Nearing the end of the project.... And just a few niggles

[vetinry]

Hello all

Once again, I'm hoping that you might be able to give me a bit of advice

It's been a few weeks now since I last posted to the site. In the meantime, I've been having a pretty dramatic overhaul of the central heating system and domestic water.

We've changed from a gravity fed "Y" plan system to a unvented "S" Plan plus central heating and a completely seperate unvented Ecocent supplying the hot water.

The plumber has done a really tidy job on the pipework, but he doesn't seem quite so switched on when it comes to the technical side and trying to get the heating system up and running has presented a few problems.

There are now 3 CH zones, 1 for the UFH in the Kitchen, 1 for downstairs and 1 for upstairs.

The boiler is outside and 28mm pipe runs to the back of the old airing cupboard (Hot water cylinder now in garage) where it goes through a Grundfos 15-60 pump, changes to 22mm pipework and to the 3 2 port valves. Each of these is switched by its own Honeywell CM907 programmable thermostat.

The UFH circuit runs to the utility room and supplies a continental manifold with its own grundfos 25-60 pump.

The upstairs circuit is fairly small (2 rooms with thermal skirting boards, 1 radiator in hallway and 1 or 2 radiators in bathroom, not yet installed) and seems to heat up no problems at all.

The downstairs circuit has 11 radiators and the main circuit runs around the upstairs bedrooms (bungalow extended upstairs) and has 4 drops to different radiators. The final 3 radiators are right at the end of the run and we've been having problems getting any heat into them.

Now that there isn't a primary radiator circuit, there is also an ABV fitted immediately after the pump.

The plumber has been trying all sorts of different things like swapping the pump because he thinks that the one I have isn't big enough, and yesterday we changed all of the lockshield valves and the old Honeywell TRVs with Drayton TRV4s (some of the radiators didn't switch off previously) - it's felt a bit like clutching at straws.

So today, I went to Maplins and bought a infrared thermometer to try and balance the circuit a little bit more scientifically. I hadn't realised that each radiator should have such a large temperature drop between flow and return.

With the pump running on level 3 and all TRVs and lockshields fully open, I did begin to get heat into all of the radiators but the final 3 were only luke warm.

I also found it really difficult to reduce the flow in the first radiators on the circuit sufficiently to create an 11 degree drop.

So then I switched to position 2 on the pump and then found it easy to balance all of the radiators apart from the last 3. They were pretty hot and flow temperature was 70 degrees, but the temperature in the return pipe was only 45 degrees - does that mean that not sufficient hot water is getting to the radiators.

Questions:

How important is the pump speed - will the system heat up more quickly on level 3, and should I try harder to reduce the flow through the earlier radiators on the circuit to see if better flow gets to the final radiators.

What is the importance of achieving a temperature drop over the radiators - is this how they work most efficiently?

It would be possible to re plumb the system so that the pipe run to the final 3 radiators was only about 10 metres (currently it's about double that from the 4th last radiator in the system).

Why do grundfos domestic pumps only have 15mm bore, and is this important. Most of the system is either 28mm or 22mm and it seems strange that the flow is restricted so much through the pump. The pump on the UFH is a 25-60 but I know whether this would be any better)

Is the pump simply not big enough for the house - total floor area is approx 300 sq metres but the pump is the same one as being used previously - 2 of the rooms have had radiators replaced by thermal skirting, and 2 of the rooms downstairs are now one room with UFH.

Finally, the auto bypass valve (Honeywell DU145) appears to have flow through it even when all of the system is running and TRVs open. I looked earlier and it appeared to be set on 6 - I tried to adjust it (loosing the screw) but as the outer red dial rotated, so did the internal black on with the numbers on - should this happen, and without knowing the minimum flow rate for my boiler (a boulter camray 5), how do I know how to set it (in fact, how do I set it, or is it broken?)

Sorry about all of the questions. It feels like I'm almost done - just want to get these final things sorted

Thanks in anticipation

Steve

 

[TasMan]

Hi Steve,
I am not on the tools but have a lot of design experience on large domestic and commercial systems.

