Balancing the CH system and BYPASS issues

[Chameleonclaude]

So, i balanced my rads today so all rad returns are returning the same temperature which is 20 degrees below the primary flow. Before anyone says it should be 11 or 12 degrees, my worcester bosch boiler specifies that optimum performance is achienved when there is a 20 degree difference between flow and return.

Having closely watched the behaviour of the boiler during balancing i believe that the boiler will deliver a minimum temperature of 55 degrees (probably due to legionaires) and will only increase in temperature if the return is greater than 10 degrees difference or so.

My dilema is this;

With my alpha 2l pump on proportional curve setting the pressure delivered reduces with falling demand. This means there will be a greater flow rate through the bypass (fitted with a taconova flow setter rather than an auto bypass valve) ehen the ch demand is high and a lower flow rate through the bypass when the system is on overrun. This is the oposite of ideal!

The problem with this is that the hot water which passes straight into the return raises the temperature of the return thus causing the boiler to not burn as hot as it perceives the demand for heat to be less.

Fitting an auto bypass valve would clearly not resolve this issue either!

My question is this;

If i fitted a motorised valve on the bypass wiring it to open when the CH valve is closed and close when the CH is open would the 20 second period or so when both valves are closed (given that it takes longer for a valve to open that close) be danerous to the pump or the boiler, or would this brief period be irrelevant.

I know this is unconventional but it Could potentially optimise the boilers performance as return temperatures would be the full 20 degrees less.

If this worked i could direct the flow of the bypass through a rad (of sorts) that was outide of the house on the coolest shadiest part of the house. Fortunately for me this would be out of site in my home. This would be a suberb heat sync in the winter.

Ps. The HW system would similarly be wired to the bypass valve.

Am i conmpletely bonkers ir is this idea a stroke of genius?

Love to hear

 

[D_Hailsham]

The links in //www.diynot.com/forums/viewtopic.php?p=2153002#2153002 may help.

 

[swbjackson]

So you want to optimize the performance of your boiler by heating the garden. The boiler will be more efficient in its use of gas with a lower return temperature but at the cost of the efficiency of the whole system.

 

[ALEC1]

the way to maximise system efficiency is to use a compensating controller, that varies the flow temperature in such a way that the heat requirement matches to the heat generated.

By passes are only used to satisfy the minimum flow rate of the boiler, as the system flow rates with comprnsation controls do not vary that much or at all.

it appears that the delta t of 20c on radiators is a curious british invention to justify avoiding adopting compensation controls with condensing boilers.all condensing boilers work down to delta t of 7-8c


the flow temp then is generally below 55c....
Granted some boilers dont like a delta t bigger than 20c, but the software in the boiler looks after that...

 

[Chameleonclaude]

brilliant

Well it wouldn't be at the cost of the whole system as the bypass would be closed completely when the CH system is open.

The outside rad would only come into play when there a need to diserpate heat during overrun.

 

[swbjackson]

But you're throwing that heat away. Keep it in the property. The only energy wasted during pump overrun operations is a small amount of power to run the pump and you will waste a great deal more than you save.

If you're worried about it look into intelligent controls. You don't have to go the whole WC route. I use a Siemens REV23 programmable room thermostat which learns how long the house takes to heat up and cool down. Instead of setting the times when I want the heating to turn on and off I set the times when I want a certain temperature and the thermostat turns the boiler on and off depending on the ambient temperature.

 

[D_Hailsham]

it appears that the delta t of 20c on radiators is a curious british invention to justify avoiding adopting compensation controls with condensing boilers.all condensing boilers work down to delta t of 7-8c

The British originally used 20F, which became 11C. 20C is the continental standard, probably chosen to maintain reasonable rad sizes in the coldest weather.

A 1kw rad would need to be run with an 88C flow temp to give 1kW if the return was 55C; that's a 33C differential! Reducing the differential to 20C means the output will reduce to 850W, so a 1170W would have to be installed to provide an actual 1kW.

