Auto bypass always open - help!

[Chris Clarke]

Hoping that someone might be able to help me.

I have a sealed heating system comprising 15 radiators and hot water cylinder. In the last 18 months all radiators were replaced and plumbed using 10mm microbore pipe back to manifolds on 22mm flow and return pipes. The system is S plan with two heating zones and hot water.

The problem I am having is that the auto bypass lets hot water through in any combination. When both upstairs and downstairs heating is on the boiler stays on and good heat is achieved, although bypassing. When upstairs is on in isolation the bypass is open and the boiler starts short cycling and also does the same with hot water only.

Initially I was concerned that the microbore pipe was causing too much resistance of flow which was resulting in the auto bypass opening. However, the bypass also opens for hot water only which is not in small bore pipe so makes me think it is not this.

I have had the auto bypass replaced, so it is not a faulty valve.

The pump was re-used from the old system and is a 15/50, I wonder if this is not man enough to overcome the resistance of the new small diameter pipes? But this would not explain the bypass being open for hot water only?

My other line of thinking is that the heat exchanger may be clogged up causing a restriction, we are in a hard water area and had a period where the boiler was kettling last year.

Overall the heating and hot water work, but not as well or efficiently as I believe they could. Really I'd like to find a heating engineer in Norfolk who really knows their stuff, a couple of people local to me have been round and I've been advised to change the boiler which I just don't believe.

 

[chris watkins]

If I am looking at your boilers M.Is then 4.3 - "a variable duty pump should be set to give a temp diff of no greater than 20deg C"
"Important F&R pipe sizes - up to 18kw = 22mm, >18kw =28mm"
The flow rate required to provide 18kw @ 20 degC would main that your pump would loose approx 1M head just pushing that amount of water through the heat exchanger if you needed it full 30kw you would loose over 2.5Metres.

Your existing pump is a 5M head but that is the max it can push with no or little flow. Moving approx 0.215L/S it would only develop a pressure of approx 3.7M head, less the boiler 1M, would leave approx 2.4 M for the rest of the system. Not likely to be enough to overcome the resistance you have put in by re-piping in 10mm.

 

[Chris Clarke]

Yes so pump size needs to go up but this will play further havoc with the auto bypass? Although it does say variable duty?

 

[chris watkins]

Fit a Grundfos pump with proportional pressure function? God knows, that's why I'm asking! Someone else must have come across this before?

Yes Chris thats why they were invented.
If you fit one would you need a by-pass ??
I don't think you do & have had my one at home closed for couple of years now with an Alpha 2 installed.
But if you do want to provide the minimum flow then why not use close coupled tees instead of LLH (to expensive) to provide hydraulic seperation. You are going to buy a new pump anyway so give it a go!!

 

[Chris Clarke]

Don't you need the bypass to prevent the pump getting dead headed on the pump overrun?

 

[chris watkins]

Effectively that is what your by-pass is providing at the moment you just don't have a pump on the other side of it to push water around the heat emitters.

 

[chris watkins]

Don't you need the bypass to prevent the pump getting dead headed on the pump overrun?

Pump reads the increase in pressure and just slows down to 5Watts.
But if boiler does need to clear away heat then yes.
Forgot my one always has open circuit as I have WC.

 

[Chris Clarke]

Something like this?

 

[chris watkins]

Yes just like that.
Good old Andrew M he pointed the way for me, also have a look at Caleffi - section 5 Page 27
https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_15_na.pdf

 

[doitmyself]

Thanks, makes sense to me. Although is 3.5m head sufficient for the longest microbore to manifold circuit of 18m? My rough calculations would suggest not.

It's not just the length of the pipe which is important. You have to take into account the flow rate, which depends on the size of radiator and the temperature differential. For example:

Resistance of 18m of 10mm pipe connected to a 2kW rad with a 20C differential is approx 0.58m, with a 10C differential it is approx 1.94m. The flow rate has doubled so the resistance increases by four times.

Upload your rough calculations and I can take a look at them.

 

[Chris Clarke]

So presumably the two pumps are wired to the same point and both overrun? The intelligent pump shuts down itself when the valves close?

 

[Chris Clarke]

It's not just the length of the pipe which is important. You have to take into account the flow rate, which depends on the size of radiator and the temperature differential. For example:

Resistance of 18m of 10mm pipe connected to a 2kW rad with a 20C differential is approx 0.58m, with a 10C differential it is approx 1.94m. The flow rate has doubled so the resistance increases by four times.

Upload your rough calculations and I can take a look at them.

It's not just the length of the pipe which is important. You have to take into account the flow rate, which depends on the size of radiator and the temperature differential. For example:

Resistance of 18m of 10mm pipe connected to a 2kW rad with a 20C differential is approx 0.58m, with a 10C differential it is approx 1.94m. The flow rate has doubled so the resistance increases by four times.

Upload your rough calculations and I can take a look at them.

I won't embarrass myself as they are obviously wrong and too basic. It's plastic pipe too which might make a difference?

 

[Ben Slade]

Won’t the flow go straight through the bypass valve as it is on its max setting now and it would have the path of least resistance than the rest of the system?

 

[Chris Clarke]

Won’t the flow go straight through the bypass valve as it is on its max setting now and it would have the path of least resistance than the rest of the system?

Hi Ben,

I might be wrong but I think the hydraulic separation using close coupled tees would mean the bypass could be removed.

 

[Ben Slade]

Hi Ben,

I might be wrong but I think the hydraulic separation using close coupled tees would mean the bypass could be removed.

Right I’m with you! Cheers for the enlightenment.

 

[Ben Slade]

Are you going to use both the tees and the hydraulic separation I was under the impression it’s one or the other with the tees being the cheapest?

 

[chris watkins]

Hydraulic separation of the boiler from the system is achieved by using a CCT's or LLH

 

[chris watkins]

I won't embarrass myself as they are obviously wrong and too basic. It's plastic pipe too which might make a difference?

Not wrong just has advantages / disadvantages = smoother bore less fitting but at the end of the days it is a small pipe which limits the heat flow to each rad to something like 3 - 3.5kW but each is fed directly from a 22mm manifold.

 

[Ric2013]

To highlight a commonly held misconception that a differential pressure automatic bypass valve works well with a proportional pressure pump (e.g. Grundfos Autoadapt), please note that Grundfos does not recommend that the two are used together.

 

[fezster]

Indeed: http://www.newtonnet.co.uk/permanent/GrundfosDocs/BypassCommissioningAlpha2or2L.pdf

A fixed bypass is a manually adjusted valve which is set to give the required bypass flow. It could be a simple valve, but Grundfos recommends the use of the Taco Nova Setter valve with flow gauge.

 

[Chris Clarke]

Thanks for the extra info. I hope to update this thread once I have (hopefully) found a solution.