You mention the temperature difference accross the radiators, have you asked your plumber how the radiators were sized ? dT of 11 is usually 82/71oC whereas dT of 20 can be 80/60, 50/30 etc. The higher the dT accross the radiator the smaller the pipe sizes, flowrates etc so perhaps this needs to be considered when balancing the radiators in relation to the duty of the pump. One of the common mistakes I come accross is people installing a condensing boiler then running them at 82/71, or even 80/60 and hence you will rarely get the condensing effect as optimum return temperature is around 55oC, plus there is the issue of running the DHW off the boiler which must be above 60oC..... Of course, if your boiler isn't a condensing boiler then the 55oC doesn't apply.... Every rad will give off a certain amount of heat at a certain dT, and hence this can affect the physical size etc. When designing a heating system selecting your dT and knowledge of what boiler you want to install is one of the first things you do, and then you can go about sizing rads, calculating flowrates etc, which will ultimately give you a pump selection. Bigger dT's will give smaller pipe sizes and different rad sizes compared to typical dt 11oC.

It does seem a big system with 11 rads, cylinder and underfloor circuit, has the pressure drop been calculated and compared against the pump curve on the various speeds ?

Did you install a new boiler ? Modern boilers with stainless or ally heat exchangers can have large pressure drops through the heat exchanger, great at extracting heat but the downside is the pump head requirements need careful consideration, this should be checked.

You mentioned the bypass valve running all the time, this should really only open when the design pressure in the circuit is overcome by the pump, i.e when TRV's shut down, valves close etc. Perhaps this needs checking as if it is running all the time you will be short circuiting the pump and not shunting water round the system. Perhaps there is dirt lodged in the bypass valve preventing it from closing down...? Are the motorised valves working ok, again if the cylinder is not calling for heat the valve will shut and the water had to go somewhere.....

From what you are saying it would seem that pump duty, specifically available pressure could be the issue, and the required minimum flowrate for the boiler should be checked, along with flowrate for the UFH, radiators and cylinder requirements....but this can really only be determined once you have picked your operating dT based on the boiler manufacturer's recommendations.

Not sure if this helps but just some passing observations, with commercial design bias....!!
Cheers
Dave

 

[Mrs Tara Plumbi]

...hence you will rarely get the condensing effect as optimum return temperature is around 55oC

Is it the optimum return temperature is:
1. around 55 C or
2. 55 or less, with the lower the better?

I thought the lower the more it would condense so lower temperatures better?

 

[TasMan]

Is it the optimum return temperature is:
1. around 55 C or
2. 55 or less, with the lower the better?

I thought the lower the more it would condense so lower temperatures better?

Anything below 55 will aid the condensing effect but you need to stay within the parameters of the boiler and avoid huge differences in flow and return temps. You will definitely get better efficiencies running with lower overall system temperatures but you can't really go with systems running 80/30, not really practical and the boiler manufacturer is likely to say no!! Your typical wall hung condensing boiler will be happy with 80/60, 50/30 etc but you need to watch the hot water side if you are running a cylinder as you need to allow for the boiler to run to 60+ to heat the DHW when it is required. If you can design your system to run with a mean water temperature as low as feasible to obtain the same output you will allow the boiler to condense as much as possible. You really only get the full benefit if you are running dedicated heating circuit from a boiler, say underfloor heating which lends itself to low mean water temperatures, hence you can get really low return temps and good condensing effects.

There is no right or wrong answer that I am aware of, its all down to the application and most benefit will be gained by installing a new system from scratch and sizing radiators to allow for the operating temps and accounting for how the DHW is generated. Most condensing is likely to occur on cold start when design conditions are being met, thereafter when the rooms are up to temperature the boiler is just ticking along and the flow and return temps are much closer. This is where the subject of weather compensation comes into play and further benefits can be had by compensating the boiler flow temp to the outdoor condition....but that is getting into a whole new discussion !
Cheers
Dave

 

[vetinry]

Hi Dave
Thanks very much for your comprehensive reply. I probably needed to give a little bit more information. The recent work that's been done has primarily been for 3 reasons

1. The old hot water cylinder was too small (approx 150 litres) for our 5 bedroom house
2. Although the house is almost 300 sq m, the system was a Sundial Y plan with no seperate control of different zones
3. Last year, the oil bill was almost £2500 and I'd really like to reduce it considerably.

Therefore, I don't know very much about the original design parameters. I don't believe that the boiler is of a condensing type and so I'm not sure that the 55 C is important.

With all of the valves fully open, I did try and measure the temperature difference between the flow and return by the boiler but I'm not sure that it was fully warm. The difference was about 12 C with the pump on maximum and this initially increased to about 16 C with the pump on level 2.

Since the pipework is mainly 28mm (from boiler to zone valves - approx 20 m) and then 22mm for most of the circuit and drops to the downstairs rooms, do I assume that a lower dT was anticipated?

