Flow rate in central heating system

[Biggles..]

The pump needs to match the load and be sized to give the right delta t the auto bypass and boiler modulation avheive the rest.

1.6m3/hr for 38kw @ delta t of 20degc

 

[edb49]

Put into simple terms the system has such a large volume that when starting up there is so much cool water returning to the boiler that the differential remains high for longer than the boiler is programmed to expact so it thinks there is a fault.

There is a simple work around to your problem.

As its a large system it should be devided into several zones and the UFH is a typical example.

All you need to do is to seperately time part of the system so that it does not all start at vthe same time ( or put a few minutes delay into part ).

In any case the UFH should be timed to start an hour or more before the rest of the system as it takes longer to warm up.

Tony

This is how the controls were programmed - UFC would come on at 5:00am, with the rads coming on at 6:00am. Unfortunately the problem still existed with this - so it's not a case of doing the zones independently will get around the problem.

 

[edb49]

What is the flow temperature when the house is up to temperature?

It is set at 80c currently.

What happens if you reduce the max flow temperature from 80 to 70?

The problem still occurs, even if I lower the flow temperature.

When the boiler goes into slow mode with S53 showing, does the pump continue to run? How long does it take before it goes back to normal?

Yes, the pump runs whenever there is demand from the heating controls, so even when the boiler is in slow S53 mode, the pump runs. (This rapidly brings down the flow temp from the boiler.)

TRVs on all rads?? Don't you have one rad without a TRV where the room stat is located?

There is no stat for the rads! There are indepedent stats for all the rooms with UFH, but the rads just rely on the timer in the heating controls.

 

[edb49]

Dont fit a magna or an alpha variable speed pump this will cause the same problems due to the pump modulating down for the load,the boiler wiil see this as an increase through the HEX and trigger S53.

Fixed speed pump UPS25-80.

I've been reading through the spec of the pumps, I seem to have two choices. I want to get about 2.5m^3/hr flow at a 5m head. (To give approximately at 15c temperature differential across the boiler.) The figure for the head is derived from current pump performance graph - see post by D. Hailsham on page 1 of this thread. So my two choices are:

MAGNA 25-100 (Product No. 96281015)

UPS 25-80 (Seem to be 4 separate product numbers, 2x 230v and 2x 240v)

The Magna has lots of clever functions but let's ignore those for a minute. The Magna seems to be "A-rated" for efficiency, so if I'm going to get a new pump then I assume it will last 10+ years, and on this basis I want to get one that will cost me least over this time period.

The ideal thing would be for the Magna to have a "constant flow" setting, so ensure we're always getting 2.5m^3/hr through the boiler, and to vary its output dependent on pressure. There doesn't seem to be this option according to the docs at http://net.grundfos.com/doc/webnet/magna/downloads/Magna_databooklet.pdf page 10. I can set it to have a constant output, and there is more adjustability than the three speeds on the UPS 25-80. (It has 9 speeds.)

I don't quite understand proportional pressure, constant pressure, and the AUTOadapt function. I suppose my question is, which pump is most suitable for my system and why?

 

[edb49]

Quick online search shows the Magna is £500, and the UPS £200....

Quite a big price difference! Would need to save £30/year on electric, so call it 350kwh, to make the Magna worthwhile.

 

[Biggles..]

as previously posted id go with the UPS25-80 or UPS25-100(lowest pump setting)

 

[edb49]

as previously posted id go with the UPS25-80 or UPS25-100(lowest pump setting)

With UPS25-80 on middle setting, it uses 155w and will provide 2.2m^3/hr flow at a pressure of 5m.

With UPS25-100 on lowest setting, it uses 280w and will provide 3.3m^3/hr flow at a pressure of 5m.

The 25-100 looks like it will be overkill, and uses a *lot* more power than the 25-80 for the desired effect.

If I go to the Magna 25-100 page on Grundfos.com, and select 5m and 2.2m^3/hr (to get an equivalent with the UPS25-80), it indicates power usage will be 77w - so about half what the 25-80 uses.
(http://net.grundfos.com/Appl/WebCAPS/ProductDetailCtrl?cmd=com.grundfos.webcaps.productdetail.commands.ProductDetailCommand&ProductNumber=96281015&freq=50&resultGridId=8&selectedRow=0&fileid=QU1RIE1RU1BNQUlOICAgIEr39z4kF1RH)

So if I can save 75 watts with the Magna... heating for 8 months of year, hot water for other 4 months. So say 10 hours for 8 months of year (due to underfloor) and 4 hours for 4 months of year. So total annual usage is about 3000 hours.

Therefore savings: 3000 hours & 0.077kw = 231kwh, and at about 8.5p/kwh that's £20/year.

