Delta T

[gordonspants]

I'm still trying to get my head around the best way to get a condensing boiler to condense as much as possible. Delta T of the boiler seems to be an important part of this. We have a Vaillant condensing boiler ready for fitting. At the same time, the plumber got me a Vaillant weather compensator, as we were originally going to use an unvented cylinder. Now we're going to use a thermal store (due to a planned woodburner with backboiler, possible solar when it gets cheaper, and as many other gas-bill avoidance plans as I can come up with including an extra jumper or three).
Do we still need a weather comp with a thermal store? Plumber says we don't.
Idea of thermal store with direct boiler connection (no coil) is the boiler runs long and hard, condensing as much as possible, whereas with a boiler just feeding rads and standard cylinder, the weather comp modulates the boiler down for the rads on a warm-ish day? Is this correct? Or does it blend the boiler flow or return to give cooler rads?
I have also read of having a blending valve on the boiler return - is this to keep the boiler return hot, cold, or close to the delta T of 50 degC for ideal condensing?
What is most efficient - flow very hot and return very cold, or warm-ish flow, 50 degC return - big temp difference, or 20 deg difference using a blending valve or using the weather comp on very cold days... or arrghh?
Should I get a blending valve for the return to the boiler if it's heating a thermal store, if so what would it do?
My head hurts. Ta in advance for any clarification!

 

[superdupergasman]

the delta t your looking for across the boiler is 20 degrees, you will struggle to get 50. the longer the boiler runs for the less delta t becomes and hence efficiency goes down.

 

[gordonspants]

Hi, thanks for replying!
So delta T is the difference between flow and return. I'm getting it confused between actual return temp.

So is delta T better when bigger?

50 degC return temp has been mentioned as being good for condensing - is this why a blending valve is used, to get the return temp up to 50 degC by blending hotter flow water into the too-cool return, or is the return temp better as cool as possible?
If return temp is best as cool as possible, I therefore don't need weather comp?

 

[superdupergasman]

Blending vlave is actually an auto bypass valve.

It will open when system resistance increases, less rads, and close when there are more rads open. use the rads to do the balancing, and adjust them to 20 degrees dif in the flow return temp.

 

[superdupergasman]

and yes you do want weather comp....its meant to increase efficiency, but have only read about it....never used it.

I have a programmable room stat and it never switches off the heating off, just keeps the house ticking along at different temps for different times of the day, again more efficient than heating from stone cold.

 

[Onetap]

The boiler's efficiency increases as the return temperature decreases.

The largest increase in efficiency is at the transition from non-condensing to condensing.

The weather compensator will drop the flow temperature to the lowest possible whilst still providing the required room temperatures.

The Vaillant does this by modulating/ turning-down the burner; this is boiler re-set, i.e., the boiler flow temperature is reset.

You don't need a thermal store until you install solar or/and a wood burner.

With a thermal store system you need a blending valve; the proper name is a mixing valve. It's usually a 3-port valve with a pump on the mixed outlet side of the valve.

This is system reset, i.e., the flow temperature is reset by blending hot flow water and cooler return by modulating the mixing valve. System reset is more common with non-modulating boilers. You can get 4-port mixing valves (uncommon) or you can use an injection mixing pump (very uncommon). It is not an automatic by-pass valve and has nothing to do with system resistance; someone is confused.

The wood burner is non-condensing; you must not allow it's return temperature to drop below 60 degC, or it will cause corrosion (back-end corrosion) and deposit excessive amounts of tar in the chimney. You can maintain the return temperature with a shunt pump (pumps hot flow into the return) or a 3-port diverting valve. The latter is the same in principle as a mixer, but is set to maintain the wood-burner's return temperature.

And you thought you were confused?

 

[gordonspants]

Am I right in now thinking on a thermal store, a 3 port blending valve (eg TMV3 bathroom antiscalding type thing?) is used to lower the temp of fluid entering the store down from the flow temp of the boiler, so that what returns to the boiler is at the correct non-condensing/condensing transition temp, which must be this 50 deg C I am reading about?

That must mean weather comp isn't required, as what will affect return temp is actually partly the temperature of the store, and partly the flow temp that the boiler is set at?

Eg boiler flow towards the store is 80 degC, blended down to 70 degC before entering the store, so that the return temp back to the boiler is then closer to condensing point of 50 degC?

 

[gordonspants]

I've discussed the woodburner bit with a stove/fire shop owner locally, and mentioned we will need a pump (with open vented system to feed the store coil) as thermal siphoning/heat rising to the store won't work due to the stove being same level as the store - he didn't mention an extra blending valve... am I confused? Oh yes.

