.

It is possible to avoid condensation you will need specialist distribution grilles to throw the air to all corners of the room. You may also need to operate at a lower humidity because you are not covering the windows with dry warm air.

You can operate the VF25, VF50 and VF100 from a 13 amp supply the bigger units will require a larger supply.

Yes the unit is there to protect the building structure but if the hall is well insulated and you use a pool cover then the unit will slow down to it minimum speed and use only a small amount of electricity to power the return fan.

Yes to avoid excessive losses from the pool and prevent condensation we need to keep the temperature in the pool 24 hrs/day. It is therefore necessary to have the hot water from the boiler available on command.