magnaclean filter and low loss header

[Onetap]

If the gate valve was an open pipe then it would better correspond to Keston's secondary pump scheme, so I don't see that as a problem.

The gate valve will have a very small hydraulic resistance; the 'gate' should be out of the water stream when the valve is open. The LLH still works as Keston's recommendation, albeit that the pipe should be larger.

You are wasting 50% of the boiler energy to keep the delta T from going off the scale.

Apart from extra pumping, no energy is wasted; half the flow twice the delta-T = same energy.

It lowers the efficency, I'd think by about 5%, but you can't avoid that for short periods if you should need 75degC flow for a HW demand or to supply the space heating demand.

I suspect that most balanced modern domestic CH will use a by-pass to increase boiler flow once the TRVs kick in and increase delta-T above the safe limit for the boiler.

If you look at your temperature curve, the problem is that Fpri > Fsec. The excess primary flow goes back into the boiler return. If you increased the secondary flow, it would alleviate the problem. A weather compensation system will do this. 100% flow at half the secondary delta-T, but still at maximum efficiency because you keep Tr below 55 degC by lowering the flow temperaure. The boilers' delta-T is still controlled at 15 degC by the boilers' controllers. The secondary flow temperature is reduced by mixing in the LLH, since Fpri < Fsec.

The system will deliver the design flow at 70 degC flow at 100% load, i.e, on a cold day.

On a mild day, 50% load, the existing system will deliver 70 degC; the TRVs will throttle the flow down to 50% or less.

A WC system will reduce the flow temperature to the minimum required to supply 50% load and the flow rate will increase to supply the same 50% heat load. The flow temperature should vary to maintain the room temperature in the one room with the sensor. The TRVs in other room will only trim the flow slightly. You should still get 90%+ or so of the design flow rate.

OK, many systems are not balanced, so the total flow through the radiators can be more than the design flow for the boiler and the by-pass need not open. Even when the TRVs reach their set points, the delta-T from the radiators might still be lower than the limit for the boiler. But you wouldn't leave radiators not balanced, would you?

The LLH system disconnects the boilers and radiators. The flow through the boiler is set by the pump speed and the boiler's controls. The flow through the rads can be one tenth of this, or ten times this and the boiler flow rate is unaffected. The boilers' delta-T is controlled at 15 degC by modulating the burners.

An infinitely variable pump would be ideal but with a suitable analogue controller would be much more expensive. The 3-speed pumps provide adequate control and are cheap, being a standard domestic CH pump. It's a clever system.

 

[Onetap]

I get the impression that the best thing I can do now is to start digging for a new soak away in anticipation of a new boiler in the spring.

No.

You would have to spend £2.5k+ on a new boiler if you had only one boiler that was broken beyond economic repair, or if you had an inefficient, open-flued, unbreakable cast iron lump, or if your boiler was unreliable and expensive on parts and repairs.

I don't know these boilers; those who do are not complimentary. They do have three points in their favour. They are efficient (SEDBUK A rated), they work and there are two of them. They aren't broken, so why fix them? Why scrap them? Whether they're unreliable or costly to maintain, only you know.

You have two boilers, either one of which will keep you warm in all but the very coldest weather. If one breaks, it is not an emergency. When one is beyond repair, you have a set of spares for the other and can organize a new boiler at your leisure. You could keep the working Keston for when the new boiler breaks.

You need a proper WC controller, not a new boiler. Controlling a modulating boiler with an on/off timeswitch and on/off thermostat is lunacy. You didn't get a suitable controller when the Kestons were installed because the installer only understood thermostats and timeswitches. A boiler installer will recommend you install a new boiler. If he installs a new modulating condensing boiler, you will still need a compatible WC controller.

 

[ajrobb]

You need a proper WC controller, not a new boiler. Controlling a modulating boiler with an on/off timeswitch and on/off thermostat is lunacy. You didn't get a suitable controller when the Kestons were installed because the installer only understood thermostats and timeswitches.

I've not seen any controller options for these boilers. I think we're stuck with a single flow temperature, governed by legionella (60°C DHW). I want a 4-pipe system boiler with weather compensation (e.g. Veissmann Vitodens 200-W 35 kW) and multiple zones. I'll probably get a 2-pipe heat-only boiler because of cost; it only needs an ABV on the existing pump to complete the installation.

 

[Onetap]

I've not seen any controller options for these boilers.

Ask Keston; there is/was an option for control by an external controller, maybe connected to the SL terminals. This is usually a 0 -10V DC input signal.

I think we're stuck with a single flow temperature, governed by legionella (60°C DHW). I want a 4-pipe system boiler with weather compensation (e.g. Veissmann Vitodens 200-W) and multiple zones. I'll probably get a 2-pipe heat-only boiler because of cost; it only needs an ABV on the existing pump to complete the installation.

You need a control system that will give you both;
a high flow temperature to the DHW cylinder
and
a reduced flow temperature to the space heating, as required by a WC controller.

There are two ways of achieving that.

