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What connection has the cold water distribution to a washing machine and laundry sink have to do with my heating? There is a tap-off from one of the flow pipes for the heating which goes through what I've been told is a 'balancing valve' to the cold water supply to these outlets. To have hot supply to heating connected to cold supply sinks etc. doesn't make sense to me but, as with other things plumbing (and electrical), what seems illogical to me is totally correct to those who know what they're doing. I would be grateful if anyone could give advice on this.

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If you post a link to your picture we can edit your question to include it –  Steven Dec 17 '12 at 16:57
    
Radiant cooling? ;) –  The Evil Greebo Dec 17 '12 at 17:20

3 Answers 3

As longneck mentions this is a pressure reducing valve and this connection of the domestic cold water to your heating system is called the system make-up. It used to fill the system and to maintain the heating system's pressure. The knob on the top can be turned to adjust the system's pressure and should be done while the system is static.

However, a pressure reducing valve is not a cross connection device and should not be considered as one. While by its nature it creates a lower pressure on the heating side then that of the domestic side (therefore not allowing the water to back feed due to the pressure difference), if there is a drop of pressure on the domestic side (the required significance would depend on the domestic system pressure) or if the safety mechanisms on the heating side fail (such as the boiler fails to cut off and the pressure relieve valve fails to open) the heating side can result in a higher system pressure then that of the domestic side. This valve will not prevent back-flow in such scenarios. For true cross connection prevention you would need a double check valve or a reduced pressure back-flow assembly.

One last note: if you shut off the system make-up be sure to periodically check that the heating system maintains its set pressure. If the system drops too low you could cause damage to your heating source if it doesn't have built-in low water cut-off.

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are you saying that the particular component used is not the proper one for the application because it can allow backflow under certain circumstances? –  mac Dec 20 '12 at 19:50
    
All I am saying is that I agree that the valve shown in the photograph looks to be a pressure reducing valve and this would be expected in this application. However a pressure reducing valves should not be considered as a form of back-flow protection as they are not designed as such. –  pdd Dec 20 '12 at 21:37
    
In addition, depending on location/year of install it is quite often the case that in a residential application no back-flow devise would be installed as there is no chemical treatment being added to the heating system and risk of back-flow is minimal. In a commercial application there would be chemical added and thus a reduced pressure back-flow assembly would be required. –  pdd Dec 20 '12 at 21:38

The looks like a pressure reducing valve. Is there also a shutoff valve on the cold side? There should be, and it should be closed.

The connection to the cold water plumbing is used to fill the radiant heating system with water. The pressure reducing valve serves two purposes: 1) it limits the incoming water pressure to the level required by your heating system, and 2) it acts as a one-way valve to prevent the water in your heating system from contaminating your domestic hot water.

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A pressure reducing valve is not a cross connection device and should not be considered as one. See my answer for explanation as it was too long to be posted as a comment. –  pdd Dec 20 '12 at 18:55

Here's an attempt at a simplified answer. Your heating system has to get filled with water at some point. In normal operation, it will be re-heating and re-circulating the same water, and have no interaction with your domestic water supply. However, if the heating loop loses water (e.g. if there is a leak, or if it is drained, or if you extend the system) there needs to be a way to add water into the system.

Because the heating loop should be kept at a limited pressure, this is typically done with a pressure-reducing valve, as you have pictured. This lets the higher-pressure cold water supply flow into the heating system, but only when the heating loop is below a certain pressure. This avoids over-pressurizing your heating system, which would lead to premature wear and failure.

In addition, there is usually a backflow prevention valve installed between your cold water supply and the pressure-reducing valve. This ensures that even if there is an unexpected drop in cold water supply pressure, your heating water is not sucked into your domestic water supply. You want to avoid that as your heating water has been sitting for a long time in pipes and fixtures that may not be rated for domestic water, and you don't want to drink that. A backflow prevention valve is purely a safety device, though, and may not have been required when your system was installed (although US building codes do now require them for new work).

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