Inadequate Circulator Pump?

[JC22024]

My house is a 1970s build fairly well insulted detached 2 storey. Split in 2no zones which are controlled independently with digital thermostats and rarely run at the same time.

Downstairs has 9 double rads and 5 single rads.

Upstairs has 5 double rads and 1 single rad.

Open vent system std heat only oil condensing boiler 26kw.

About 10 years ago I swapped in a Grundfos Alpha 2 smart circulator pump. At the time this was in part due to radiator noise after installing TRV valves throughout the property and it did cure the noise.

Unfortunately I did not pay great attention to the balancing of the system. And only now when a new condensing oil std boiler has replaced the old one am I properly balancing the system again.

The pump is set to max constant speed 3 and showing about 38wattts power consumption on the display. Trouble is it does not seem to have nearly enough oomph to drive the heating system!

I noticed that of I turn most rads nearly off but still warm to touch I can get them all to show heat. But when I start to set the first ones to approx 11c drop across the other rads quickly die again and the boiler outlet pipe sits piping hot, return cool to touch and heat not going anywhere.

I think either the grundfos is starting to fail or I simply need a larger spec circulator pump in order to drive the water sufficiently around the house. Boiler located at one end of the house. 6 bedrooms house. So it has quite a path to go around.

Any recommendations for a replacement pump? Probably larger 180mm wide 8m head std unit not a smart one for a large property?

 

[ShaunCorbs]

Grundfos upm3 25-70 should help

 

[JC22024]

Grundfos upm3 25-70 should help

Thanks for that I looked it up there. 6.5m head is slightly higher than the Alpha 2 but some other pumps at 180mm wide have much higher output:

Central Heating Hot Water XTRA Large Capacity Circulation Pump by Trident. | eBay

TRIDENT WRS25/80-180 Central Heating Hot Water Large Capacity Circulation Circulating Pump. PREMIUM WATER PUMP. Max flow: 130L/min. Mode: 3 adjustable mode(power:248/200/150W,flow:60/70/80L/min,head:8/6/5M).

www.ebay.co.uk

Eg the grundfoss upm3 has 3.5m3 per hour capacity.

7.3m3 per hour capacity for the Trident pump above at max speed. Which I could turn down as needed.

I like Grundfos brand but I may need a larger flow pump.

 

[John.g]

If the new boiler is also 26kw then there should be no change in rad outputs with existing pump. If the boiler is running at 26kw continuously then a flow rate of 2.0M3/hr @ a dT of 11C which even a 6M pump will circulate at a head of ~ 3.5M. You possibly have a failing pump, can you post a photo of your one as there are a few different models, one at 38W on full speed = circulation rate of 5LPM at a 6M head, highly unlikely.
IMO a 8M Grundfos or preferably a 8M DAB Evosta should be quite sufficient for your needs bearing in mind that you have a 26kw boiler.

 

[ShaunCorbs]

Thanks for that I looked it up there. 6.5m head is slightly higher than the Alpha 2 but some other pumps at 180mm wide have much higher output:

Central Heating Hot Water XTRA Large Capacity Circulation Pump by Trident. | eBay

TRIDENT WRS25/80-180 Central Heating Hot Water Large Capacity Circulation Circulating Pump. PREMIUM WATER PUMP. Max flow: 130L/min. Mode: 3 adjustable mode(power:248/200/150W,flow:60/70/80L/min,head:8/6/5M).

www.ebay.co.uk

Eg the grundfoss upm3 has 3.5m3 per hour capacity.

7.3m3 per hour capacity for the Trident pump above at max speed. Which I could turn down as needed.

I like Grundfos brand but I may need a larger flow pump.

Where did you find the m3/h as the listing says it replaces the 15-60

The upm3 is roughly double the flow as your existing one 15mm - 25mm opening and impella

The thing you have to be careful of is your open vent so with too big of a pump it will pump over

 

[Chuck]

Just a thought but... does your system have an automatic bypass valve that might be short-circuiting the circulation because it hasn't been properly adjusted with the new boiler in situ, perhaps? Symptoms can be superficially similar to a weak pump.

 

[JC22024]

Where did you find the m3/h as the listing says it replaces the 15-60

The upm3 is roughly double the flow as your existing one 15mm - 25mm opening and impella

The thing you have to be careful of is your open vent so with too big of a pump it will pump over

Fair point on open vent - I found the specs on screwfix direct website. Grundfos are not very good at supplying figures in the data sheets! I will take a better search later today on specs of the upm3 it does sound good.

This is the old pump

 

[JC22024]

Just a thought but... does your system have an automatic bypass valve that might be short-circuiting the circulation because it hasn't been properly adjusted with the new boiler in situ, perhaps? Symptoms can be superficially similar to a weak pump.

Thanks- there is no mixing or bypass valve. The house has open vent system. In boiler room there are 28mm pipes which serve ground floor. Teed off of that is 22mm pipes for first floor. Motorised zone valves provide the control.

The house used to be one zone no stats. I updated it years ago to 2no zones for more control and efficiency. The TRV valves were also added and at some stage noises were heard from valves with old pump. A smart pump was solution or so I thought may have to consider a bypass valve if new pump doesn't play well.

 

[JC22024]

Boiler room is being tidied up, painted and pipes insulated now. But here is older photo showing install generally-

Return goes into bottom right of boiler. Flow out at top left. Vent is coming in at top right. Negative pump pressure at it with pump on outlet side of boiler. So no overflow issues at present. (Used to be years ago until pump was moved)

 

[ShaunCorbs]

Fair point on open vent - I found the specs on screwfix direct website. Grundfos are not very good at supplying figures in the data sheets! I will take a better search later today on specs of the upm3 it does sound good.

This is the old pump

View attachment 67134

Screwfix don’t stock Trident pumps ? Also they don’t stock upm3