Best way to set system pump speed

[Kilt]

We replaced our thermal store last year due to a coil breaking, the return system pump was also changed. 
We’ve a Grant vortex oil boiler and 2 UFH manifolds. 
 

since the new thermal store went in, it struggles to get up to desired temperature. 
The thermal store company recommends a tank temp of 80°c but the max we can achieve from Grant is about 65°-68°. 
I’m asking on here, as due to the time of year original store went, we were extremely limited on available plumbers.. and who we had out, let’s say “didn’t inspire confidence”. 

The calculations show our Grant is a little underpowered for our house, however the system (return) pump is blistering, to hot to touch. 
Could system pump be sending water back to fast, so store doesn’t get chance to warm up or not fast enough?

Should there be a thermostatic valve limiting the water temp back? Seems daft sending +60°c water back to Grant. 

What’s best way to work out pump speed? Is there a desired temp drop between flow and return?

 

Diagram below is roughly our system. We don’t have crossed out sections. 

[PeterW]

Ok so.. is the boiler turned up all the way..? A Vortex should be able to get to 80c. 

 

And is the pump the right way round ..?? I’ve seen pumps installed the wrong way round and that would be meaning a very hot pump also pushing into the lower connections on the TS. Can you measure the inlet and return temps on the TS..??

 

 

[Kilt]

Pumps right way round. Grant is almost fully up, about 75%. 
It’s currently on its middle kW setting (18.5kW), we’ve scope to pump it up to max kW (21kW) on next service, just trying to balance out oil consumption. 
Low kW for long period or high kW for shorter!
 

After boilers had a good run +4h and all the UFH manifolds are satisfied, flow is about 69°c on outside of pipe, and return is about 60°.

Grant schematics suggest pump should be close to boiler on flow, whereas the thermal store (old and new) state pump should be on return. 

Edited November 4, 2020 by Kilt
Typo

[Fly100]

Had a few probs like this myself.Ive learnt a great deal. I aim to have my boiler flow 10-15 C above the UFH manifold setting, this has helped me out now end. You need to get that return temp down to below 47c or the boiler wont condense and you are losing a lot of efficiency.

 

Have you tried altering the pump speeds?  What was the old one set too ? 

 

[PeterW]

How big is the TS..? Grant will be out of condensing mode at 60c return and not sure what / where the tank stat is on the tank or what it’s set to in order to satisfy the boiler. I’ve seen TS systems with a blender to stop cold shock into the boiler but usually on solid fuel - is it purely hot to top and cold from the bottom on yours and the pump in the return ..? 

[Kilt]

TS is 210L by McDonald. 

 

manifolds are set to 40-45°c for downstairs scree and 47-50°c for the upstairs biscuit/timber. 
These were all messed up when we got bought house. (UFH manifolds are 20 years old and have a terrible thermostatic valve. Very accurate control, but no way of monitoring actual temp). 
We used to run out of Hot water with old tank, as TS stat was set to be 60°c. Great idea if you like legionnaires disease. 

 

 

Cylinder stat was set to 80°c by factory, but i set it to 70°c, in hope of satisfying the call for heat. 
ive re-done all the central heating wiring as it was an utter disgrace.

(Electrician had signed it off). 
 

yes, flow (hot) into top, return (cold) out bottom with pump. 
 

no pump on hot. 

[Kilt]

1 hour ago, Fly100 said:

Have you tried altering the pump speeds?  What was the old one set too ? 

 

Haven’t want to muck about with to many options at once. 
 

just recently bought a Testo clamp thermometer to try and trouble shoot UFH temps and flow rates. 
it was clearly just chucked in, with little thought to flow rates, of manifold. 
 

now I’m getting ontop of that, I’ve not started looking at the system. 
 

Around here I haven’t found a plumber who does Grant servicing, or a service engineer that does plumbing. Seems utter madness..

Edited November 4, 2020 by Kilt

[Kilt]

1 hour ago, PeterW said:

Grant will be out of condensing mode at 60c return

So what would be best practise to lower this?

 

slow the pump? I’ve an Alpha 2L 15-60 which was a direct replacement for the old Alpha. 

[Kilt]

Ok, so slowing pump to slowest speed achieves this diagram:

(Diagram with just with Hw on, no UFH pumps on, no hot taps on etc)

 

With old speed setting, I was getting max flow (hot) of 65-69°c. 
 

new (slowest) speed I’m getting these peaks and troughs of 65-85°c (on flow/hot). 
 

Boiler ok with these peaks and troughs? Would have thought it would be continuous output?

