Help to understand heating system with two controllers and two circulation pumps

[optilude]

Hi,

I’m trying to get my head around the plumbing in a new house. I intend to install a Tado smart thermostat system, but unlike my previous home, this one has two programming units and I am confused about how they interact and what one of them actually is for. The first part of my question is to understand the function of each pipe around my hot water cylinder and specifically the pump that is controlled by the secondary programming unit. The second part is to consider how best to install the Tado system.

There is a Gloworm Ultracom 38hxi gas boiler in the garage, and a Megaflo cylinder (I believe CL300, but I can’t see an actual model number, just a handwritten serial number) in the airing cupboard upstairs.

Next to the cylinder, there are two programming units. A Danfoss FP715 is the primary unit with timers for both radiator heating and hot water. I can see cables coming from it going to two separate (metallic) motorised valves, one (red) Grundfos circulator pump, and the indirect thermal controls on the Megaflo. This unit will be replaced by the Tado wireless receiver unit, so that it can control heating and hot water in accordance with the Tado app, coordinating with individual smart radiator thermostats for each radiator in the house (replacing their manual TRVs).

A separate Danfoss TS715SI unit has radiator timer controls only. It is connected only to a second (yellow) Grundfos circulator pump. My main concern is trying to understand how this interacts with the primary unit.

I have identified where the mains cold water comes in, through a 22mm (I think) pipe with a stopcock. It appears also to continue to another part of the house down a second 15mm (I think) pipe, before going up into the filling loop and a pressure vessel mounted on the wall, and into the water inlet at the bottom of the Megaflo.

I have also identified the hot water flow from the gas boiler, which is circulated by the red Grundfos pump and enters the Megaflo indirect heating coil, and also runs to a bypass pipe, a pipe labelled “radiators”, and a pipe I don’t understand (see below). One of the (metallic) motorised valves controlled by the FP715 primary unit sits on the pipe labelled “radiators” and the other on the flow into the indirect heating coil of the Megaflo, which makes sense.

I’ve also identified overflow pipes from both the Megaflo and the pressure vessel and the way in which the filling loop adds extra water to the boiler circuit.

However, two pipes/areas confuse me:

The blue line drawn here shows the mains cold water in and the yellow circulation pump that appears to be wired to the secondary programming unit, with red manual valves above and below it. It comes off the same pipe that is feeding the Megaflo inlet (bottom right). This pipe above the valves just above the stopcock, is warm to the touch (but not as hot as the hot water coming out of the top of the Megaflo ). I don’t understand what the yellow pump is for, or even whether the water is flowing “up” from the floor or “down” through it.

Can someone explain what’s going on here?

The second point of confusion is the red line. This is connected after the red circulation pump (which is connected to the primary programming unit), in parallel with the pipes going to the indirect heating coil in the Megaflo, a pipe labelled “radiators” and a pipe labelled “bypass”. This mystery pipe is labelled “TSAIR/RADS” (I think, it’s hard to read).

What is this pipe for?

And finally, the real exam question… if I replace the primary programming unit with the Tado receiver, what should I do with the secondary one that is controlling the yellow pump?

The Tado can open and close the motorised radiator and hot water valves, and determine the operation of the (red) circulator pump. One option would be to leave the secondary unit in “always on” mode (it is set to work on a timer currently) but this feels wasteful or worse. Each radiator will have its own motorised TRV, which will coordinate with the central Tado thermostat to turn on heating if one or more of them need it, but (unless I wire them in parallel?) Tado won’t be controlling the second pump. Another option might be to have a second Tado receiver unit replace the secondary controller, though I’m not sure if Tado supports this. Tado customer service is great at answering questions, but at the moment I don’t understand enough about what is going on to formulate a precise question to them.

Thank you for any insight and advice! Some extra photos below, including a top-down view.

 

[DP]

Instal your Tado on FP 75.

Yellow pump is secondary circulation. Has nothing to do with heating or hot water
Its function is to keep the hot water circulating in the hot pipes to the hot taps so you do not have to wait for hot water when hot tap is run and timer associated to it would be programmed when you are at home and likely to use hot water

Cold mains is not circulated by that pump as shown by blue line. Yellow pump is connected between end of the hot pipe run and cold water into the base of the cylinder. Circulation is set up in this loop and also ensures complete cylinder is now at set temperature. Yellow pump is known as bronze pump.