The Grundfos 15 - 60 pump was on the system that I inherited, and so I assumed it was the right one for the house.

With the renovation, we have effectively lost 7 or 8 radiators from the original circuit and replaced with mixture of UFH (for 3 rooms), thermal skirting for 2, and I'd like to put a couple of fan convectors in the study and conservatory. Plus, the system is now only CH, not DHW.

That's why I assumed that the pump would still be ok.

The plumber did try what he said was a bigger pump (25-60) but I thought that this was the same pressure with a wider bore.

So, how do I know if I've now got the right pump, what the correct dT should be, and how to set up the system correctly?

The radiators in the main lounge (2 of the 3 on the end of the system, and one of which seems to be the index radiator) have warmed up tonight, but still much more slowly than the first one or 2 on the system.

When I install the fan convectors, they will be immediately before these final 3 radiators on the circuit, with about a 10 metre pipe run between them and the final drop. I'm worried that their thirst for heat might compound the problems with these final 3 and therefore whether I'd be better to install some much more direct pipework to these 3 radiators and then the fan convectors (which are heating much smaller rooms) would then be at the end of the circuit? Do you think this seems like a reasonable idea?

Finally, I've read a couple of different sources about flow and return systems. One of which shows the flow pipe continuing into the return pipe which therefore allows continual flow through the system.

The other shows the flow pipe running into the final radiator and then the return pipe originating from there. This seems more sensible because then, if the final radiator doesn't require heat, then there won't be flow (and unneccesary heat loss) through the circuit.

I've just been looking at the grundfos site and think I'll sign up for the academy to try and better understand the pump side of things.

I'm think I'm getting to grips with the basics here but wish I could understand the technical bits a little better.

Sorry for all the questions

Steve

 

[TasMan]

Hi Steve,

Its sounding like a fair sized system you have there and I have looked at the pump in a little more detail. Using Grundfos Webcaps you can find the details on the 15-60 pump, specifically the pump curve showing the flowrate versus head at the 3 speeds. Taking a nominal 2 m3/hr as an example flow on the curve equates to 0.55kg/s which in kW terms is about 25kW. At 2m3/hr this is getting near the upper end of the useable pump duty on speed 3, specifically 3metres head which is approx 30kPa. The 25-60 pump, from the information I have appears to be exactly the same pump and the reason I know is that I have a 25-60 in my house and have the pump details to hand which appear the same as the Webcaps info for your pump, but worth verifying with Grundfos.

If you are aware of typical pressure drops through systems 30kPa is probably fine for a typical house but your house is large and has reasonable pipe runs with many fittings valves etc and this will quickly eat up the available head of the pump, this is even before considering adding in fan convectors which can have high pressure drops through the coils. Anyway, digressing slightly, it would be useful to know the kW rating of your boiler and we can then work out a typical flowrate which I would assume to be based on an 11oC dT given it is an oil boiler. As you will be aware to get the full heat rating of your boiler you need to have a pump capable of the required flow that will overcome the system pressure drop. I suspect this is where your issues continue to lie. But you also need to look at the heat demands of your house which appears larger than average and may in fact outstrip the pump capacity in terms of flowrate around the system to carry the heat required.

When sizing a typical heating system we design on a two pipe flow and return and calculate the approx total pipe length to the index point which is usually the furthest point in the system. So if a radiator is 20 metres from the pump you will have 40 metres of pipe, allowing 25% extra length for fittings, at 200Pa per metre gives 10000kPa, or 10kPa. That is only usually a good guide if the system has been sized correctly, i.e pipes have been sized no greater than 200Pa/metre for the required flow, but it gives an indication. A radiator/valve can typically be 4 – 6 kPa, boilers vary with manufacturer but allow say 10kPa and we are now sitting at 10 + 6 + 10 kpA = 26kPa…….as you can see if your pump can only do 30kPa we are starting to get close to the limit.

It sounds like your pipes are sized ok and you have no huge runs of 15mm serving lots of radiators so I suspect your pipework is sized ok, but I can’t be sure on the original dT of the system, but most likely to be 11oC.

