Flow rate in central heating system

[Biggles..]

Calcs appear to done at 30degc delta t.

Boiler if like the vaillants probably goes in to s code waiting period

presume house/rads not upto temp

what pipe size is the primaries to the pump,for a 25-60should be minimum 28 might be worth lolling into a 25-80 or 25-100 greater flow against resistance. Don't come cheap but you should be able to sell your pump on eBay.yours cost new is between 2 and 4 hundred depending on discount by merchant

 

[D_Hailsham]

Going back to the beginning for a moment,

I've got a problem with the return temperature dropping too much for my boiler, and the safety cut out comes on.

Which Fault Codes (F??) appear when this happens?
Which Fault codes are stored in the memory (page 45, para 13.3?
Which State List code(s) appear(s) (see pages 50 & 51 of manual)?

 

[edb49]

You say you are getting a 30°C differential. What are the flow (d.40) and return (d.41) temperatures?

I'm measuring the flow and return temperatures from these two menus in the boiler. I've confirmed the sensors are accurate with a temperature probe.

What seems to happen is the temperature differential will start off at 20c and then climb up to 30c after a few minutes as the boiler gets itself going. (The boiler seems to start off on 4/5 heat output and then work its way up to 5/5.) The safety cut out seems to trigger when the 30c difference has existed for 10 seconds or more.

Just reading the rest of your post now and trying to get an understanding of it.

 

[edb49]

Going back to the beginning for a moment,

I've got a problem with the return temperature dropping too much for my boiler, and the safety cut out comes on.

Which Fault Codes (F??) appear when this happens?
Which Fault codes are stored in the memory (page 45, para 13.3?
Which State List code(s) appear(s) (see pages 50 & 51 of manual)?

I should clarify this - the boiler doesn't cut out entirely, but it goes into a "limp home" mode. This is in the D.99 menu as state S.53 which the manual describes as: "waiting time: temperature difference flow - return sensor too high". I appreciate this description makes it sound the wrong way around - I have implied that the problem is the difference between the flow and return temperatures by monitoring D.40 and D.41 immediately before boiler goes into S.53 mode.

There aren't any fault codes that appear - and nothing is stored in memory either, it's just the S.53 waiting time.

 

[D_Hailsham]

You say you are getting a 30°C differential. What are the flow (d.40) and return (d.41) temperatures?

I'm measuring the flow and return temperatures from these two menus in the boiler. I've confirmed the sensors are accurate with a temperature probe.

What seems to happen is the temperature differential will start off at 20c and then climb up to 30c after a few minutes as the boiler gets itself going. (The boiler seems to start off on 4/5 heat output and then work its way up to 5/5.) The safety cut out seems to trigger when the 30c difference has existed for 10 seconds or more.

Very interesting - but what are actual flow and return temperatures when the fault occurs?

How are you estimating 4/5 and 5/5 output?

 

[Biggles..]

S53 on GW or Vaillant is air in the Hex or in correct pump setting or performance

 

[edb49]

Why would the safety cut out activate with low return temperatures?

I don't think low return temperature is the problem, it's the temperature difference across the heat exchanger. I'm not 100% sure why it's undesirable to have a large differential, but maybe something will expand or contract??? Just guessing!

What F&R temperatures is your boiler achieving when system up to temperature?

About 20c - when it is up to temperature flow is 80c and return is 60c.

What F&R differential temperature did you base the flow rate calculation on?

The temperature the boiler cuts out at - it's 30c, and it only just seems able to get this 30c difference. E.G. 80c flow, 50c return, 70c flow, 40c return - same thing happens with each.

And finally, what exactly is this thread about? A cold house or a hot house?

Cold house - the boiler will not heat the house up when it goes into the safety cut out mode.

 

[edb49]

what pipe size is the primaries to the pump,for a 25-60should be minimum 28 might be worth lolling into a 25-80 or 25-100 greater flow against resistance.

I've just measured the flow to the pump at 26.5mm on external diameter - so I guess is is 28mm and I'm not measuring properly.

 

[edb49]

Another avenue to explore is the output of your radiators, to make sure they have the capacity to dissipate 35kW. Provided you have the standard flat steel rads, you can use the Stelrad Elite Catalogue to get a good idea.

Yes, I've checked the output of the radiators and there's sufficient capacity there to warrant the 38kw output.

You said earlier that you were 99% sure that the problem was not due to poor balancing. However the pressure through the heat exchanger of the 38hxi is 4.05 metres (see diagram 5.1 in the installation manual). The maximum the pump can handle is about 5 metres, which leaves only 1 metre for the pipework and radiators. This is a very low figure - most small systems are about 2 - 3 metres; yours could be much higher. A slight error in balancing the system can have a significant effect on the pressure loss in the system. This is particularly true of a mixed radiator - underfloor heating system.

To summarise: I don't think it's a boiler problem - it's a pump problem. You really need an engineer who can calculate the pressure loss round your system and advise on the correct pump to use. If you want to have a go, read Copper Tubes in Domestic Heating Systems, which tells you how to calculate the "index circuit".

I think this is about where my understanding runs out. What is clear is that the 38hxi boiler has a much higher resistance than the 30hxi, and I would imagine the pump has been sized on the previous boiler, and old non-condensing floor-standing one. I guess this had less resistance with a less sophisticated heat exchanger etc.

