Oversized boiler - efficiency issues?

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8 Nov 2016
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United Kingdom

I've done a fair bit of googling and reading posts in this forum regarding my current setup. Probably stuck with what I have, but would like to be 100% sure. So any extra insights would be very welcome. Thanks in advance.

I have a large 5 bed detached house, 16 radiators in all, with a Vaillant Ecotac 637 (37 kW) boiler and a 210 l Vaillant unistor hot water cylinder. In S plan with 2 motorised valves, all radiators TRV (except bathroom (3) and hall (1)) and thermostat (on/off type) in hall.

The radiators can heat the house (some rooms faster than others, but all get there) so are near the heat requirements, as far as I can understand. Total maximum heat output of the radiators is around 16 kW (Some bigger ones at 1700 W, some smaller ones at 500 W).

This boiler is oversized I guess (for some strange reason - I guess the previous owner thought bigger is better);
This is even more so as I don't use a few of the rooms, so their TRV's are set lower, say setting 2, and close down quickly once the chill has been taken off the rooms.

So on normal, I estimate only to use a sustained 6 to 9 kW once all is warmed up.
The boiler can modulate down to 6 kW so that should be fine (and it does, no cycling), and I've maxed/downrated the boiler to 18 kW (setting d.00) in the hope it helps with efficiency.


With house reasonably warm, the TRV's closing up just enough to keep everything warm, the radiators only outputting about, say, 8 kW that means only around 700 litres of water (at around 10 degree drop, I've balanced as well I could) flows around the radiator. Even though the boiler can modulate down to 6 kW; the pump doesn't. So it pumps around circa 1300 litres of water per hour minimum (I guess to keep the heat exchanger safe) and so the internal bypass is open most (of not all) of the time, mixing hot water straight back in to the return:

Boiler puts 6 kW into heating 1300 litres of water by about 4-5 degrees. Even though the return is 10 degrees cooler, this is mixed with half of the flow resulting in a return of only 4 to 5 degrees below flow.

I've read somewhere the bypass opening means the efficiency of the boilers drops (like a rock, I think was the exact frase used). Sounds believable to me.


I'm surprised they (Vaillant) even sell a boiler that can modulate down to as low as 6 kW when that results in bypasses opening up and reducing efficiency?!

Is that correct?

Are there ways to adjust the setup to increase efficiency, i.e.

Can one have the pump replaced (downsized), effectively changing a 637 into a 618 for at reasonable cost?

Or would weather compensation help (still having the bypass open all the time but at least the return temperature would be in a more efficient range most of the time).

Any other suggestions?

Thanks in advance,

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Vaillant sell a 37 kW boiler to be used in a property that needs 37 kW and not 18 kW.

Their 37 kW boiler will modulate down to a minimum of 6 kW so that when it is not so cold it can continue firing in the 37 kW property.

I don't understand your comment on 700 litres. Usually 16 typical rads and pipework might have a total volume of not much more than about 120 litres.

You have already set your boiler to 18 kW and it will still modulate down to 6 kW. Until recently that all most 18 kW boilers would do.

So apart from perhaps minor balancing improvements I don't see there is much that you will be able to do.

Thanks for your reply Agile.

Where i have written litres I should have written l/h; the litres per hour pumped around. As far as I can see it is at *minimum* of around 1300 litres/hour.
But flow through system at 10 to 12 degrees difference is around 600, 700 litres per hour.

So even though the boiler will modulate down fine, it stills pumps water around at the same speed as on 37 kW. Reason unknown but I assume to protect the heat exchanger from overheating. It would have been nice if it would go down to, say, 500 to 700 litres/hour at 6 kW. Which I think is the specs for a smaller boiler like the 618 (which lists 774 l/h as opposed to the 1591 litres/hour for the 637). I guess the pump is just chosen to be able to work with for the highest heat output possible and does not adjust when the boiler modulates.

At first I thought my radiators might be clogged up but doing the math tells me that is probably not the case; it's just a case of how much capacity the radiators are and this translates in a number of litres/hour at the desired temp differential of 12 degrees. Which I estimate at around 600 to 700 litres/hour.

So with the pump at 1300 litres/hour that means the bypass valve opens up. Meaning I can't get anywhere near 12 degrees differential between flow and return as over half goes through the bypass.

Whereas a properly (at least IHO) sized boiler like the 618 lists 774 l/h and that would fit with the radiators' capacity, meaning bypass would open a lot later.

I am curious about

a) what is the impact on inefficiency, as I've read in this forum this bypassing will make the efficiency go down a lot.
b) anything I can do about it - short from swapping the boiler?

If you have the newer 637 which I think you do as it's modulating down to 6kw it has a two speed pump and if set to auto (if memory serves correctly it's D19 to check the current setting) when running under approx 50% power output the speed of the pump reduces.
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Minimum load for a 637 would be around 6.5 kW/hr , a deltaT of 10c would yield a flow rate of around 560 l/hr.
How have you concluded a 600/700 l/hr flow rate?
Not sure why your being so anal with regard to efficiency?...
Certain parameters can be altered although best left alone unless you know what you are doing.
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Hi hossacd thanks for the reply.

Yes, I read that bit about two speed pump. I think high speed is around 1560 litres/hour, and lower speed around 1260 l/h. I have set it at lowest speed but as I downrated the boiler to 18kW this was already the case automatically.

Still way more (water, that is, the 1260 litre/hour) than I can shift through my radiators in an average situation...

You should be aiming for a difference in temperature between flow & return of 20°C for best efficency.
Hi Steelmasons, thanks for your input.

Yes I agree, 560 l/h at 6.5 kW with a delta t of 10. That is about what my radiators 'use up' to keep the house warm.

I estimated on an average winter day I needed around 8.5 kW to keep the house on temperature, guessed around 600-700 litres.

And that's what I was aiming for. But with 12 degree drop and around 600/700 litres/hour through my radiators, the boiler still wants to pump a fixed min. amount of water. 1260 litres/hour.

So the bypass opens. And I'm curious to hear from anybody what impact that has on efficiency. I'm thinking it can't be good...

@muggles thanks

Yes, assuming it would keep the house warm - but that would mean restricting flow further; the 8.5 kW to keep the house warm would even less water, around 350 litres/hour.

But again the pump still pumps 1260 litres/hour as the bypass will accommodate and mix hot water from flow back to return.

I just can't see how to get a big boiler like this anywhere near condensing mode if it insists on pumping so much water around!

Have an RGI increase CO2% , this would yield a noticeable increase in efficiency.
Hopefully down rating the boiler won't compromise HW recovery rate.(y)
Hi Steelmasons thanks but could you elaborate?

Had a Gas certified gentleman service the boiler recently, all measurements within specs, so assume efficiency from that perspective should be optimal?
and no idea what a DRV is...

HW recovery is fine, probably slower than it was at full power but it's on timer so only on during the night anyway.

Just decrease the flow temperature as low as you can to get best efficiency. Basically you need low return temperatures to get the best efficiency, so if your Delta t is 20c and all your trvs close then tour efficiency will drop due to the return being 20 Warner than it was as it's using the bypass. But that's immaterial because it'll use next to nothing anyway. 90% of very little is still very little.
Just don't worry about anything complicate just go for the low water temperature. And that goes doubly so for your hot water for your shower, so many people heat it to 60 and then mix with cold when setting the boiler to 40c and using hot only would save a lot.

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