New DHW pump making radiators hot

[MNW67]

This is about my elderly auntie.

She lives in a very large dormer bungalow. There is a 45KW conventional boiler. There are three pumps, one for downstairs, one for upstairs and one for the hot water cylinder. The cylinder is on the same floor as the boiler and the pumps, and is only about ten feet away from them.

In December, BG replaced the pump for the cylinder, under a home care plan. Now we are in the summer, she has realised that when the cylinder is on, the downstairs radiators get red hot. This has only happened since the pump was replaced. Unfortunately, in the period in between, she cancelled the BG plan because she had a new boiler fitted last year and didn't think she needed the plan.

My auntie doesn't know what sort of pump she used to have for the cylinder circuit. But I have looked today, and the replacement is not a new part. It is a 90W single speed OEM Grundfos pump which might have been salvaged from inside an old Baxi boiler. It says "SPARE PART" on it. It is the silver pump in the photo.

Is it possible that this pump is just too powerful and this is the cause of the hot water from the cylinder circuit going into the downstairs radiator circuit? Being a single speed pump, it is obviously running at the full 90W, and there is no way to turn it down. Wouldn't a modern, variable speed pump have been more appropriate?

I don't know what that motorised valve does, but when I tested it today the pipe below it wasn't hot, even though the radiators were.

Layout photo and details of the pump below.

Baxi Combi 105HE Pump 248042 GC-47-075-18

Baxi Combi 105HE Pump 248042 GC-47-075-18 59200702 Fully refurbished It comes with 12 months warranty and the product with good condition.

www.heatingandcateringparts.com

 

[terryplumb]

The zone valve for downstairs rads is probably passing water when it shouldn't.

 

[MNW67]

The zone valve for downstairs rads is probably passing water when it shouldn't.

Is that the motorized valve below the big red pump? How can I test if this is happening?

 

[terryplumb]

Is that the motorized valve below the big red pump? How can I test if this is happening?

That is indeed a motorised valve. Is it the only one ?
If the two radiator circuits , and the hot water cylinder circuit can be used independently of each other ,I would expect to see 3 motorised valves .

 

[cross thread]

Is the silver pump not pumping in a different direction to the ot 2 ?. They have a direction arrow on the body.

 

[conny]

Is the silver pump not pumping in a different direction to the ot 2 ?. They have a direction arrow on the body.

All the indicators are saying they run anti-clockwise. (Doesn't necessarily mean they are though, ).

 

[Harry Bloomfield]

I don't know what that motorised valve does, but when I tested it today the pipe below it wasn't hot, even though the radiators were.

Could the pump have been installed, so it is pumping in the wrong direction?

 

[MNW67]

That is indeed a motorised valve. Is it the only one ?
If the two radiator circuits , and the hot water cylinder circuit can be used independently of each other ,I would expect to see 3 motorised valves .

There's three pumps, but only that one motorised valve.

All circuits can be used independently. It's an older system, installed just before zone valves became common.

 

[MNW67]

Is the silver pump not pumping in a different direction to the ot 2 ?. They have a direction arrow on the body.

I'll check that tomorrow or this weekend.

 

[MNW67]

Could the pump have been installed, so it is pumping in the wrong direction?

I'll check that tomorrow or this weekend.

 

[MNW67]

I've just been to my auntie's again. It's obviously not a good day to test with it being so warm. But this is what I've found out so far.

The DHW circuit gets hot very quickly on the flow side. Nothing is leaking through the pump (the big red one) that works the downstairs radiators or through that motorised valve.

Very slowly the downstairs radiators seem to heat up. But I think it is happening on their return side. The common return is getting red hot, way further back before the return from the DHW circuit joins it (I can't test any further back). It is almost as if the water is flowing backwards through the downstairs radiator system, when the DHW pump is on. That sounds mad to me. Is it possible?

How do I check whether the pump head is on the right way round? Where is the arrow and where should it point? Thanks

 

[Harry Bloomfield]

How do I check whether the pump head is on the right way round? Where is the arrow and where should it point? Thanks

There should be an arrow cast into the pump casing. It should be pumping in the very same direction as the other pumps, either all towards the boiler, or all away from the boiler.

 

[Johntheo5]

The two red pumps havn't been touched? so if top one arrow is pointing upwards and the the bottom (red) one arrow is pointing down then the silver pump arrow should be pointing downwards. If the top one arrow is pointing down and the bottom red is pointing up then the silver ones arrow should be pointing upwards. Its probably the former., some systems are just zoned without any M.valves the call for heat just starts the pump but they will then have a non return valve on the pump discharge to prevent gravity circulation when off.