 

[Onetap]

Am I right in now thinking on a thermal store, a 3 port blending valve (eg TMV3 bathroom antiscalding type thing?) ?

The TMV3 is a blending valve, intended for use on domestic hot water systems (hot taps); it uses a thermostatic device to control the mixed water flow temperature. They are used as a cheap option on many underfloor heating systems. The set-point/ flow temperature is set manually and then doesn't change.

A 3-port mixing valve has a motorized actuator. The flow temperature is set by an electronic controller. The flow temperature varies throughout the day, e.g., hotter on start-up, reducing as the house warms up, lower in the afternoon if the sun shines. Simple weather compensation resets the flow temperature accoding to the outside air temperature.

If you invest in programmable elctronic controls, you can also use the 3-port to maintain the return temperature above 60 degC, i.e., restrict the opening of the valve and so the amount of heat taken from the boiler as the return temperature drops towards 60 degC.

Someone will try to sell you a TMV; this is because they don't know any better or , more usually, they know it's carp but are happy to rip you off and won't tell you about the deficiencies inherent in it.


TMvs are usually used on the heating flow.

That must mean weather comp isn't required, as what will affect return temp is actually partly the temperature of the store, and partly the flow temp that the boiler is set at?

The return temperature depends on Flow temparature, system flow rate, and system heat emission.

Q = m x C x dT = m x C x (Tf-Tr)

Tr = Tf - Q/(m x C)

C is a constant, specific heat capacity of water

Tr can be varied by changing Tf, Q and/or m.

Eg boiler flow towards the store is 80 degC, blended down to 70 degC before entering the store, so that the return temp back to the boiler is then closer to condensing point of 50 degC?

No, TMVs are nothing to do with the return temp. You need to control the return temp on a non-condensing boiler only, purely to prevent condensation. The return temp to the store or a condensing boiler is better when lower. I must go and do some work.

 

[gordonspants]

I must go and do some work.

Shift work - been on 12 hour nights, happy days

So,
1 with the condensing boiler, I need the lowest return temp I can get.
2 I don't need a 3 port mixing valve because the boiler is a condenser?
3 The weather comp IS un-necessary.

Best for efficiency when 'charging up' the thermal store would be to set the boiler flow temp on the boiler to maximum so it burns for a shorter period, but might sacrifice low return temps towards the end of the re-heat, or am I thinking in the wrong direction - slightly lower boiler flow temp would give better/lower return temp (more condensing) but it'd run for longer until the store stats reach shut-off temperature. Argh.

 

[Onetap]

So,
1 with the condensing boiler, I need the lowest return temp I can get.
2 I don't need a 3 port mixing valve because the boiler is a condenser?
3 The weather comp IS un-necessary.

1) Yes

2) You don't need a 3-port mixing valve because the boiler has a modulating burner (as against an on/off burner). It turns itself down to get the right Tf.

3) Not necessary, but desirable to get lowest possible Tf and so lowest possible Tr. See 1 above.

Best for efficiency when 'charging up' the thermal store would be to set the boiler flow temp on the boiler to maximum so it burns for a shorter period, but might sacrifice low return temps towards the end of the re-heat, or am I thinking in the wrong direction - slightly lower boiler flow temp would give better/lower return temp (more condensing) but it'd run for longer until the store stats reach shut-off temperature. Argh.

I can't see any logic in operating a thermal store with a modulating condensing boiler; maybe someone else can explain it.

The store is useful with solar and the wood-burner.
The condensing boiler will just burn gas as needed to generate heat. There is no advantage, but some disadvantages, to burning more gas than needed immediately and putting the excess heat in a heat store.

 

[gordonspants]

I can't see any logic in operating a thermal store with a modulating condensing boiler; maybe someone else can explain it.
The store is useful with solar and the wood-burner.
The condensing boiler will just burn gas as needed to generate heat. There is no advantage, but some disadvantages, to burning more gas than needed immediately and putting the excess heat in a heat store.

Thankyou again!
I don't have a choice with the boiler - we need one that doesn't fall to bits, it has to be a condenser to meet regs, and the plumber got a really good deal on a Vaillant.
I agree you're right now I understand the basics - there isn't any benefit in a modulating condensing boiler with a store - boiler load will always be maximum, with low return temp right until the end of the burn when the store is almost charged up, so clever low-load modulation isn't an advantage, and neither is weather compensation. The store will be linked to the boiler directly, rather than thru a coil, partly to get the best cold return temps.
But, hopefully I'll be chopping wood rather than paying gas bills... when the boiler is used, I just want to make sure we get the best from it.
Thanks for the advice re the 60 degC return for the backboiler as well - something I've not heard of (yet!) but makes perfect sense