42mm piping?

[JamesPa]

34 minutes ago, ReedRichards said:

If the boiler sets its own delta T it must be able to read the room temperature. 

It only needs to measure flow and return temp which it can do at the boiler.  Then it adjusts the pump speed to get the desired deltaT.

[markocosic]

10 hours ago, ReedRichards said:

Whereas for a gas boiler the opposite is true, the lower the return water temperature the better the efficiency so for a gas boiler 52.5/32.5 is more efficient than 47.5/37.5 which is more efficient than 45/40.  Correct me if I'm wrong. 

 

You're correct.

 

Return temperatures to condense great out of the flue gas matter.

 

Hence wider dT on the water side for condensing boilers (usually 29C rather than 20F/11C) even though this increases radiator size for a given for temperature etc.

[markocosic]

9 hours ago, sharpener said:

The words of mine you have signally failed to understand are <the return to the buffer tank>.

 

I missed that entirely because it wasn't in this thread / considered relevant to the previous queries as to why pipes need to be the size they are.

 

And if you have a 3/4 pipe buffer, which means two pumps, then you don't need an auto bypass either. (modern pumps of the sort you would use on a secondary circuit will happily turndown or indeed deadhead)

 

They have no place in variable flow system design any more. Not since electronic pumps became a thing. 

 

The place I see them all the damn time is people fitting heat pumps wrong and thinking that tricking the flow switch into there being sufficient demand (flow) to justify firing up the compressor is a good thing. Some less ethical distributors even suggest it.

 

9 hours ago, sharpener said:

 

No idea what you mean, why don't you proof read your postings?

 

My brain isn't good with words. 

 

It reads that as "one rad on a heat pump" because that's what it is expecting to see in context - unless someone else points out the typo or I reread some hours after initially writing it.

 

This is not an uncommon condition.

 

 

Looks like you're having a bit of a huff at the moment generally though.

 

I'll leave you to your own 42 mm pipes and other fudge ups in that case..

 

If on reflection my time spent reading your musings and offering advice has any value you might try to appear less ungrateful about it.

[markocosic]

2 hours ago, dpmiller said:

A heat pump with it's circulating pump under PWM will control flow rate to assist in setting the deltaT. It's under maximum heat demand that things get sticky as pipe restrictions might be fixed but flow though the UFH mixers is not. I'm using an auto bypass to help out in this situation.

 

Why mixers at all?

[JohnMo]

3 hours ago, ReedRichards said:

If the boiler sets its own delta T it must be able to read the room temperature. 

Why - don't follow your logic. Please expand.

 

If you are flowing a set temperature, the amount of work being done by the radiators is reflected in delta T.  As room gets closer to the radiator temperature the radiator has less work to do.  So delta T decreases, as less energy is transferred to the room.  The boiler then cuts back flow to manage delta T.

 

Test your own system if you are sure, I have.

 

If you choose to run the radiators at 70 degs flow temp the delta T will stay at about 20.  If you run the temps lower than 70 very rarely will you see a delta T of 20.

[ReedRichards]

19 hours ago, JohnMo said:

If you are flowing a set temperature, the amount of work being done by the radiators is reflected in delta T.  As room gets closer to the radiator temperature the radiator has less work to do.  So delta T decreases, as less energy is transferred to the room.  The boiler then cuts back flow to manage delta T.

As the room gets closer to the radiator temperature (i.e. hotter) the radiator has more work to do, because the work it has to do increases with the difference between the outside temperature and the room temperature.  

[JohnMo]

23 hours ago, ReedRichards said:

the boiler sets its own delta T it must be able to read the room temperature. 

So back to your comment

 

You the user on a modern modulating boiler or ASHP cannot choose the delta T, that is predefined within operating software. The in-built circulation pump will change speed to maintain the delta T it wants. The only way to change it is add hydraulic sepereration between heat source and heating circuit, such as a buffer or LLH.

 

[ReedRichards]

9 minutes ago, JohnMo said:

So back to your comment

 

You the user on a modern modulating boiler or ASHP cannot choose the delta T, that is predefined within operating software. The in-built circulation pump will change speed to maintain the delta T it wants. The only way to change it is add hydraulic sepereration between heat source and heating circuit, such as a buffer or LLH.

 

Okay, I'll buy that.  With an ASHP we know it is trying to achieve a small delta T, typically 5 C.  So you can size your radiators accordingly and if Delta T is actually 3 C or 7 C it won't make much odds.  With a gas boiler, the object is to achieve a much larger delta T.  If this is fixed and you know what it is then you can still size your radiators accordingly.  But you indicated that it is variable and the difference between 20 C and 10 C could make a much more significant difference to the radiator output.  So what value do you use?  If, for example, the gas boiler uses 10 C delta T to heat the room up but drops to 20 C to maintain the room temperature then you would size your radiators based on the 10 C delta T.  But that mode of operation requires that the gas boiler knows the room temperature and how far that differs from the set temperature and can therefore do a form of Load Compensation.  So my thinking is that variable (and large) delta T is only viable in a gas boiler that does Load Compensation.     

[JohnMo]

For a gas boiler I would simply design it to run exactly the same as a heat pump, big radiators, big coil in cylinder. Install buffer if required.

 

For rads design around a delta T of about 5. Run then at 30 to 40 degs, let the boiler condense as much as possible and have the efficiency uplift. Our UFH is running on a delta T of about 4 fed via a buffer.  Average boiler efficiency in this mode is around 110%.

 

For radiators I would engage load compensation, but not for UFH, as system response time messes things up.