Most boilers are now designed for a 20C differential so if you try to run them at the traditional 11C diff you will meet problems due to the increased pressure drop across the hex caused by higher flow rate. For example the Vaillant 418 has a drop of 1.38m at max output with a 20C differential. The drop for an 11C diff is not shown on the graph, but is probably more than 2.5m.

Do you mean a 7-8C differential at minimum output?

 

[ALEC1]

yes

 

[Chameleonclaude]

Ok, so the outside rad idea is a bit bonkers. I'll run the bypass through either through the bathroom or something like that.

Hailsham, why would decreasing the rad flow and return diff to 20 lower the output. Surely it would be achieved by increasing the flow and thus raising the temperature of the rad?

I have no doubt your right, just want to understand it.

What about the motorised valve idea. The main thrust of my question is about closing the bypass during CH open times?

 

[ALEC1]

because the mean temperature of the radiator drops

 

[D_Hailsham]

I assume you have come up with this idea because you are having problems getting the correct settings for your pump and the Taconova Setter valve.

You are not the first person with this idea! See the topic (14 pages long!) Bypass when using Grundfos Alpha2 pump. The link I gave earlier is to a post from that and gives links to Grundfos's advice on the problem.

You said that WB told you that the minimum flow through your boiler is 5l/min when in overrun mode. This seems very high.

Which boiler do you have - 18Ri or 24Ri?

Which pump do you have?

 

[Norcon]

A taconova setter is just a gate valve with a flow reading.
No substitute for a spring loaded tap.
What the op needs is a big vessel of water between the boiler and the heating system.

 

[D_Hailsham]

A taconova setter is just a gate valve with a flow reading.
No substitute for a spring loaded tap.

Grundfos actually suggest an Auto-bypass in series with a Fixed bypass such as the Taconova Setter.

What the op needs is a big vessel of water between the boiler and the heating system.

Or a Low loss header or close coupled tees?

 

[Chameleonclaude]

As always, many thanks for your interest.

I must be honest, I'm a little out of my depth with the technical side! Hailsham, thanks for links. I am trying to digest it all but there's alot to take in and understand.

What i am concluding so far though is that an auto bypass is completely ineffective with a pump that reduces the head pressure with falling demand. For this reason i'm seriously considering controling the bypass with a motorised valve.

I know now that honeywell do thier 4043 model in a 'usually open' setup meaning that the valve opens quicker than it closes. This would pair up beautiflly with the usually closed valves.

I'm also going to look into weather compensation as i wasn't aware of this technology. Are these basically room thermostats with an outside measure or can they take other forms?

Finally, with my fully balanced radiators and my boiler on maximum heat setting, my flow temperature is 68 and my return is 55. The return would certainly be lower if the bypass was closed during non overun periods. But why am i only getting flow temperatures of 68 on maximum. I would have thought that a return temperature of greater than 11 degrees would prompt the boiler to increase heat until the difference is something like 7. It makes me wonder whether my gas/air ratio is set up correctly.

Hailsham, my boiler is either a 24ri or an 18 ri. For the life of me i cant remember which and don't know how to find out. Worceter bosch do insist on 5 litres per minute for both these models mind you so it doesn't really make much difference.

 

[D_Hailsham]

What i am concluding so far though is that an auto bypass is completely ineffective with a pump that reduces the head pressure with falling demand. For this reason i'm seriously considering controlling the bypass with a motorised valve.

If you read Grundfos's info about bypass commissioning (link earlier), you will see that they say:

If an automatic bypass valve must be used, then a manual fixed bypass valve should be installed in series with the automatic bypass valve. The fixed manual bypass will restrict the flow as the automatic bypass valve opens.

This may be the solution you are looking for. Put the ABV first, followed by the Taco Setter.

I'm also going to look into weather compensation as i wasn't aware of this technology. Are these basically room thermostats with an outside measure or can they take other forms?

The boiler must support weather comp; unfortunately yours does not.