I have a 12 year old Pottington gas combi-boiler running my radiators and water tank. I was checking the central heating loop pressure today and I discovered a twisty valve I never used before. I think this valve has been incorrectly set since I bought the house, and it appears to connect the boiler water OUT to the boiler water IN, short circuiting the radiators. It is a smaller pipe than the main one. It is located beside the boiler.

  1. The radiators don't heat up very quickly, I had assumed it was just because the weather was extra cold
  2. The boiler powers down every few minutes, including soon after a cold start

Please see the diagram below.

  1. I found the valve fully open (anticlockwise) and it was stiff, not recently turned.
  2. I measured temperatures, see diagram:
    1. 47°C
    2. 27°C
    3. 47°C

In this configuration the water seems to be going through the short circuit and so the boiler turns off soon. The radiators still work and have done for 6+ years but they could heat more quickly.

Turned the value to fully closed (clockwise), and now

  1. measured temperatures:
    1. 47°C
    2. 33°C
    3. 33°C
  2. boiler ran much longer even when the thermostat dial was turned down

So these are my conclusions:

This valve allows a significant amount of hot water to short-circuit into the return from the radiators, limited by the smaller diameter of the pipe. The boiler thus receives a warmer return water and cycles off quite quickly, limiting the speed at which the system can heat. Is this good for the boiler?

And my questions are this:

  1. What is the purpose of this valve?
  2. Should it have been fully open?
  3. Where should it normally be placed and why?
  4. Will it do any damage to place it fully closed?


2 Answers 2


It's a boiler bypass loop. These can be installed for various reasons. I can't tell which one(s) apply to your case:

  • preheat the incoming cold water a little to reduce temperature fluctuations in the heat exchanger and to reduce condensation within the boiler
  • provide a water circuit for the pump to keep circulating if/when all other circuits are closed by zone valves or other valves. Various reasons for this related to the design of the overall system.

The valve is to tune the bypass. It should not be fully open or closed. An experienced boiler installer should adjust it in order to achieve whichever of the above goals is needed.

  • 3
    "for reasons you can Google" shouldn't make it into an answer. Imo your answer should either only contain the name of what it's for, or the full description of what it does. Commented Jan 30 at 7:51
  • 1
    Fair enough @DaniëlvandenBerg. "Full description", no .. that would be a two-volume set. Hopefully now an appropriate level of summary. And no hiding it, still a little evasion in the second bullet. :)
    – jay613
    Commented Jan 30 at 14:07

I think you are correct that the valve helps the boiler to turn off sooner. You typically want a delta T of 15F such that the water has lost 15F of heat after coming back from your zones before being heated again. Heating water that is already at temperature doesn't make sense.

116F to 91F is a delta T of 25. This means your heating your water too high. The boiler can't choose how hot to heat the water only whether to heat. If the water comes back hot enough the boiler turns off. If the water comes back cold then the boiler will continue to heat. The boilers are often over sized to account for the coldest day of the year and to avoid any call backs for heat issues.

If you open the short circuit valve then the water is coming back warmer which turns off the boiler and colder water is going to be pumped through your radiators. The less the boiler runs the more cost efficient your bills will be. If you want heat the quickest then pushing water at the highest temperature will get you that but that also costs the most amount of money.

Radiators in older drafty houses are also placed near windows to make those areas comfortable.

I run my radiant in floor heating at 90-95F and it returns at 80F but I have multiple zones and I am not heating up windows. I also have a variable speed pump which adjusts speed based on the number of zones calling for heat.

Cycling the boiler quickly probably isn't great but are your measurements based on trying to heat the house from a cold state or from once the heat has been on for a while and the house has heated up to a generally acceptable temperature? I guess what you are aiming for is that the boiler doesn't quick cycle during the majority of it's operation. The only thing that is likely to take wear with the cycling is the gas valve but if this results in the boiler running less frequently then you'll likely save the value of the gas valve very quickly.

One other note is the general consensus is that the circulator pump should be on the cold side as it will last longer.

Let's say you close the valve and you push 47C water through your radiators. The house reaches temperature quicker - say you have the thermostat set to 22C.

The thermostat stops calling for heat, the pump stops, the 47C water at the radiators is allowed to cool but while cooling is also still heating until is chills from 47C to 22C. The house over heats to 25C and you overshoot your temperature - maybe ok.

The house cools to 21C and your thermostat calls for heat. 47C water goes back out through your whole loop and you overshoot again maybe by more this time as everything in the house is already warm and the heat loss is lower.

So it can depend a bit on how far you'll overshoot based on the 47C and 22C requested temp. The closer the water temp is that you can push and still deliver heat in a reasonable amount of time the better and the lower chance of overshooting your design temperature.

  • 1
    Don't mix your units. First part of the answer is using fahrenheit units as temperature measure and then the last four paragraphs switch to celsius, SI units are always the way to go.
    – D Duck
    Commented Jan 30 at 9:09
  • 2
    95F (35C) to 80F (26.6C) is a delta T of 15 which is the literature ideal. 35C to 26.6C is 8.4C delta T. 115F (46.11C) to 100F (37.78) is 8.33 delta T. I am not sure what the metric boiler people use as a delta T is it 8.4C? I work in metric temperature but in North America our components and engineered systems are mostly not metric and I've only heard/learned the delta T literature in F. OP posted in C but had to convert to F to do the delta T work - I am not fluent in celcius delta T =) So I've mixed the units to irritate everyone! Commented Jan 30 at 9:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.