 

Return (cold) is still well above 60°c, closer to 70°c now. 
 

Edited November 4, 2020 by Kilt

[Kilt]

5 hours ago, PeterW said:

Grant will be out of condensing mode at 60c return

 

Ok... bear with me, I'm trying to learn more here.

 

The alpha has 3 mode settings, Proportional pressure, Constant Pressure and Constant speed.

 

The original plumber had pump set to constant speed (mid speed), which was achieving the low flow temp. Slowing constant speed has improved flow temp.

 

However.... Grundfos recommends Proportional Pressure for Rads, Constant Pressure for UFH and Constant speed for Hot-water (showers).

Grant recommend a modulation circulating pump.

 

 

Looking at the duty point, I can't quite figure out which setting is best/makes sense.

• Proportional Pressure -  Duty point moves up or down, and pressure reduces with demand.
• Constant Pressure - Duty points moves in or out, but head pressure remains same.

 

Constant pressure seems to make more sense in my head....

Edited November 4, 2020 by Kilt

[Temp]

I have a similar system but with a bigger boiler.

 

I run my store at about 60C to keep the return to the boiler low enough to be in condensing mode.

 

My understanding  is that the knob on the boiler sets the peak flow temperature .. the burner runs until it hits that temperature then shuts off. It will cycle lon and off like that if you let it until the store stat is satisfied. My preference is to not do that. I turn the boiler right up and increase the pump speed up until the boiler never quite starts cycling. Then when the store calls for heat the burner runs until its satisfied without cycling.

[Temp]

My feeling is that you could do with a pump speed between medium and slow. Not so fast that the flow is too cold. Not so slow that it cycles.

[Kilt]

On 05/11/2020 at 11:22, Temp said:

 

I run my store at about 60C to keep the return to the boiler low enough to be in condensing mode

Is yours an unvented system?

 

ours is vented so legionaries is always a concern. 
 

do you find full power best for your oil consumption?

 

always that dilemma, put your foot down and get there quicker.. or slow and long. 

Edited November 7, 2020 by Kilt

[SteamyTea]

1 hour ago, Kilt said:

always that dilemma, put your foot down and get there quicker.. or slow and long. 

From my understanding, condensing boilers are more like turbocharged cars.  They reclaim some of the energy that usually goes to waste.

If you run a turbocharged car at too low an engine speed, it is basically 'running rich' as there is not enough air to burn all the fuel.

A similar thing can happen at the top end as the fuel injection system tries to put in more fuel, but the turbo has 'run out of puff'.

So there is a sweet spot where all the fuel gets burnt, and maximum torque is produced.

Now a boiler is obviously not the same as a TDi.

Rather than having a 'rev range' is has a thermal range.  This is just a range of temperature differences where the boiler can transfer the most energy for the least fuel.

This is easier as a formula:

 

Q = U×A×ΔT

where, U is the overall heat transfer coefficient
A is the overall heat transfer surface area
and ΔT is the mean temperature difference between hot and cold side

 

The trouble is that the cold side i.e. the water is not a constant, it varies, and as it varies in temperature i.e. gets hotter, the coefficient U would have to changes to keep the efficiency the same.

As a rule, the flame temperature will be constant (within 10°C or so), A, the area of the heat exchanger within the boiler will stay constant, so all that can be changed is the temperature difference ΔT.

 

As heating is not a linear process but follows an exponential curve:

 

T(t) =1- t- exp(-k * t)

Where T is temperature, t is time and k is a constant.

This produces a chart that shows that when there is a large temperature difference i.e. cold store, the energy transfer is large, but as the store warms up, the energy transfer decreases, even though the fuel burnt per unit time, is the same.  Or in simpler language, the hotter you get, the more fuel you burn.

The trick there is to find a lower bound temperature that satisfies your hot water needs i.e. 40°C and an upper bound for safety i.e 60°C

Using the made up chart below you can find out the amount of time the boiler needs to run at (bare in mind this is showing % change, not absolute temperature).

 

So what you need to find is the range where the boiler is condensing, the water never goes below an acceptable temperature, and never goes above a safe temperature. And runs for the least amount of time, as I think oil boilers do not modulate (I may be wrong here, but the physics is basically the same, just another variable in the mix).

I have highlighted this with coloured lines.  So for the same temperature change between the blue lines is 7 times units, and between the yellow lines is 22 time units.

 

 

 

 

Edited November 7, 2020 by SteamyTea

[Temp]

 

1 hour ago, Kilt said:

Is yours an unvented system?

 

ours is vented so legionaries is always a concern. 
 

do you find full power best for your oil consumption?

