Additional pump on a combi system

[Agile]

Can you explain why the boiler would be "prevented from working properly" bearing in mind the software wont 'see' an increase in flow rate only head due to the pump being fitted in series as opposed to parallel.

I cannot explain it in detail in words but I do know that a Biasi M90/96 loses its pump proving pin extrusion when a second pump is added in series to the flow!

Went to one under warrantee like that.

People dont seem to accept that the makers fit a pump which will be totally sufficient for its full heat output ( thats excepting Atmos of course ).

Tony

 

[mysteryman]

Fix the disease, not the symptoms.

 

[LondonChap]

I friend had this problem with a combi when the system was extended with rads in the attic extension. Replacing existing pipes under floors was not an option because of new flooring. The loop for the attic rads was a spur off the existing rad loop. The new rads had thermostatic valves on them. The engineer said there is two choices of pump location. One on the flow of the new spur to the attic or on the return to the boiler at the boiler. The pump was to be a Grundfos Alpha that turns off when all rads are throttled down. The engineer preferred having the pump on the spur loop as this is where it needed boosting and there was space for it. It was put there and switched from the programmer clock. It worked well and would wind itself off when the attic rad's thermostatic valves shut down. When the combi was on DHW the pump would stop if there was no water flow. The attic rads needed their lockshield valves screwing down to prevent robbing too much flow from the rest of the system.

The engineer said there may be some reverse circulation when drawing off DHW when the 3-way valve in the combi moved over, but none when in normal CH mode, so only a very minor problem.

It worked and gave no problems at all. He did offer a cheaper Wilo Smart pump, but the Grundfos was used because of reputation.

 

[Dan Robinson]

I suppose the other option is a plate heat exchanger and pump wired back to the boiler.

 

[LondonChap]

I friend had this problem with a combi when the system was extended with rads in the attic extension. Replacing existing pipes under floors was not an option because of new flooring. The loop for the attic rads was a spur off the existing rad loop. The new rads had thermostatic valves on them. The engineer said there is two choices of pump location. One on the flow of the new spur to the attic or on the return to the boiler at the boiler. The pump was to be a Grundfos Alpha that turns off when all rads are throttled down. The engineer preferred having the pump on the spur loop as this is where it needed boosting and there was space for it. It was put there and switched from the programmer clock. It worked well and would wind itself off when the attic rad's thermostatic valves shut down. When the combi was on DHW the pump would stop if there was no water flow. The attic rads needed their lockshield valves screwing down to prevent robbing too much flow from the rest of the system.

The engineer said there may be some reverse circulation when drawing off DHW when the 3-way valve in the combi moved over, but none when in normal CH mode, so only a very minor problem.

It worked and gave no problems at all. He did offer a cheaper Wilo Smart pump, but the Grundfos was used because of reputation.

I forgot to mention, the engineer said that it would be 100% safe in avoiding reverse circulation if the attic rads spur loop was teed off the flow of the existing system and the return from the spur loop teed back into the flow a few cm downstream from the flow tee. He said reverse circulation is eliminated 100%, but the rads further downstream on the main existing system would be cooler than the others on the warm up as the spur loop would be pumping its cooler water into the flow. When the system and house was near to or up to temperature, this was not an issue.

It reminds me of those old 1960s one pipe CH systems that were fitted upstairs to bedrooms while two pipes, flow and return, was used for downstairs rads which needed more heat to them. They worked fine, my mother had one that was faultless.

The engineer did tell me that he knew of one "cowboy" CH fitter who would fit all a domestic house in 15mm plastic pipe to the rads with a pump on the flow at the boiler and one on its return, in a push me pull you, with both on full speed. He would use a small amount of 22mm pipe at the boiler. The plastic pipe reduced water noises and vibration and as it bends easily few elbows were used improving flow. It was far cheaper for the fitter, saving money in 22mm pipe and fitting and saved him a day or two in installation time. The customers liked the quick installation. The engineer said he never once had a call back and they always worked, although a little slow in warm up. He said the quality of his work was fine, he just cut corners in cost. The home owners had to pay twice the electricity costs in pump running.

 

[Agile]

I forgot to mention, the engineer said that it would be 100% safe in avoiding reverse circulation if the attic rads spur loop was teed off the flow of the existing system and the return from the spur loop teed back into the flow a few cm downstream from the flow tee. He said reverse circulation is eliminated 100%, but the rads further downstream on the main existing system would be cooler than the others on the warm up as the spur loop would be pumping its cooler water into the flow. When the system and house was near to or up to temperature, this was not an issue.

Thats quite a good way of doing it and as he states will prevent reverse circulation if the pump is not working. The pump can then be stat controlled and creates an independent zone although only useable when the supply zone is heating.

Tony

 

[LondonChap]

I forgot to mention, the engineer said that it would be 100% safe in avoiding reverse circulation if the attic rads spur loop was teed off the flow of the existing system and the return from the spur loop teed back into the flow a few cm downstream from the flow tee. He said reverse circulation is eliminated 100%, but the rads further downstream on the main existing system would be cooler than the others on the warm up as the spur loop would be pumping its cooler water into the flow. When the system and house was near to or up to temperature, this was not an issue.

Thats quite a good way of doing it and as he states will prevent reverse circulation if the pump is not working. The pump can then be stat controlled and creates an independent zone although only useable when the supply zone is heating.

Tony

I thought so as it created a sub CH zone not an independent one. In doing it was more work in grabbing pipes and diverting them under floors. He said at least one rad had to be non-thermostatic down stream of the sub zone. The great thing was that as an Alpha pump was used even if all rads were thermostatic the pump would wind down in supplying the attic until the main zones thermostatic valves downstream of the sub zone opened up again.

As the attic rads had thermostatic valves fitted he said a wall switch could be in the attic to isolate the attic 100%. He mentioned a check valve may be used or zone valve for full isolation. It was all more complexity and needing space to fit the stuff. Attics tend to have void space nearing the eves, so in most cases that would be fine. If I was extending into an attic I certainly would consider a sub zone taken off and returned to the main zone's flow pipe.

EDIT: added some stuff