Controlling return temperature of CH boilers

[Loggy]

We've just had a new combi installed - a budget version Worcester Bosch Greenstar 1000 30kW. This replaced an old Baxi 105E 31kW that was falling apart. The WB unit is a nice boiler overall - the DHW arrives almost instantly - but I am disappointed to find it doesn't report the return temperature, which is crucial in a condensing boiler to achieve the claimed efficiency. At the moment I set the flow temp to 60C and I have a clip-on thermometer on order. There is a modulating pump but that can only be set manually from the controls to somewhere between 20% and 100% but it's not automatic so I don't expect to save much money at all.

We do need to change our radiators - we know that - and were hoping our old Baxi would hold out for a year or so when we would go all heat pump - but we had to change and the 1000 and it was far cheaper than we had feared.

So I was wondering whether anyone has tried adding a throttling (ball) valve on the CH (return or flow - water is incompressible!) that reduces the flow to keep the return temp down below 53C or so and dT <= 20C so flow temp should be ~70C, all controlled by a couple of thermistors and a SBC like a RasPi or Arduino? Obviously there would need to be minimum flow and if the flow temp was too high this would not be achievable but low return temperature is essential for condensing boilers.

 

[ericmark]

I am an electrician, so could be wrong, but my understanding is the boiler adjusts output to ensure the return is cool enough.

The lock shield valve on each radiator has two uses, one is to ensure water reaches all radiators, the other is to give time for the TRV to adjust. But as the TRV closes the by-pass valve opens, this is an analogue process, and it needs time to adjust.

So the big thing is not to switch the boiler off/on when not required. Clearly we need programmers and wall thermostat to turn off boiler when not required, but unless analogue not to maintain room temperature with modulating gas boiler.

So main control is with TVR's, but for these to work lock shield valve needs setting, since mine are electronic I have current and target temp displayed on PC, phone etc, so if current exceeds target, lock shield needs closing a bit, how to do with manual valves not sure, as *123456 tells one very little.

 

[Harry Bloomfield]

So I was wondering whether anyone has tried adding a throttling (ball) valve on the CH (return or flow - water is incompressible!) that reduces the flow to keep the return temp down below 53C or so and dT <= 20C so flow temp should be ~70C, all controlled by a couple of thermistors and a SBC like a RasPi or Arduino? Obviously there would need to be minimum flow and if the flow temp was too high this would not be achievable but low return temperature is essential for condensing boilers.

A variable speed circulating pump, could achieve some of that..

 

[Loggy]

A variable speed circulating pump, could achieve some of that..

They are more expensive than dumb pumps, difficult to fit post-installation, will violate any warranty, and make maintenance difficult.

IMHO a variable speed pump controlled to match eg max(flow temp-20, 45) or with some profile designed according to the boiler and installation characteristics should be mandated for all boilers. There is little point in running the boiler at 80C for example where the efficiency is down to 80% then wondering why the bills have not gone down compared to the old boiler.

Other things can be done pending replacement of old radiators like ensuring that any fins are clear of dust, cobwebs etc, adding reflective material behind the radiators, adding simple fans to the top of radiators, and floor insulation on the ground floor of houses at least.

 

[Johntheo5]

It's much easier to just reduce the boiler flowtemperature you will then get low return temperatures = good condensing effect. Foe example if your rad requires 100% output at a very low outside temperature of say -10cC then you would require flo/return temps of 75C/65C to maintain a room temp of 20C but at a OT of 10c you may only require 25% rad output so you just set the flow temp to 39C this will result in a return temp of 36C with excellent condensing effect, 35% output requires flowtemp of 44C= return of 40.5C, 40% = 46C/42C, excellent condensing effect but of course if a system boiler then HW heating g
Has to be programmed separately with a boiler flow temp of around 70C

 

[Loggy]

Having been running the boiler for a bit, I now have an analogue thermometer thermally attached to the return pipe. It seems (in these cold days where the boiler is working hard) that the return temperature is always about 20C below the flow temp. As this isn't being done by the pump varying the flow, it must be by throttling the gas flow to the burner. I have had the flow temp set to 60, 65 and 70C and this always appears to be true, I suppose that is probably cheaper to do than have an automatic variable speed pump (although that can be set manually). If the boiler had reported the return temperature as well as flow, I would not have worried. This doesn't seem to be written anywhere, hence my concern. Now all I have to do is to balance the radiators but I will wait until we have updated them to newer (thinner) rads and not set the flow temp above 70C (it will go to 80C which would mean a return of 60C so not condensing although I haven't tested that yet!).

 

[MNW67]

IMHO a variable speed pump controlled to match eg max(flow temp-20, 45) or with some profile designed according to the boiler and installation characteristics should be mandated for all boilers

I think most, if not all, modern combi and system boilers already come with pumps like you describe.

Your brochure says:

"Low energy ErP modulating pump"

EDIT: Can you help me find the setting you mentioned which lets you change the pump speed? I've been searching through the settings menus but haven't been able to locate it!

 

[MNW67]

This boiler is new to me but a quick glance inside shows that it seems to have a stainless steel heat exchanger. Is that a first for WB?

 

[Johntheo5]

I think all modern combi and system boilers already come with pumps like you describe.

Your brochure says:

"Low energy ErP modulating pump"

EDIT: Can you help me find the setting you mentioned which lets you change the pump speed? I've been searching through the settings menus but haven't been able to locate it!

Some boilers like Vaillant now have flow/return dT control and is settable, default dT is 20C, I think.

 

[muggles]

This boiler is new to me but a quick glance inside shows that it seems to have a stainless steel heat exchanger. Is that a first for WB?

Yes, but don't believe they make it. As far as I'm aware it's an off the shelf item from an Italian manufacturer.

 

[Loggy]

EDIT: Can you help me find the setting you mentioned which lets you change the pump speed? I've been searching through the settings menus but haven't been able to locate it!

From the Installation and Maintenance Instructions, 1-C2 enables you at least to see the current pump output as a percentage of the the rated output. This is a bit ambiguous - I assumed it can be changed from 100% otherwise why is it there?

 

[MNW67]

1-C2 enables you at least to see the current pump output as a percentage of the the rated output. This is a bit ambiguous - I assumed it can be changed from 100% otherwise why is it there?

Presumably it is just there for diagnostics and for anyone who is interested to see what is going on. On the 2000, 4000 and 8000 there is an option to change the pump profile at 3-D in the settings menu. But that parameter is missing from your model. At 1-b8 you can view how much the boiler output is modulating. As the boiler modulates down, I would expect the pump output to modulate down automatically as well. I would be interested to know if this is, in fact, happening. In your OP, you seem to mention seeing the pump output as low as 20%?

As far as I can see everything is working pretty well because your dT is 20, as intended?