True - part L 1.38
I also notice many don't notice the independently timed requirement of part
1.39 and fit simple roomstats in the other zones when full timed controllers are legally required.
I don't see how that helps! It's the *boiler* which needs the flow to keep going after it stops firing in order to carry the residual heat away and prevent the over-heat cutout from tripping. No amount of electronic sensing can maintain the flow if there ain't anywhere for it to go!
When a boiler needs pump over-run, you *don't* wire them in parallel. The rooms stats and zone valve simply control the boiler, and the boiler controls the pump. The boiler has dedicated terminals for pump connection. The boiler also needs a permanent live as well as a switched live - so that it can run the pump when required even when it isn't firing. [A bit like an electric radiator fan on a car, which continues to run for a bit after you switch the engine off].
Well, Keston doesn't, and I don't recall it on others I considered at the time. An internal bypass would have difficulty meeting the requirements for minimum pipe run length before the bypass, which several boilers have (needs to be enough volume of water to absorb the heat produced before the burner shuts off and cools).
I designed my system to use a mid position valve between the two heating zones (and a radiator in each zone is the bypass with no TRV or hand control), so there is no shut-off position and always a water flow path.
Some (not Keston) have an extra electrical output for connecting to an additional pump, which might be useful for detecting pump run-on.
Another related issue... Many boilers have frost detection built in, and can fire up by themselves when there's no call for heat, and again will need a flow path. Also, some boilers will run the pump for a short time if it hasn't been used for 24 hours to stop the rotor from sticking.
Given that the bypass will usually only come into play when the pump is on overrun and there is no call for heat from any stat, so the boiler will be off. So who cares?
Utter nonsense...
calling
That's why you have 2 lockshield valves on the rad(s) in the room with the thermostat.
Yep
I think the pumps are okay running into a dead end provided there's no actual heat involved.
The DIYer voices his uninformed view. Yes, he does.
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You really don't know.
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"1.39 In most dwellings one timing zone divided into two temperature control sub-zones would be appropriate."
That is one time clock and TVRs on all rads, except the rad where a room stat is. Over 150 sq M floor area and it needs a separate times zone.
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Or when most of the TRVs are off but the stat is calling for heat?
Dave S
Having all the TRVs off tends to have the same effect. The purpose of having a bypass in to ensure a flow is maintained through the boiler and to cool hot-spots during the pump run-on.
An auto by-pass will open when the TVRs close down. Best have a Grundfoss Alpha pump and flow switch. The flow switch off the burner when the flow is below a certain setting. The pump continues.
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Yes precisely. So Drivel's argument that "By-pass valves defeat the purpose of a condensing boiler" is nonsense. As you say, the only time it will be passing water (assuming its correctly adjusted) are in situations where there is no alternative flow path and the boiler ought not be firing anyway.
This can happen when there is no call for heat on pump overrun. It could also potentially happen if the installer has stuck a TRV on the rad in the room with the stat - then there is the possibility that you will still have demand for heat with all TRVs closed. That is a problem with the installation though, and not a basic incompatibility between bypass valve and boiler technology.
Total tripe. All TVRs could be down and only the downstairs toilet slightly open. A call for heat. The by-pass will still open as the flow through the rad will not be enough for 90% of boilers. Many rads may be calling for heat and the by-pass can still be open. Then the spring wears in time, making it open even more. They are a waste of time and should be avoided. Makers put them on boilers in order to protect themselves from idiots. It is easy to design them out.
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Ah - you've sussed out the reliability of these matches certain makes of boiler electronics :-)
So you routinely fit the main room stat in the downstairs loo then?
Yup that figures.
You really don't understand.
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