Look at Honeywell Evohome. This is my preference and you also have the option to control hot water from the controller etc

Leave the TS ( single channel programmer) to control the bronze pump. You will gain little by fitting a Tado.
You could replace this timer with a stand alone Tado programmable thermostat to control the bronze pump ie say set at a high temperature. The advantage would be geofencing the programmer has so bronze pump runs when you are home

Bypass. It is there to allow circulation of heating water from the boiler when both valve shut down and pump needs to circulate water since pump run is controlled by the boiler and boiler is hot so pump keeps on running to dump excess heat. The bypass allows that.

Something you have not noticed is existence of balanced feed which often is not provided. Almost all of your hot and cold water taps will be running at the same pressure

 

[Johntheo5]

Is that yellow pump just doing destratification or de stratification or/and secondary recirc?, normally secondary recirc returns the water ~ midway to the cylinder.

 

[optilude]

Instal your Tado on FP 75.

This explanation is brilliant. Thank you!

Yellow pump is secondary circulation. Has nothing to do with heating or hot water
Its function is to keep the hot water circulating in the hot pipes to the hot taps so you do not have to wait for hot water when hot tap is run and timer associated to it would be programmed when you are at home and likely to use hot water

That makes sense. And means I can just keep it as it is.

Cold mains is not circulated by that pump as shown by blue line. Yellow pump is connected between end of the hot pipe run and cold water into the base of the cylinder. Circulation is set up in this loop and also ensures complete cylinder is now at set temperature. Yellow pump is known as bronze pump.

Look at Honeywell Evohome. This is my preference and you also have the option to control hot water from the controller et

For what it’s worth, Tado also has hot water control.

Leave the TS ( single channel programmer) to control the bronze pump. You will gain little by fitting a Tado.
You could replace this timer with a stand alone Tado programmable thermostat to control the bronze pump ie say set at a high temperature. The advantage would be geofencing the programmer has so bronze pump runs when you are home

I don’t think this is worth the cost but interesting to consider.

Something you have not noticed is existence of balanced feed which often is not provided. Almost all of your hot and cold water taps will be running at the same pressure

Could you help me understand where it is and how this works, out of curiosity?

I am an also still confused about the pipe that is labelled “TSAIR/RADS” (possibly misread, it’s partially worn off) that runs down into the floor in parallel with the bypass pipe. This was drawn with a red line in the photo. Any ideas?

 

[DP]

Deleted

 

[DP]

Could you help me understand where it is and how this works, out of curiosity?

Blue line.
Pipe comes up from the floor, terminates on the stop valve. From here it connects to pressure reducing valve which reduces ( hopefully excess pressure) to working pressure around 3.5 bar. The pipe that exits to the left is balanced cold supply to cold taps.

I am an also still confused about the pipe that is labelled “TSAIR/RADS” (possibly misread, it’s partially worn off) that runs down into the floor in parallel with the bypass pipe. This was drawn with a red line in the photo. Any ideas?

Outlet from red pump connects to three components, two motorised valves, one for heating and other for hot water. Third connection is to bypass valve that will allow circulation to the return to the boiler

Evohome hot water kits has better facilities.
Whole system is more versatile than Tado
For example, I have full functionality at the control that Tado does not

 

[optilude]

Outlet from red pump connects to three components, two motorised valves, one for heating and other for hot water. Third connection is to bypass valve that will allow circulation to the return to the boiler

There are actually four. Better picture here:

The hot water comes in from the floor into the red pump.

On the left it goes down through a motorised valve into the floor. This i assume is for radiators.

Further to the left it bends around through another motorised valve and into the primary flow of the cylinder.

On the far right is the bypass pipe with the pressure reducing valve, labelled thus.

But the second pipe from the right confuses me. This is the one that has “TSAIR/RADS” scribbled on it.

 

[DP]

There are actually four. Better picture here:

How many room thermostats do you have?

 

[optilude]

How many room thermostats do you have?

Only one, in the downstairs hallway.

Also note that - in the airing cupboard at least - there is no motorised valve on this mystery pipe.

There is underfloor heating in all the bathrooms with their own thermostats but I have always assumed they are electric. Each bathroom has a small display and an isolator switch at floor level. I’ve not actually used them yet owing to the season!