Speak with your plumber again about the pump sizing. I had a look at a 25-80 pump curve and this gives a much bigger pressure for the same flowrate. Comparing the 2m3/hr scenario this pump can deliver 2m3/hr at approx 52kPa on speed 2, speed 3 is sitting up at 65 kPa. In theory you will therefore be able to drive a flow of 2m3/hr through a system up much greater pressure drop. You will also tend to find that even if a pump is set to a certain speed, it will produce a certain flowrate depending on the system loss, this is the duty point, therefore excess pressure available isn’t necessarily a problem, just means the water will get shunted round the system at a much higher rate and allow maximum transfer of heat from the boiler. Changing to a larger pump may however affect the pipework, often the port to port length is bigger and modifications would be needed.

Sorry if I am rambling on but am just trying to paint a picture here of pump sizing etc. The only way to know if you have the right pump is to have it calculated out properly based on the system pressure drop and the flowrate required to transfer the kW of heat required to your house. The dT is ‘likely’ to be 11 but can’t guarantee it, however until you get adequate heat through to all parts of the house there is little point worrying about that just now. When the system is delivering adequate heat you can then ask the plumber about balancing the system and improving dT’s etc.

I would be careful about bolting on fan convectors into a system. They will have heavy coil outputs and hence high required flowrates which will further impact on your pump requirement. You could possibly run a dedicated circuit to the FCU’s but would need to include some sort of flow regulation device to ensure only the required flow is going to the FCU circuit and not starving the rest of the radiators and UFH, we would commonly install a commissioning set to achieve this, but this is starting to get into commercial heating system territory. Before jumping in and installing FCU’s I would try to sort out the existing distribution first and then consider where to go from there.

Finally, the piping design you mention sounds like single pipe and two pipe systems. Modern design is two pipe flow and return, older systems often have a loop with radiators branched off at various points. Fine for small applications but doesn’t lend itself to good distribution and can be tricky to set up, with two pipe you can easily work out the pump head required to the far end of your circuit, as described earlier.

I hope you find the above useful, obviously only my thoughts and only your plumber can verify and carry out the work and you will ultimately need to decide the best way forward. (that’s my disclaimer over!!)

If you can post more info on your boiler and obtain total rad kW outputs, UFH circuit flow requirements (manufacturer usually can confirm flow to the manifold) and perhaps work out your approx system pressure drop I will try to help further.

Cheers
Dave

 

[vetinry]

Hi Dave
Thanks again for such a comprehensive reply. I've managed to dig out a little bit of additional information so far, which should hopefully help.

Firstly, the boiler is a Boulter Camray 5 65/95 - does this mean it supplies between 65,000 BTU, and 95,000 BTU? It was installed in June 2000 and I think is rated at about 87% efficient.

I don't know what the UFH requirements are but can ask Continental for the details on that. The installer said that he works on the basis of dividing the total length of pipe installed per circuit divided by 4 to give a flow rate. I have 2 circuits each with 100m of pipe installed and so have set the flow rates to 2.5litres per second. Interestingly changing the UFH pump speed controller between 1 and 3 doesn't seem to make any different to the flow meters. Is this what you mean?

I can make a fairly good guess as to the length between the pump and the index radiator based on the length of the upstairs rooms its going around. Including the pipe drop from upstairs to downstairs, I reckon it's probably about 28m of pipe - can't see under the lounge floor downstairs, but all the rest from the 2 port valve is 22mm.

Whats the best way of estimating the radiator outputs - do I need to measure them and then look up some outputs?

I've found the leaflet that came with the pump and it says that it shouldn't ideally be installed in horizontal pipework and if it is, the spindle should be horizontal rather than vertical. Mine is currently vertical but I can easily twist it around. The only place that it could go in vertical pipework would be to move it close to the boiler but this would move it about 15 to 20m further from the zone valves, and from the index radiator. Would this all add to the pressure requirements?

Cheers

Steve

 

[SimonJohns]

Is this a very elaborate way of saying;
A low loss header would be best

??????

 

[vetinry]

I've just had a look at the camray 5 manual and found the following information:

[FONT=Arial,Bold][FONT=Arial,Bold]

3:2 BOILER TECHNICAL DETAILS​

[/FONT][/FONT]

Maximum Boiler working pressure 3 Bar - 30.6m Water Head.Minimum recommended return water temperature 60ºC.Maximum hearth temperature Less than 85ºCMaximum side panel temperature will be less than 35ºC above room temperature.Water Resistance Less than 300 m.m.w.g.​

With 11ºC temperature rise across the boiler.

does this mean that it is designed to have a dT of 11? And if minimum flow return temperature is 60 C, can it be any 11 degree difference above this minimum temperature? It's got a dial on the front with 5 positions - are these the equivalent of a temperature control for the boiler?

