Intergas Xtreme combi with rads, UFH and evohome Opentherm

[thecarfixer]

Hello all,

After speaking with Intergas technical about running an Xtreme combi with rads and UFH, they suggested that a NO zone valve on the inlet to the UFH powered by X4 terminal 3 so that when the DHW is called for, the UFH is isolated to avoid undesirable flow through the heat exchanger (other than the DHW which isn't pumped). They then suggest a NC zone valve on the rad circuit so that when the controls call for radiator heat, the heating circuit is opened up.

They gave me a wiring diagram which would work with a BDR91 as that output could pull the valve open, which would then call for heat from the boiler. This would work for radiator heat, and I plan to use a HCCR80 for the UFH, so would have to use the boiler output of this to ensure that the UFH could independently call for heat without driving the rad zone valve open.

I would like to run Opentherm, though, given it's a bang up to date Intergas boiler. My question is this - how would I drive the motorised zone valve for the rads open in this scenario? How do people use evohome, with an opentherm bridge, when running zone valves in a system?

[thecarfixer]

This might belong in the Honeywell + Opentherm forum, please feel free to move..

[Richard]

If you are getting cross flow through the heat exchanger, then something is wrong with the system? In terms of an UFH system, it should be hydraulically separated from the heating system. Sounds like Intergas are trying to solve system design issues with a brick, rather than fixing the issue with the system?

[thecarfixer]

Thanks, this system hasn’t been built yet, I was just querying what is suggested in the manual.

What would you suggest, low loss header?

[Richard]

Thanks, this system hasn’t been built yet, I was just querying what is suggested in the manual.

What would you suggest, low loss header?

Any UFH system attached to a gas boiler will flow at least twice the flow rate that the boiler needs to. Therefore, you normally need hydraulic separation to cope with the mismatch in flow rates. Personally I prefer small buffer cylinders than LLH, as they help to prevent cycling as well as hydraulically separate.

[thecarfixer]

Thanks, I’m interested to understand the approach. Doing research it seems that most people (underfloor heating suppliers, boiler makers etc) propose using an S plan style system with a two port valve on each ‘zone’ (ie one for UFH and another for rads in the case of a combi boiler system)..

My current system hasn’t even got this, just a two port valve in the teed off flow leading to the UFH manifold. So presumably that’s not the best approach?

[Richard]

Thanks, I’m interested to understand the approach. Doing research it seems that most people (underfloor heating suppliers, boiler makers etc) propose using an S plan style system with a two port valve on each ‘zone’ (ie one for UFH and another for rads in the case of a combi boiler system)..

My current system hasn’t even got this, just a two port valve in the teed off flow leading to the UFH manifold. So presumably that’s not the best approach?

Its a broad subject, but in my experience even the UFH suppliers don't know what they are doing. I define myself as a system designer, so I think of all the components and how they operate together. Just bolting things together won't make the system efficient. Needs designing from the boiler downwards.

[thecarfixer]

Makes a lot of sense, and not surprising that UFH manufacturers are not designing as a whole; they’re just thinking of ‘their’ system.

In terms of a buffer cylinder, what sort of size are you generally thinking of? I’m presuming that like the use of a LLH, you’d then be running a pump/circulator for the rads? I’m not sure I’m following how it would look on a schematic.

Thanks for your help so far, hopefully this discussion will help others too.

[Richard]

Makes a lot of sense, and not surprising that UFH manufacturers are not designing as a whole; they’re just thinking of ‘their’ system.

In terms of a buffer cylinder, what sort of size are you generally thinking of? I’m presuming that like the use of a LLH, you’d then be running a pump/circulator for the rads? I’m not sure I’m following how it would look on a schematic.

Thanks for your help so far, hopefully this discussion will help others too.

Correct. Got a customer at the moment who has just had all the UFH fitted and now asking about an Intergas boiler and cylinder, but all controls are on/off switching.

It all depends on your heating load and how low the boiler can modulate. But in my experience a 50-70 litre buffer is sufficient to prevent short cycling with evohome and low load conditions (you can have much bigger if you want and add solar thermal or PV diversion to it). But yes, you would need a secondary system pump like a LLH would.

[thecarfixer]

Thanks, that makes sense.

And in terms of controls; I’m thinking through - how would the secondary pump be activated when using an opentherm gateway? I know I’d drive the UFH pump from the HCC80 pump output, but not sure how to drive the rads secondary pump when any radiator is calling for heat?

[edit] doing some reading and thinking; I am guessing what I use is the opentherm gateway on the boiler, then a BDR91 run in a zone valve configuration to open a valve and run the pump from the switch contacts in the valve?