Central heating thermostat replcament - 2 wire/3 wire confusion!


Hi there,
Can anyone help me - I would like to replcae a 15 year old Potterton thermostat in my house. When I opened the thermostat up I found 3 wires: L, N and a mysterious 'H'. These go to terminals labelled TP1, TP2 and TP3 repectivly.
The thing is the new thermomstat I purchased (Danfoss RMT-230) has 4 terminals but the instructions only shows how to wire the L and N. What do I do with the 'H' wire and what is this mystery wire for??
Any help would be most appreciated!
Thanks Steve
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First paragraph (Thermostats and TRVs) may help: <http://www.diyfaq.org.uk/plumbing/controls/controls.html#thermostats
The following is a different brand but explains that the 4th terminal isn't used. <http://www.horstmann.co.uk/downloads/ElectronicDocuments/ThermoStats/HRT2UserandInstallationGuide.PDF
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Thanks for the information Phil.
So I have 2 options if I understand this (which may be doubtful!).
1. Connect L wire to terminal 1 and N to terminal 2. Presumably I just tape up the 'H' wire. If I do this then the resistor/accelerator is not used.
2. Connect L to terminal 1. Connect N to terminal 4 and H to terminal 2. Then the accelerstor/resistor gets used.
Is that correct? and is 2. th better choice?
Thank you Steve
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"Steve" wrote:

If you are referring to the terminals shown for the Horstmann thermostat, T2 is not used. L wire connects to T1 and N wire connects to T4. H wire if used connects to T3. Your option 2 would be the best choice apart from H wire should go to T3 and not T2.
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<http://www.danfoss-randall.co.uk/default.asp?strSearchResult=tem1019.asp?intPage12|intLang=2|strLang=|intLevel=2|intLevel2ID12>
shows you how to wire it up.
Adam
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