System Circulator Advice Required - Four Storey House

[snodge]

I have a new-build property with underfloor heating on four floors. The boiler is in the basement, and the height between the basement circulator and the top floor manifold is around 6.5m. I estimate the total potential flow of the UFH system at 75 litres / minute based on the settings of the various flow gauges on the UFH manifolds (it's a big house). The supply loop from the boiler to the manifolds is 25mm. The problem I have is that under high demand (around 50 litres / minute - I don't heat some of the rooms) I can't get the heat to the upper two floors until the lower floors are satisfied and some zones shutdown. Example on recent testing being 45 degrees basement manifold, 45 ground floor, 40 first floor, 35 second floor.

The circulator that has been fitted by the builder's plumber is a Selectric 15-60 130, running at speed 3. I've checked the specs of this circulator - am I right in thinking it is woefully under-specified to meet peak demand?

The manifolds have their own circulators, Selectric 15-50 130, running at speed 1. Changing the speed of these doesn't make any difference to the problem.

I have suggested to the builder that something like the Magna 25-100 would be more suitable to cope with full demand and provide the necessary head. Am I heading in the right direcction with this suggestion, or is it over specified?

Any advice would be appreciated so that I can point the builder in the right direction of what needs doing to sort it.

 

[kimbo]

A 15/60 is only good for a 6m head I would say you would need a 25/80 (8m head) but if you ring grundfoss they will tell you exactly.

 

[snodge]

A 15/60 is only good for a 6m head I would say you would need a 25/80 (8m head) but if you ring grundfoss they will tell you exactly.

Thanks for the reply. I've just had a look at the performance graph for the 25/80 and at 1.25 l/s (75 l/m) it can only manage 5m head. That's why I suggested the Magna 25/100 which looks like it can do 7.5m at that flow rate. As you say, Grundfos will be able to give a definite answer, was just hoping to get my facts right about the cause of the problem so I can point the builder in the right direction.

 

[kimbo]

I didnt look at the graph but you are absolutely right a 15/60 is out of the question

 

[Howsie]

Awful builder if you have to do his job for him IMHO

Sent from my GT-I9100 using Tapatalk

 

[snodge]

Awful builder if you have to do his job for him IMHO

Sent from my GT-I9100 using Tapatalk

To be fair to him he's being helpful over it - they're only a low volume local builder and my house is an exception to what they normally build. The plumber has been twice to have a look at things but I think the size of the system and the head requirement are a bit out of the ordinary to the his normal work. He did say to me that the 15-60 was what he normally fitted to UFH sytems - it's not until you start to add up the peak flow requirement that you realise how inadequate it is.

 

[Howsie]

Well it sounds like you have a good relationship with him but wouldn't you expect him and his plumber to be doing the donkey work? After all you're paying them... 😉

 

[Gray0689]

I'd be going with what you have said grundfos magna 25-100 as there modulating set it to auto adapt
I'd also be going for alpha pumps on the manifolds
If you can I'd love to see some pictures of the set up they have gave you

 

[doitmyself]

The boiler is in the basement, and the height between the basement circulator and the top floor manifold is around 6.5m. I estimate the total potential flow of the UFH system at 75 litres /minute based on the settings of the various flow gauges on the UFH manifolds.

1. The distance from the circulator to the top floor manifold is completely irrelevant to the required pump head. The head delivered by a pump is the pressure it supplies to overcome the friction within the pipes, which depends on the size of pipe and the flow rate through the pipe. You can work out the head by using the instructions in Copper Tubes in Domestic Heating Systems

2. A flow rate of 75 litres/min is equivalent to a 104kW boiler, assuming a 20C differential. Are you sure you have this right?

 

[snodge]

1. The distance from the circulator to the top floor manifold is completely irrelevant to the required pump head. The head delivered by a pump is the pressure it supplies to overcome the friction within the pipes, which depends on the size of pipe and the flow rate through the pipe. You can work out the head by using the instructions in Copper Tubes in Domestic Heating Systems

2. A flow rate of 75 litres/min is equivalent to a 104kW boiler, assuming a 20C differential. Are you sure you have this right?

Thanks for explaining 1. - I'll read up on it.

Regarding 2. - its a 12kw ground source heat pump, feeding a buffer tank, which then feeds the system. Supply is 45C, return is around 8C lower. The boiler has been sized for the property's peak heat loss of 17.5kw, with the addition of a gas boiler connected to the buffer tank to cover the shortfall at peak demand. The figure of 75 litres/min is what I have totalled up from the current settings of the flow gauges on the UFH manifolds.

 

[doitmyself]

Regarding 2. - its a 12kw ground source heat pump, feeding a buffer tank, which then feeds the system. Supply is 45C, return is around 8C lower. The boiler has been sized for the property's peak heat loss of 17.5kw, with the addition of a gas boiler connected to the buffer tank to cover the shortfall at peak demand. The figure of 75 litres/min is what I have totalled up from the current settings of the flow gauges on the UFH manifolds.

A 12kW heat pump working with a temperature differential of 8C, will have a flow rate of 21.5 litre/min. Assuming the boiler supplies all the 17.5kW with a 8C differential, the flow rate will be 31.4 litre/min.

Modern condensing boilers are all designed to work with a differential of 20C. In this case the flow rate for a 17.5kW boiler would be 12.6 litre/min.

Without seeing a diagram showing how everything fits together, I don't see how you can just add up the flows from the various UF sections.

Have you designed the installation yourself?

 

[snodge]

A 12kW heat pump working with a temperature differential of 8C, will have a flow rate of 21.5 litre/min. Assuming the boiler supplies all the 17.5kW with a 8C differential, the flow rate will be 31.4 litre/min.

Modern condensing boilers are all designed to work with a differential of 20C. In this case the flow rate for a 17.5kW boiler would be 12.6 litre/min.

Without seeing a diagram showing how everything fits together, I don't see how you can just add up the flows from the various UF sections.

Have you designed the installation yourself?

No I haven't designed it myself - if I had it would be working properly! I've already surmised the problem from the previous post - too much flow and the flow gauges need trimming so the flow is correct for the length of pipe, which isn't how it's been setup. I think the plumber has thought "that's a big room, stick that on 2½ litres, that's a small room, stick that on 1 litres"! With 45 loops in the building that's how I've arrived at 75 litres.

I've already made some changes to the flow rates this morning, estimating the length of pipe in a room, and adjusting according to the manufacturer's recommended flow for the length. Some of them were way over. I'm pleased to report that as a result of this, I've managed to get the heat to the top floor, and the temperatures in the rooms I've adjusted are maintaining. Result! It still needs sorting properly by the builder though.

 

[doitmyself]

I think the plumber has thought "that's a big room, stick that on 2½ litres, that's a small room, stick that on 1 litres"!

Similar to the way most installers go about "balancing" a rad system: open the nearest rad to the boiler by half a turn and increase it a bit at a time until the furthest rad is fully open.