So I think the Magna doesn't really pay for itself over 10 years - maybe closer to 15.

 

[Biggles..]

Sorry didnt have the exact figures to hand for the 100.

Bear in mind if you did your calcs on seperated timed heating and then put them into the graphs your actual KPa may be higher than what it looks which is why a true KPa would have been better.

If you took your figures with all calling from cold at the same time then you have roughly your figure to work from.

ups25-80 will surfice based on your original posted figures.The magna will still give problems during pump modulation with your boiler product and isnt cost eefctive.

have fun.

 

[edb49]

I've been reading through the spec of the pumps, I seem to have two choices. I want to get about 2.5m^3/hr flow at a 5m head. (To give approximately at 15c temperature differential across the boiler.) The figure for the head is derived from current pump performance graph - see post by D. Hailsham on page 1 of this thread.

Have just realised I'm wrong on this - based on current performance of the pump, boiler, etc, the system is flowing 1.15m^3/hr. The Glowworm pressure loss graph says at this rate the boiler is giving about 2 metres head of water.

Taking the boiler out of the equation and looking at the pump spec, at the maximum setting at 1.15m^3/hr the head is about 4.9m.

Therefore at 1.15m^3/hr, we know:
Boiler pressure loss = 2.0m
Rest of system pressure loss = 2.9m

At the desired flow rate of 1.6m^3/hr, the boiler pressure loss is about 4m. If the rest of the system doesn't change then the total pressure loss would be 6.9m, but I am pretty sure the pressure loss would increase with flow rate - is that correct? If so, is there a rule of thumb I can use for central heating systems to work out the pressure loss change with flow rate?

 

[edb49]

ups25-80 will surfice based on your original posted figures.The magna will still give problems during pump modulation with your boiler product and isnt cost eefctive.

I think the Magna can be set up in the same way as an "old-fashioned" pump though? Looking on www.colglo.co.uk, pump prices are:
£330 for UPS 25-100
£475 for Magna 25-100

Difference is £145, which would pay for itself much more quickly on the UPS 25-100 versus UPS 25-80. I think if the Magna can be set up to act the same as a UPS, then it would make sense to go for the Magna - more finely adjustable and also seems to save anywhere from 70 - 110 watts against the UPS, which is worthwhile.

 

[Agile]

I am surprised that Lee does not agree with me that the problem is solely caused by the whole cold water content of the rads taking longer than the boiler expects to allow the return to come up to temperature.

Just because the UFH is timed earlier, it is insufficient to solve the problem if you have a large capacity of rads all on the same circuit and not zoned as the regulations require. Even odder that you have no room stat. On a large system the potential savings from proper controls are very large.

Upsizing the pump to increase the return temp faster on start up will, if uprated sufficiently, solve the problem. However, its also likely to reduce the temperature differential across the rads.

With a condensing boiler the normal flow temperature should be about 70-75° for efficient operation as long as the flow/return differential is 20°.

The best way to estimate the flow rate through the boiler is to measure the power input and flow/return temperatures.

Tony

 

[Agile]

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

About 20c - when it is up to temperature flow is 80c and return is 60c.

That implies that the flow rate is correct to achieve the desired temperature differential when its all up to temperature.

But by then the boiler will have modulated back and you need to measure the power input at that stage to have all the details needed to decide how to best to deal with the problem.

I am also surprised that you are running the boiler at a flow rate of 80° when this will mean the boiler is hardly condensing and you are losing up to 12 % of the efficiency.

Another interesting way to bypass the problem effect would be to reduce the power output of the boiler in d0. I would suggest to about 20% above the steady state power output as measured when up to temperature.

This is an installation where weather compensation would be expected to offer a significant improvement in efficiency and comfort.

Tony

 

[Biggles..]

What exact regulations specify zoning of a heating system.

We're not changing our mind on what the problem is are we Tony, the system has UFH,heating and hotwater you have suggested segregating the 3 with differing call times which doesnt work.

As for the delta t im pretty sure that for some reason anything greater than 15deg will signify a wait status in the boiler, S53. Its down to the sensors and their position and the bad software in the boiler(probably the most annoying design error with them)

the op has checked the input and the temps through the d menu, the problem is the boiler cant shift its heat quick enough.

 

[Biggles..]

Edb-appolagies i'm going to leave it to MR G to continue to let you chase round balancing,derating and seperating your heating times.

Personally your calcs seem quite reasonable and your pump suggestions are spot on, spending and saving i cant help you with as i dont know your financial situation.

Lee

 

[Biggles..]

The pressure loss will increase in the2nd power compared to flow

flow doubles pressure loss quadruples