Pump overrun “working” but not giving up heat

[WiltshireWarrior]

I have a complicated/big system. 3 pumps and a mix of ufh, a few rads and an indirect HWC using a Worcester Conventional

With recent help from some guys on here I’ve got it working almost perfectly....the pump overun “works”, but doesn’t dissipate enough heat quickly enough to actually be of use.

because of multiple pumps I have selected one of the pumps, with a bypass circuit, that via a clever relay comes on whenever the boiler call moves (electrically) to low (off).

This all works electronically. What isn’t that effective is the actual cooling of the boiler by cycling water through it for a couple of minutes when the boiler is no longer called for.

I think this is because there really isn’t much opportunity for the heat to be given up. The flow/return are both 28 copper and highly insulated. The total loop I’d guess is 16-20m in length (from boiler to overrun pump and back). So that’s pretty useless, as is shown by the boiler temperature not decreasing when the overrun comes on.

previously (before the system was completely changed as part of house rebuild) the pump overrun did take the heat away quite nicely, but that flow/return run was problem 3 times as long and not very well insulated.

options I’ve thought of:

1. remove insulation from flow/return. Seems counterintuitive
2. Add to length of current via bypass circuit with more pipe and leave that uninsulated. Seems a hack/looks rubbish
3. Add a dedicated device to overrun current design to give up heat. Small rad or is there a specific device, something made of alu with fins etc. A heat sink essentially

any thoughts?

 

[Terrywookfit]

Your post suggests that you have already made the decision in your own mind .

How about your option 1. To see if it has the desired effect.... Before complicating matters.
Your options 2 and 3are just expensive methods of achieving option 1 !!!

 

[WiltshireWarrior]

I certainly do not have a preferred solution in mind. So you’ve drawn the wrong inference.

My post really was to ask some experts/someone with more experience/someone who has faced this problem and has a clever solution, to suggest something new to me. Or to suggestion a better amendment to my suggestions.

Yes, I do realise the fundamental solution is to have some kind of “heat sink”. That much is pretty obvious.

Cheers

 

[Harry Bloomfield]

Would increasing the overrun timer not solve it more easily, or am I not understanding the problem?

Or add a pipe stat on the flow, which makes on heat rise, to power the pump. That would run the pump until the boiler is below a set temperature.

 

[Terrywookfit]

Would increasing the overrun timer not solve it more easily, or am I not understanding the problem?

Or add a pipe stat on the flow, which makes on heat rise, to power the pump. That would run the pump until the boiler is below a set temperature.

The overrun is controlled by the boiler Pcb which turns off when cool enough

 

[Harry Bloomfield]

The overrun is controlled by the boiler Pcb which turns off when cool enough

That is what happens on my system, but I understood the OP had to devise another way. If the OP's pump is controlled by the boiler, then what is the issue?

 

[WiltshireWarrior]

The overrun is controlled by the boiler Pcb which turns off when cool enough

no, it isn’t controlled by the boiler PCB

It’s controlled by a dedicated relay in my system.

The electrical side is completely under my control. I can run it for as long as I want by changing the delay settings on the relay.

 

[WiltshireWarrior]

That is what happens on my system, but I understood the OP had to devise another way. If the OP's pump is controlled by the boiler, then what is the issue?

see other reply, it’s not controlled by the boiler pcb

The issue is not electrical, that all works fine. It’s water based issue and the water not giving up its heat.

yes I can run the overrun pump for longer but what concerned me is the temp doesn’t drop as fast as it previously did (for the reason I explained in the original post) hence the exchanger is under stress for longer than ideal

whether it’s controlled by the boiler or not is not relevant to this particular issue.

 

[WiltshireWarrior]

Reckon this might be the least worst option.

https://www.ebay.co.uk/itm/SK-623-5...-/193743506350?_trksid=p2349624.m46890.l49292

attaching this to the bypass part of the circuit so that in normal situations it doesn’t “waste heat” but when the pump overrun circuit is being used it helps dissipate the heat to help save the heat exchanger

I wrote my original post because I’d hoped some plumber would point me to a proper product. Can’t be alone with this problem

 

[Mottie]

Couldn't you plumb in a small radiator or even a series of copper pipes into the bypass circuit to dissipate heat on the overrun?

 

[WiltshireWarrior]

Yep, see the 3 options I stated in my first post.

I suppose I was looking for more elegant solutions rather than a “hack”

See the eBay link in above reply I made. Something like that might be the solution. Dunno

 

[Lower]

Do you actually need to cool the water on the over run? Isn't it just a case of keeping the water moving to make sure you don't get any kettling?

 

[WiltshireWarrior]

Do you actually need to cool the water on the over run? Isn't it just a case of keeping the water moving to make sure you don't get any kettling?

maybe you’re right, but I always thought the idea was to bring down the temp slowly of the exchanger and then switch off when temp is lowered

my post was also to elicit replies like yours, questioning the premise of what I’m trying to do. Because yeah, maybe I don’t quite get the point of overrun

 

[Terrywookfit]

Which boiler are you using.?

Over run is used on high output low water content boilers to dissipate the latent heat to prevent the over heat stat from operating.
Larger boilers usually have a shunt pump into a header.

 

[WiltshireWarrior]

Worcester 40CDi Conventional

I’ve had it for, what is it now, over 13 years and it’s still working great.

I’ve had the PCB replaced by Worcester once I think. But the heat exchanger seems fine (even though the Worcester engineer told me categorically that “the heat exchanger is knackered mate, it’s gone!” And that was 4 years ago.)

Takes less than 15min to heat up my high recovery HWC etc

probably change when needed in next 5y maybe