I've already had three plumbers come out to try and fix the system unsuccessfully, if it's allowed within the rules of this forum can anyone suggest one who could come to the Maidenhead area to sort this out please?

 

[edb49]

The fault happens when the temperature difference between flow and return is 30c or more for 10 seconds. The fault happens when the system is relatively cool (e.g. 25c and 55c) and relatively hot (50c and 80c).

The 4/5 5/5 output I'm just reading the LCD display on the boiler, it has an indicator of a little flame with 4 arrows above it.

 

[Biggles..]

From memory these boilers like no more than 15seg across them before triggering S53.

Normal course of action for this is bigger pump,top speed and down rate the boiler.

Don't know anyone in your area unfortunately

 

[D_Hailsham]

I should clarify this - the boiler doesn't cut out entirely, but it goes into a "limp home" mode. This is in the D.99 menu as state S.53 which the manual describes as: "waiting time: temperature difference flow - return sensor too high".

Have you carried out the checks suggested for this fault?

I'm not 100% sure why it's undesirable to have a large differential

The higher the differential, the more heat is required to raise the temperature at a given flow rate.

Am I understanding this correctly? The boiler heats up to 80, with a differential of 20. The flow stays at 80 but the differential increases until at 30 the boiler goes into limp mode until the temperature diff has dropped.

If the flow is 80 and the return 60, the boiler is not condensing , so it is only producing 38kW, not 41kW. The flow rate will be 1.633m³/hr. For some reason this flow rate is not being maintained, which suggests an increase in the pressure in the system.

Do you have thermostatic valves (TRVs) on your radiators?
Do you have a bypass? If so, is it manual or automatic?

Yes, I've checked the output of the radiators and there's sufficient capacity there to warrant the 38kw output.

What did the rads add up to?

 

[edb49]

Have you carried out the checks suggested for this fault?

Yep.

Am I understanding this correctly? The boiler heats up to 80, with a differential of 20. The flow stays at 80 but the differential increases until at 30 the boiler goes into limp mode until the temperature diff has dropped.

Not quite - once the system is up to temperature and the boiler starts modulating, everything is fine. The problem is when the system is coming up to temperature - the boiler on its maximum output causes a temperature differential between flow and return of 30c, which causes the S.53 error state. Once the system has warmed up, the boiler will modulate and so burn less gas and the differential will be < 30c.

Do you have thermostatic valves (TRVs) on your radiators?

Yes, all rads have TRVs.

Do you have a bypass? If so, is it manual or automatic?

Yes, automatic.

Yes, I've checked the output of the radiators and there's sufficient capacity there to warrant the 38kw output.

What did the rads add up to?

Including the underfloor (which I guessed at 150w/m^2) it came to 36kw.

 

[D_Hailsham]

I hope this is not too disjointed.

Not quite - once the system is up to temperature and the boiler starts modulating, everything is fine. The problem is when the system is coming up to temperature - the boiler on its maximum output causes a temperature differential between flow and return of 30c, which causes the S.53 error state.

That's interesting. It means the flow rate, for some unknown reason, is lower than required. Normally, when a house is below temperature and all TRVs are open the system pressure is at its minimum. As TRVs come into operation the flow rate reduces and the pressure increases, which is where the Automatic bypass valve come into operation. The valve opens to allow some of the water to flow back to the boiler. It could be that the valve is set to open at too high a pressure.

What is the flow temperature when the house is up to temperature?

What happens if you reduce the max flow temperature from 80 to 70?

When the boiler goes into slow mode with S53 showing, does the pump continue to run? How long does it take before it goes back to normal?

TRVs on all rads?? Don't you have one rad without a TRV where the room stat is located?

What happens if you remove all TRV heads?

Ignoring the underfloor, which is controlled separately by a mixer valve etc, what do the rads alone add up to?

The reason I ask this is that rad output varies with flow, return and room temperatures. The output is always quoted for 75/65/20; 80/60/20 is virtually the same. However the output at 70/50/20 is about 73% of the nominal. This is not relevant to underfloor heating. However it could account for the house feeling cold.

 

[Biggles..]

You have the classic vaillant gloworm S53 problem.Had you poster just the model of boiler and the S code all the guys on here would have given you the same advice.

I'll explain what your boiler is doing. Boiler sees demand for heat,fires and modulates to full.Pump runs(your fixed spped set to max).Flow temp rises(temp on display when not in I mode) but the pump cant shift the water quick enough allowing the flow temp to rise more than the tolerated delta t that is set into the boiler.Boiler pcb uses flow temp and return temp plus software to calculate this.Boiler then goes in S53, with the pump continuing to run to try and lower the flow temp and raise the return.Once delta t of the boiler is within the tolerence set by the software boiler fires to max and the circle goes round again.

The circle is only broken when some of the load is reduced either rads closing or underfloor upto temp.

Cure for this is simple,down rate boiler so it matches your pump performance(you cant you need 36KW),increase pump speed(you cant its on full),increase pump size

You can do all the maths you like,unfortunately its a common problem with these boilers,your old boiler might have been happy with that pump and such a big difference initially in flow and return but your new product will not.

Could be totally wrong but a vaillnat installer i know how i'd solve your problem.Look on the website for an accredited engineer near you, phone the manufacturer who will advise an engineer near you.

Lee