Dumb OT question

[skwerl]

Been doing some reading abut OT as will soon be installing a new boiler and want something that's compatible (leaning towards Viessmann at the moment as they seem to have the better OT integration, although happy to hear alternative views).

I have an S-plan setup with CS92 hot water control (Megaflo cylinder) and HR92s on all rads. Current boiler is a pretty old "regular" boiler, i.e. external expansion vessel, pump etc. and DHW and CH are controlled by the respective BDRs via one zone valve each.

I've read a few comments that, with OT, it's best to lock out/remove the CH zone valve:

Why is that? Surely this will mean that when the heating is off (ie all HR92s closed) and I have DHW on, the primary CH circuit will still be heated. Isn't that inefficient?

Without a CH zone valve is there really a need for an ABV? Doesn't the primary circuit give enough flow to negate the need for a bypass?

Thanks

[Lexden]

You might wish to post your question on the Automated Home heating forum as there are a couple of guys there who have a Viessmann 100/Evohome and Opentherm.

I am currently looking at replacing my boiler with an Atag. I already have Evohome connected to a 'S' plan. My chosen installer has quoted for removing the CH zone valve and fitting a two port valve to provide HW priority. He will also fit an ABV which the boiler manufacturer recommends.

[Richard]

Trying to divert my posts to AH Hengus!

I am sorry the forum seems like a ghost town at the moment, but we are currently inundated with orders and my attention is upon satisfying orders!

You misunderstand what the 2 port valve does (you are not alone). The 2 port valve just simply stops the flow of water around the heating circuit, just the same as the HR92's do when fitted to all radiators! HR92's fitted to all radiators means NO FLOW and when one does open the water flows just to this radiator because it needs to! So the ideal OpenTherm spec does not include 2 port valves, hence we recommend the one on the heating side is removed (and HR92's fitted to all rads).

The best setup for OpenTherm when you have stored hot water is a 3 port 'diverter' valve orientated to shut off the hot water port (best using hot water priority with OpenTherm).

When the issues Intergas have are resolved for OpenTherm multi zoning, I will be leaning very much towards Intergas for my go to OpenTherm boiler. Ideal and Viessmann are both very good but Intergas is my favourite because it is unique in its patented design and the only domestic boiler out there with a 20 year design/service life.

I hope that helps.

[Lexden]

Thanks Richard - I think. Glad that you are so busy.

As per the quote:

Install new 2 port valve to heating circuit to allow for a hot water priority set up to enable low
modulation flow temps for open therm control of central heating.

Modify wiring to suit new hwp and OpenTherm setup.

Comments?

[Richard]

Well, with another years experience and new learning with OpenTherm my personal preference is to have the 3 port diverter (https://theevohomeshop.co.uk/145-home_d ... -valve.jpg) as this gives you 'true' hot water priority (fast reheat of hot water cylinder) and means the heating port is always kept open until hot water demand is satisfied. When evohome calls for hot water demand the OpenTherm bridge tells the boiler to go to maximum output and you don't really want this water to go to the rads that may have just been controlled at 55, hence I recommend this setup now for new installations. I still very much believe in zoning all rooms with HR92's, however I am starting to recommend a small towel radiator open circuit just to allow some of the features of OpenTherm to work more in tandem with how OpenTherm systems were designed to operate. However (as a retrofit), removal of the heating motorised zone valve and HR92's on all radiators is a must.

I hope that is helpful.

[Lexden]

Richard - as always, your response is very helpful.

What I understand from my proposed installer is that his solution is a variation on the above. The heating zone valve is replaced with a two port valve which remains in the open position when heating is required. When hot water is required, the hot water zone valve opens AND the two port heating valve closes. Opening the hot water zone valve triggers boiler max output to the hot water cylinder only. Arguably, a slightly less elegant solution than the replacement of both zone valves with a 3 way diverter valve.

I have copied to your comments to my installer as I am keen to get the best solution. I should add that he is a listed as an 'ecotechnician' (as recommended by TB) and I have no doubt that he knows his stuff.