Vaillant EcoTec Plus 630 dT

[John Carroll]

This system boiler & LLH replaced a heat only Vaillant which had a 8M UPS 2 pump, a 140W giant of a pump which will circulate 52LPM @ 8M, 46LPM @ 7M & 35LPM @ 6M, there are 21 rads + UFH (+HW cylinder) and the pump had to be run at a 8M head to satisfy all the rads, so a toss up I suppose to go for the same set up or the system boiler with the LLH. The UPS 2 was installed on the secondary side of the LLH and satisfies all requirements at a 7M head, (and sometimes at 6M), the LLH takes care of the circulation through the boiler HEX which has a pressure loss of 2.6M @ the boiler's rated output of 30kw and flow of 21.5LPM at a 20C dT. The system is often run on UFH only for long periods but because of the above problems then the return temperature is 54C (flow temp 62C), if the dT control was working properly then the dT could be set to the flow temp minus the UFH return temp of ~ 42C, so to 62-42, 20C, or somewhere in between,  to give excellent boiler efficiency with a boiler return of 42C, unfortunately, this isn't happening.

On UFH only the (primary) boiler pump runs  at 80% speed with a dT of (62-54) 8C on dT control, the lowest setting on fixed speed is 50% which does give a improved dT of ~ 12C with a boiler return of 50C.

Edited March 1 by John Carroll

[JohnMo]

So you have two very different flow situations on primary and secondary sides of the system. This is cause huge flow temperature distortion. Basically you will have return from secondary side going back to the satisfy the flow to secondary side. 

 

 

[John Carroll]

Don't think so John, its the opposite, the primary side will satisfy the secondary side flowrate, the remaining primary side flowrate will recirculate on the primary side to raise the boiler return temperature?

[JohnMo]

I am reading the secondary flow is higher than the primary and that is the reason for the LLH. If that isn't the case why install the LLH?

 

If secondary flow is higher than primary then the distortion illustrated will be there. If it's the other way about primary flow is higher than secondary you get similar distortion, but with return temp to boiler higher than coming back from the secondary side.

 

Add zones on the secondary side you have one system that cannot do what you want or expect. Flow temp or return temp will always be compromised for the boiler side.

 

[John Carroll]

THe main and only reason in the above case is that because the boiler  circ pump head/capacity is too low to circulate the water at 21.5LPM through both the boiler HEX and the system. With the LLH, 80% boiler pump speed will/should circulate 21.5LPM through the boiler HEX and the secondary circ pump will circulate 21.5LPM through the system at ~ 7.0M head. Without a LLH then the (6M) boiler circ pump wll only circulate ~ 15.7LPM through both the HEX and the system. If the(with a LLH) boiler circ pump speed is increased to 100% then ~ 4.8LPM will "short circuit" from the boiler flow to the boiler return, rising the return temperature a little.

 

If UFH only on and the primary circ flow is still 21.5LPM with the flow/return to the UFH manifold at only 8.0LPM then a huge short circuit of ~ 13.5LPM which raies the return temperature considerably but if the primary flowrate was reduced automatically by reducing the pump speed via dT control then the boiler return temperature will be considerably less.

 

 

 

 

 

 

 

 

[John Carroll]

The big danger to rad output is if the secondary flowrate is higher than the primary for whatever reason, see below.

the rad output is 16.5kW if both flowrates are 20.83LPM but only 12.4kW if the secondary flowrate is double that of the primary.