Flow rate in central heating system

[edb49]

Hi everyone,

I've got a problem with the return temperature dropping too much for my boiler, and the safety cut out comes on. I've been working out the flow rate and came to the conclusion the pump was flowing about 0.32 litres per second of hot water. This is for large four bed detatched house.

The pump is a commercial one and is set on its highest setting:

Grundfos UPS 25-55 180

What sort of flow rate should I be getting in litres/second from the pump? Will the pressure in the system effect how much water the pump can flow?

Way I calculated the flow rate was the output of the boiler, and difference between flow and return temperatures:

Specific heat capacity of water:
4.2 kilojoules/kg

1 celcius raise requires 4.2 kilojoules/litre
:. 30 celcius raise requires 126 kilojoules/litre

Boiler output is 41kw in condensing mode, or 41kilojoules/second
:. boiler is heating 0.32 litres/second

 

[Biggles..]

based on a deltaT of 20degC you'd need approx 1.8cubicmetres per hour.

0.32l/S x 3.6=1.152cubicmetres/h

your pumps undersized for the KPa of your system.

I'll wait foir the knowledgeable D.Hailsham to confirm or not my calculations.

Lee

 

[edb49]

I can't seem to find much information on the spec of the pump. I've checked the system pressure and it's about 1.2 bar, will increasing this help the pump flow more?

 

[Biggles..]

system pressure wont alter anything im affraid.

try grunfos website for pump information and flow curves.be warned the sites not very easy to navigate

 

[edb49]

Yep, I've already been on the Grundfos website trying to get data, but it seems that you have to be an installer and they will post of spec data to you, I can't seem to download any public data.

 

[D_Hailsham]

What sort of flow rate should I be getting in litres/second from the pump? Will the pressure in the system effect how much water the pump can flow?

Your method calculation was correct

A 41kW boiler with a 20C differential required a flow rate of 41/(4.2x20) =0.49 litres per second or 1.76 m³ per hour

If you are getting a temperature drop of 30C then the flow rate is 41/(4.2x30) = 0.33 litres/ sec or 1.17m³ per hour.

This graph shows how the flow rate (X axis) of the 25-55 varies with pressure (Y axis). What you should be getting is the green line; what you are getting is the red line. The three lines represent the three speed settings. The pump will always work at a point on the line. So if the pressure is too high the flow rate will reduce, which is what is happening in your case.

UPS25-55

• D_Hailsham
• 23 Dec 2009
• 1

How flow changes with system pressure

However, the high pressure may be due to the system not being properly balanced. Read How to Balance a CH System, but ignore the first sentence as it does not apply in your case.

 

[edb49]

D_Hailsham, thanks very much for your detailed reply, certainly helps clarify a lot!

I'm 99% sure that balancing isn't what is causing the issue. I have three zone valves; radiators, underfloor and hot water cylinder. I've tried heating the hot water cylinder up from 15c or so, and get the same issue. (I get the same issue with all three zones independently.) The reason I tried the hot water is because it is the simplest loop of the system; the radiators go off and round the house, and the underfloor has a manifold system with an auxiliary pump - so the hot water is the simplest of the three.

 

[edb49]

I am assuming that I am right in saying that the hot water flow to a cylinder can't be balanced?

 

[Biggles..]

The hot water flow to the cylinder DEFFINATELY SHOULD be balanced.

sorry not asked but is this a system boilers own in built pump or a seperate pump.Just for ref what boiler is it.

 

[edb49]

It's a separate pump and boiler; the boiler is a Glowworm Ultracom 38hxi one, less than 6 months old.

 

[D_Hailsham]

It's a separate pump and boiler; the boiler is a Glowworm Ultracom 38hxi one, less than 6 months old.

I am beginning to suspect that your boiler is oversized.

How was the boiler size determined? Did the installer just base it on what you had previously or did he calculate the size based on heat loss etc?

If you want to check what size boiler you need, you can use either Sedbuk Boiler Calculator

or

EST online boiler calculator

Fortunately the 38kW version of the HXi can be range rated to reduce the output to what you need.

 

[edb49]

It's a separate pump and boiler; the boiler is a Glowworm Ultracom 38hxi one, less than 6 months old.

I am beginning to suspect that your boiler is oversized.

I've just run through the online calculator and got a value of 35kw for required size. The house is about 3500sqft, and it's a mix of solid wall from the 1700s, filled cavity from the 1970s, and modern 2004 construction with Kingspan insulation boards. There's single glazing that's 100+ years old, and recent double glazing. The roof insulation varies from none in the difficult to treat areas, to 300mm in the areas I've recently done.

I've tried range rating the boiler down to the minimum it will allow (7kw) but this doesn't actually seem to reduce its output. I might not be doing this 100% correctly so will check the manual now.

 

[edb49]

I've checked in the manual about range rating, and I think I have been setting it correctly. I can only think it may need a complete power off or similar for the range rating to become effective.

It does say in the manual that range rating isn't necessary as the boiler is auto-modulating. From this I take that it has some kind of logic that says "if flow temperature is below set point, output 38kw, if flow temperature starts to exceed set point, reduce output as required." From my reading of range rating, it would simply mean the boiler always outputs at X kw, either it is on or it is off. I suppose one solution to my problem would for the boiler to auto-modulate but for its maximum to be lower than 38kw - nothing in the manual about how to achieve this though.

 

[D_Hailsham]

It does say in the manual that range rating isn't necessary as the boiler is auto-modulating. From this I take that it has some kind of logic that says "if flow temperature is below set point, output 38kw, if flow temperature starts to exceed set point, reduce output as required."

You have a 38kW boiler and need 35kW, so there should not be any need to range rate, which is only necessary when there is a significant difference between the boiler output and requirement, say 38kW boiler and 10kW requirement. This often happens with combi boilers as the high output is needed to provide instant hot water.

From my reading of range rating, it would simply mean the boiler always outputs at X kw, either it is on or it is off.

That's not correct. All range rating does is to set the upper limit of modulation. So, instead of modulating between 6.3kW and 38kW, it will modulate between 6.3kW and, say, 20kW.

If you have set the output at 7kW, the boiler is running between 6.3 and 7kW. If it needs to go any lower than 6.3kW it will revert to on/off mode, which is what is happening in your case.

I would set the output to 35kW output. It should then modulate.

Another avenue to explore is the output of your radiators, to make sure they have the capacity to dissipate 35kW. Provided you have the standard flat steel rads, you can use the Stelrad Elite Catalogue to get a good idea.

You say you are getting a 30°C differential. What are the flow (d.40) and return (d.41) temperatures?

You said earlier that you were 99% sure that the problem was not due to poor balancing. However the pressure through the heat exchanger of the 38hxi is 4.05 metres (see diagram 5.1 in the installation manual). The maximum the pump can handle is about 5 metres, which leaves only 1 metre for the pipework and radiators. This is a very low figure - most small systems are about 2 - 3 metres; yours could be much higher. A slight error in balancing the system can have a significant effect on the pressure loss in the system. This is particularly true of a mixed radiator - underfloor heating system.

To summarise: I don't think it's a boiler problem - it's a pump problem. You really need an engineer who can calculate the pressure loss round your system and advise on the correct pump to use. If you want to have a go, read Copper Tubes in Domestic Heating Systems, which tells you how to calculate the "index circuit".

 

[Norcon]

I've got a problem with the return temperature dropping too much for my boiler, and the safety cut out comes on.

Why would the safety cut out activate with low return temperatures?

What F&R temperatures is your boiler achieving when system up to temperature?

What F&R differential temperature did you base the flow rate calculation on?

And finally, what exactly is this thread about?
A cold house or a hot house?