What is the max dT that a normal system boiler can run at?

[ringi]

I am playing with a system design that will need the output from the boiler at 60c; however I can get the return down to 20c or below. I can regulate the output temperature by controlling the flow rate through the boiler. E.g using a 32KW boiler, I will need a flow rate of 11 m/s.

Would a normal system boiler cope with this, or is the dT too high for the boiler to be happy?

Given an output of 60c, how much better does the boiler condense with a return of 10 rather than 20?

If I was to run a 40KW boiler at a flow rate of 11 m/s would I get more condensing then using a 32KW boiler?

What flow rate do the internal bypasses in boilers tend to be set at?

Many thanks; sorry I could not find this basic information for boiler spec sheets.

 

[Howsie]

You do not want too large a temperature differential across the F&R IMHO. Everything you need to know should be in the MI.

 

[Dotty]

Max dT iirc is 20deg. As Howsie says, all the info you need is in the MI's.

Anything else you don't need to worry about. It's your GSR's concern.

 

[oz-plumber]

Hi Ringi,

What type of system are you trying to design.
Why would you want so much temperature difference?
I don't think any boiler will be able to cope with a 40 C >50 C temperature rise particularly at that flow rate.
Some more info may help

 

[Nostrum]

Lower return will mean a lower mean water temperature, so larger radiators. Many boilers will freak out and see this as a circulation fault.

Im not sure how much gain there is to be had past 50-55 degree return. The boiler will still condense as below dew point.

 

[doitmyself]

I am playing with a system design that will need the output from the boiler at 60c; however I can get the return down to 20c or below. I can regulate the output temperature by controlling the flow rate through the boiler. E.g using a 32KW boiler, I will need a flow rate of 11 m/s.

Eleven metres/second, or do you mean 11 litres/second. Either way, both are ridiculous. A 32kW boiler with a 10C differential will have a flow rate of 0.766 litres/second and with a 20C differential it will be 0.383 litres/second, i.e half the flow rate. The velocity (metres/sec) though a pipe for a given flow rate (litres/sec) depends on the diameter of the pipe - the greater the diameter, the lower the velocity.

Would a normal system boiler cope with this, or is the dT too high for the boiler to be happy?

Short answer - NO. Too high a temperature differential across the heat exchanger will lead to damage.

Given an output of 60c, how much better does the boiler condense with a return of 10 rather than 20?

Condensing starts when the return is about 55C. The amount of condensate, and therefore energy extracted from the flue gases, does increase as the return temperature decreases. See Condensing Technology

 

[ringi]

Firstly sorry for getting my units wrong, I meant 11 litres per minute!

I was working out the largest Dt I could get for a hot water heating system, so the boiler would condense as much as possible all the time, and I would never have luke warm water in the top of the tank.

Give the helpful answers; it is clear that it is very hard to come up with a system that does better then the GasSaver without having buffer tanks heated to several different temperatures – that is just silly for a domestic system!

 

[hammers4spanner]

If you come up with a better design than what the manufacteres spend millions on then I suggest you get your urse on dragons den

 

[doitmyself]

I was working out the largest Dt I could get for a hot water heating system, so the boiler would condense as much as possible all the time, and I would never have luke warm water in the top of the tank.

So you want the water at the top of the cylinder to always be at the same temperature? The only way to do this is to have a temperature sensor at the top which turns the boiler on when the water temperature drops by, say, 1C. It's nothing to do with the temperature differential at the boiler.

In any case, there's not much point in maintaining the water temperature at 60C±1C as the hot water has to be mixed with cold to bring it down to a usable temperature, approx 40C. This means the water temperature can be allowed to drop nearly 20C before you need to reheat the cylinder. That's why most cylinder thermostats have a differential of about 10C.

If you want to heat the cylinder to 60C, the water leaving the boiler has to be higher than this, say 70C, or no heat will be transferred. So having the flow temperature at 60C is no good.

 

[ringi]

....

If you want to heat the cylinder to 60C, the water leaving the boiler has to be higher than this, say 70C, or no heat will be transferred. So having the flow temperature at 60C is no good.

I did not say DHW cylinder….

I am thinking of a thermal store, using a plate heat exchanger to heat the DHW, so provided the water at the top of the tank is at or over 60, I can get DHW at 55c on demand (so over 50c when it gets to the taps). The thermal store contains water on the primary side, so it does not need the boiler to run hotter than it needs to be.
I can extract enough heat from the thermal store for the water at the bottom of it to be at 20c or lower, but don’t wish to have the water being put back into the top below 60. Therefore as I have now learned that boilers cannot cope with a dT this high, I will just need a mixer value on the input to the boiler to increase the return temp when needed.

More to the point, there is no benefit in having the return from CH or the plate heat exchange being below 30c apart from getting a better gain from solar on poor days.

 

[Dotty]

What do you do for a living Ringi?

 

[Howsie]

Boilers modulate so I'm not sure where you're going with this?

 

[lame plumber]

where do you live ringi, just so I dont need to quote for any work, I hate being watched let alone lectured when doing installs

 

[Killy Bing]

Theres a product called a magic heating box you might be intrested in! Give it a google!

 

[Rickster123]

What do you do for a living Ringi?

Retired electrical engineer would be my bet..

 

[chris watkins]

I was going to put that old chestnut "a little knowledge is a dangerous thing" but I think it needs a bit of revision, maybe "a good amount of knowledge when turned to a subject you don't fully understand is a dangerous thing" what do you think? or maybe just "some people with to much time on there hand ________" Complete as you will !

 

[doitmyself]

I did not say DHW cylinder….

Just shows how easy it is to make the wrong assumption when all relevant information has not been provided.