 

Partly vented..

 

The boiler - store - UFH circuit on mine is vented but it can be dosed with chemicals to prevent legionares. 

The mains - heat exchanger - shower run is mains pressure and flushed with fresh water every time its used so i figure there is no time to grow bugs. Its no different to an electric shower I think.

 

The burner in an Oil boiler always runs flat out. For some reason they dont/can't turn the size of the flame down (aka modulate) like a gas boiler does. To reduce power output to match the load an oil boiler has to turn the whole flame on and off instead. This is called cycling. Rapid or short cycling can be inefficient (ike driving a car in stop start traffic) so to prevent that I have the dial on the boiler right up and run the boiler to store pump quite fast. When its working right the store calls for heat and the burner makes one long burn until the store is satisfied. 

 

 

[PeterW]

8 hours ago, Kilt said:

ours is vented so legionaries is always a concern. 

You mean the TS is vented ..? Not the cold supply to the hot water itself ..??

 

A TS cannot have legionella within it - either the coil or the flat plate heat exchanger type. Mains cold water contains enough chlorine to kill of legionella and there is no “stored” hot water in the system, it is a sealed system from supply to tap. 

[Kilt]

Thanks @SteamyTea amazing informative post. 

[Kilt]

On 07/11/2020 at 09:28, Temp said:

 

 

Partly vented..

 

The boiler - store - UFH circuit on mine is vented but it can be dosed with chemicals to prevent legionares. 

The mains - heat exchanger - shower run is mains pressure and flushed with fresh water every time its used so i figure there is no time to grow bugs. Its no different to an electric shower I think.

 

The burner in an Oil boiler always runs flat out. For some reason they dont/can't turn the size of the flame down (aka modulate) like a gas boiler does. To reduce power output to match the load an oil boiler has to turn the whole flame on and off instead. This is called cycling. Rapid or short cycling can be inefficient (ike driving a car in stop start traffic) so to prevent that I have the dial on the boiler right up and run the boiler to store pump quite fast. When its working right the store calls for heat and the burner makes one long burn until the store is satisfied. 

 

 

Sounds like we have pretty similar systems, though mine is vented. 
 

Been experimenting with speed and think I’ve stopped the cycling and achieved long burn with a faster pump speed, as you mention. Need to do a few more measurements to confirm, but TS is def getting satisfied now, so UFH manifolds can just chug away happily. 

[Kilt]

On 07/11/2020 at 16:13, PeterW said:

You mean the TS is vented ..? Not the cold supply to the hot water itself ..??

 

A TS cannot have legionella within it - either the coil or the flat plate heat exchanger type. Mains cold water contains enough chlorine to kill of legionella and there is no “stored” hot water in the system, it is a sealed system from supply to tap. 

The TS is vented, but you’re right, the manual states is a Primary Store so no chance of legionaries. Missed that bit, so thanks for confirming. 
 

We’ve a private fresh water storage break-tank, but it is council chlorinated at source, so just a concern sometimes that water isn’t maybe as fresh as that found on a street/estate etc. 

[J1mbo]

sounds like the coil might not have enough surface area.

[Kilt]

Apologies for resurrecting this old post, but I found a old photo of our old setup, and I can’t figure out why they did this. 
 

can someone far cleverer explain thinking on this pipe work?

[Nickfromwales]

Typically the pump should be pulling on the outlet of the mixing valve, not pumping into one of the inlets. Not how I'd have set this up, and I'd be surprised if that's working as intended ( however that is supposed / intended to work !?! ). My guess is that the mixing valve is installed incorrectly ( upside down ) as the direction of flow through it appears to be wrong vs the drawing at the outset of this thread. 

[Kilt]

2 hours ago, Nickfromwales said:

Typically the pump should be pulling on the outlet of the mixing valve, not pumping into one of the inlets. Not how I'd have set this up, and I'd be surprised if that's working as intended ( however that is supposed / intended to work !?! ). My guess is that the mixing valve is installed incorrectly ( upside down ) as the direction of flow through it appears to be wrong vs the drawing at the outset of this thread. 

I should clarify, above photo is of previous installation. This tank was ripped out 2 years ago. 

our new tank and pump is as per original drawing. 
 

I just recently found the old photo and couldn’t work out why the ORIGINAL system had been plumbed this way. Makes no sense to me. 

[Kilt]

2 hours ago, Nickfromwales said:

... as the direction of flow through it appears to be wrong vs the drawing at the outset of this thread. 

If You zoom in they’ve plumbed it correctly in terms of flow -

h= flow from boiler

c = return from TS

m = return back to boiler