One gives priority to DHW. When there is a DHW demand the boilers run at high flow temperature and the CH is off. When there is no DHW demand the boilers run at the Tf required by the WC controller. If you look at Fig 2 posted on P5, but delete the manifold station and 3-way valve, that would be the arrangement. Either the DHW pump runs or the CH pump serving the panel rads (higher temperature load) runs.
You could do this with one secondary pump and 2 zone valves.

This is OK for most systems. The CH is off when the cylinder heats and the space temperature drops slowly. I have a Vaillant ecoTec WC system that works this way. It is inappropriate if you have a high DHW demand (B&B, many teenage daughters, etc) or if you have air handlers that will blow cold immediately the heat goes off.


The other method runs the DHW and CH simultaneously giving two flow temperatures and 4-pipe connections. This is as Fig 2, page 5 but with the CH radiators connected through a mixing valve to give the reduced flow temperature. The 3-way mixing valve would have an actuator, with a 0-10V control signal from the WC CH controller. A cheaper option would be to use a TMV, but I would advise against this. When there is no DHW demand, the boiler flow temperature drops to the temperature required by the WC controller.

The Viessman 200W works this way, but has a mixing valve inside the boiler casing. I very nearly bought one, but decided on the Vaillant.

I don't see the point of having a mixing station crammed into the boiler casing; I'd prefer a separate one, but then I have a few spare mixing valves lying around in my shed.

 

[ajrobb]

Well, I did another check this morning. I turned up nearly all the TRVs to max (I missed the hot kitchen radiator and small cold laundry radiator) and left the system to stabilise for 15 minutes. The gas consumption was 46 seconds for 1 ft³ at the meter or about 25 kW* input. (I use 1132/t to calculated input power from seconds/ft³)

*It is not clear on the gas bill if they quote gross or nett calorific value. I'm sure I've seen over 100% 'nett efficiency' quoted with UFH temperatures (40-30). I think the nett calorific value is for gaseous products and ignores the latent heat of vapourisation in the water vapour whereas the gross calorific value includes this latent heat.

This measurement was with a flow temperature of 70°C and a boiler return temperature of 60°C (non-condensing) so running about 88% efficient or about 22 kW output. (I calculated 22.5 kW from the radiator sizes.) It is 10°C outside blowing force 3 or 4 and the heating is adequate. I increased the flow temperature to 85°C in the recent cold snap and measured a gas consumption of 31 kW with the house and radiators warm (about 28 kW output).

Continental boilers only seem to support flow temperatures up to 75°C, so I doubt I could use more than 25 kW output for the radiators with the house and radiators warm. Obviously you get more heat out of radiators when the house is cold and you need more power to get up to set flow temperature. I suspect a 32kW boiler would be adequate.

 

[Onetap]

There are also issues with the condensate drain; twin 22mm external with internal spec insulation into a soak away in chalk bedrock close to foundations.

I had never thought of that. I wonder how long it would take before we start getting subsidence problems due to condensate undermining chalk or limestone foundations? Will we be able to go potholing in caverns excavated by condensate?

I don't see the point of having a mixing station crammed into the boiler casing; I'd prefer a separate one, but then I have a few spare mixing valves lying around in my shed.

One of the valves was from the previous system, a WC system supplied from a Range Powermax (Silence, Corgi dogs!) boiler/thermal store. The control was done by an old BMS outstation. This wasn't user friendly, the program was impenetrable unless you were familiar with the configuration manual (lever arch file sized). I was and so could plug a laptop into it to monitor performance and tweak the programme. It eventually had WC control, flow temperature adjustment as required by the indoor sensor and optimum start/stop. It was the most comfortable heating system I've known, the rads were usually warm not hot, the flow temperature stayed at the setting required by the WC routine and the room temperature stayed at the set point. I did wreck one 3-port valve by making the 3-port valve over-active, wearing out the gland.

The Powermax boiler went a year ago, due to an extension requiring the flue to be re-routed. It was about 6 years old, still worked and had never broken down. It was a pig to clean though.

 

[ajrobb]

I had never thought of that. I wonder how long it would take before we start getting subsidence problems due to condensate undermining chalk or limestone foundations?

The 2002 Keston installation manual clearly states that a soak away chamber filled with limestone chippings must be sunk at least 600mm from foundations with drain holes facing away from foundations.

I have even seen one reference that claims acid condensate is bad for clay drains as well as metal.

 

[doitall]

Condensate will not harm drains including Cast iron, it it's connected into the wet system. e.g. not above the highest connection.

 

[Onetap]

Should I consider getting a magnaclean filter installed on my fragile dual Keston Celsius 25 system? If so, where? As it is primarily to protect the boilers, my initial thought is it should be in the low-loss header between the primary return and the boilers, maybe on the far right of the picture, next to the gas meter, so it is immediately before the boiler returns.

On the secondary return, where it will collect any sludge (corrosion products, mostly from the rads) before it gets into the primary system.

As discussed above, the secondary flow can be reduced without causing damage; reducing the primary flow could damage the heat exchangers. The filter will clog up in use and can completely obstruct the flow; I don't know if they have by-pass valves built-in. Maybe use one of DIA's AVDOs in parallel.

The secondary pump may already be undersized though and the filter will add some resistance, even when clean.

Got there in the end.