You've described the requirement of the pump to overcome pressure drop in the system, and I understand that using the distance to the index radiator plus connectors, radiator valves etc all add to this figure. Should there be a figure quoted for the boiler? And what about any pipework between the boiler and the pump because I have quite a long run of 28mm pipework. Or is this already factored in because in supplying the index radiator (and all others) water is automatically drawn through this pipework from the boiler?

Steve

 

[TasMan]

Hi Steve,

I have had a quick trawl of the web and found 2No. manuals on the Camray 5 boilers. I can find a 65/90 boiler and a 95/130 boiler. I will assume yours is the 65/90 which according to the info gives 26 – 27kW at 11oC dT running on kerosene. You are right about the BTU output, this will relate to the firing characteristics of the burner and converting to kW matches my findings.

Not sure on the method your UFH manufacturer is using to calculate the flowrate, seems unrelated to the heat output but perhaps he knows how much heat is given per metre length of pipe and he is calculating the flow based on that. On commercial projects I work on we always insist the UFH manufacturer confirms their total system flowrate and kW output and we can then provide the heat required via our main system design.

Estimating radiator outputs is reverse engineering the heat losses (assuming rads sized ok!) and will give a fair indication of total system load. I am assuming your rads will be the same age as your boiler, have a look at a radiator catalogue, Stelrad Elite for example. Make a schedule of your rads in each room i.e room, length x height and whether they are double panel double convector (K2), double panel single convector (P+) or single panel single convector (K1). Ignoring the factors for different temperatures at the moment check off the tables at 50oC the heat outputs. This should give you a fair estimation of your total rad output. If you want to get really technical you can use the factors to work back theoretical heat loss. Knowing your meant water temperature ((82 + 71)/2 = 76.5) and then subtracting your room temp from the MWT (76.5 – 21 = 55.5) you can then obtain the factor applied to your rad sizing (typically 1.1ish) and then work that back through your rad sizing. But don’t get too hung up on this if you are getting lost in the numbers !!

Anyway, between your UFH and rads you should be able to tally up your total system load. What type of hot water cylinder do you have ? Check with your plumber how the system is controlled i.e hot water priority or simultaneous heating/hot water and then factor in the coil rating of the cylinder.

Your pump should definitely be installed with the spindle horizontal. Canned rotor pumps are lubricated by the water and any air in the pump will cause damage to the pump, you may even find your pump performs better if it is turned to the correct rotation which will allow the pump to operate as intended.

The position of your pump in the system, in terms of pressure, doesn’t matter too greatly, it’s the overall pressure available that you are concerned about. If you imagine your pump in the middle of the pipe loop, in your instance as the pump is doing the complete system circulation the whole circuit is in effect the index. Running from the pump to the last radiator, and then the pipework all the way back to the boiler, through the boiler and then back to the pump…..this is your circuit and what should be calculated for the pressure drop….typically 200 Pa/metre. According to the boiler manual your boiler has 300 mm water gauge of resistance through the heat exchanger. This is typical of large older cast iron heat exchangers. This equates to approx 3kPa which is fairly small overall, and means your pump has more head available for the rest of the system. I would make an estimate on your total pipe length, add 25% for fittings, add in the boiler 3kPa and see what it comes out as.

With reference to SimonJohns email, yep in an ideal world what Steve is trying to achieve should ideally be a boiler with low loss header and shunt pump with separate circuit pumps off the header but this would mean a significant re-pipe of the system at the boiler.

Regarding your last post Steve, yep in theory your rads, pipes etc should all be designed to dT 11oC but in reality it has probably just been installed and balanced based on rule of thumb figures for heat losses etc. Not sure on your boiler dial but likely to be adjustment of the boiler thermostat. Your boiler should generate water at say 82oC and the dT relates to the heat losses in the system hence if correctly designed it should come back to the boiler 11oC cooler. 60oC is the minimum temperature to prevent condensing effects in your boiler which is not good in cast iron heat exchangers as sulphuric acids can form and damage the boiler, hence why older non condensing boilers all generally operate with higher flow and return temperatures to avoid condensation. Modern boilers with stainless steel/ally heat exchanges can cope with acidic formations fine, hence they can operate at lower temperatures. See above on the pressure through the pump, yes you need to include for all pipework and the boiler in this instance as your pump is circulating the complete system.

Keep us posted on your investigations Steve…..!
Cheers
Dave