Radiator sizing calculators

[Flyfisher]

Btw, its also possible to pump from the wood stove if you get one suitable for sealed systems. There aren't too many of these available but means you can site the stove anywhere you want. This is what we had to do. Predictably the first stove man who came to look swore that where we wanted the stove was physically impossible and we never heard from him again...

It's also possible to have a pumped boiler stove in a vented system, which is what we have because the boiler stove and the thermal store are both on the ground floor with a fair distance between them.

Basically, the thermal store itself is vented although the gas boiler, DHW and CH circuits are all pressurised. The boiler stove is also vented and circulation to the store is pumped. However, because the boiler stove is regarded as an 'uncontrolled' source of heat the system must still operate safely in the event of a power cut stopping the pump. In our case, we installed some 'heat leak' rads in the bedroom above the boiler stove and these work purely on convection so the boiler stove can safely 'lose' heat to them in the even of a pump failure. The sizing calculation for these rads was 50% of the boiler stove capacity of 11kW, ie 5.5kW. These rads still get warm even when the pump is running, which keeps the bedroom warm enough that the TRV on the CH rad shuts down.

System control is simple but effective enough in practice. We have a flue thermostat on the boiler stove that switches on the pump at around 120C and another stat on the thermal store that switches it off if the store temp is too hot (80C I think) and the boiler stove itself has a passive thermostat that controls its own airflow. In practice, the thermal store doesn't reach 80C when the CH is running, which is usually is when we have the boiler stove alight. On the few occasions that we've had the stove alight without running the CH, the pump stops running when the store is at max temp and the boiler stove heat all goes to the bedroom rads.

There are some other subtleties but that's the basics. I originally had ideas about a more sophisticated control system but it all works well enough that I've never got around to changing the current configuration. Besides, future maintenance would likely be impossible if it became too sophisticated.

 

[Flyfisher]

All this complexification is well beyond my pay grade, but just a couple of points:

Larger rads = more metal and more water, more mass to heat. Responsiveness is not a strong point with iron rads, all that extra mass will not help.

True, but I'm not sure it's that significant compared to the inherent responsiveness (or lack of) of cast iron rads compared to modern steel ones. Plus, if the heating also includes wood stoves then they are the ideal way to deal with sudden cold snaps.

A minor point... arrange for an 'always on ' loop in the bathroom, meaning it will be on in all year when hot water is being produced. Take it to a towel rail with a TRV. That way you will always have nice warm, dry, towels.
Could also be useful in a laundry room, if you have one.

Good point, though I've often wondered if it would be more efficient to use a towel rail plumbed into the CH as a standard rad but with an electric heating element (on a timer) for when the CH is not running.

 

[DJH]

How times have changed...when I was a self employed plumber and heating engineer I used to sub-contract to a chap who calculated the radiator sizes required in a room by measuring the length of the windows and using a radiator the same length... :roll:

This was nearly forty years ago.

 

[Owain]

Usual way to deal with this is something called a secondary return. ... It can be wasteful in terms of energy use if the water isn't consumed immediately because the pipes will eventually cool down and the pump will run again to heat them up. Typically you would put it on a timer to just run at peak times of day.

Or you could put the secondary return pump near the bathroom (it doesn't have to be near the boiler) and run it on a PIR detector so when someone enters the bathroom the pump starts and by the time they're ready to wash their hands the water at the tap is hot.

15mm pipes and double or triple lagging them will help a lot though.

 

[overlander matt]

It's been a few years since I posted this question originally. I eventually installed a system with 2 thermal stores - one 400L store for UFH and hot water and a winter only 300L store for the cast iron radiators. The help I received from Fernicarry was invaluable and I greatly appreciate the time, effort and knowledge shared.

The system was designed with a view to adding a solar thermal collector at a later date to reduce the demand on the gas boiler. That date is fast approaching and I'm very interested to know of other forum members' experiences with solar thermal. Is it worth the £2k to provide hot water for 6-8 months of the year? I really don't like the appearance of any solar related product and finding a suitable location for the collector where it's not prominent has been a challenge. Do you just learn to live with it? I've reluctantly decided to put my concerns aside and go for it. It would be good to know how anyone else has got on with anything similar.

Thanks.

Matt

 

[fernicarry]

Hi Matt, hope the system is working well. What do you have in mind for solar thermal? I wonder if the economics favour PV instead of thermal these days? Panels getting cheaper all the time and electricity is more flexible.

 

[a twig]

I’m sure I saw a paper somewhere recently where for heating purposes solar thermal was still vastly more efficient. I’ll have a dig

I guess if you have the roof space, enough PV to run a heat source pump is the solution that provides the most comfort, but costs the most

 

[overlander matt]

Hi Matt, hope the system is working well. What do you have in mind for solar thermal? I wonder if the economics favour PV instead of thermal these days? Panels getting cheaper all the time and electricity is more flexible.

Thanks Alan,

The system has worked very well to date although there are still a few bits to complete... I need to hook up the woodburning stove/boiler for this winter (and get the room it's going in finished), there are two more radiator zones to connect up, a gap in the pipework awaits the arrival of the secondary DHW pump and finally the solar thermal!

It's been such a dilemma whether to go for the solar thermal or not. I might get away with just under £2k for a 40 tube collector, pump and controller. It's complicated because I'll have to dig a 15m trench to the collector location and it needs to pass under a couple of garden walls, a driveway and avoid some tree roots. Is it really worth it? I don't have a great place to site the collector - it'll just be on a small patch of unused land.

I was looking at an open loop system into the thermal store ie a pump circulates the water in the store directly to the collector rather than using a closed loop filled with glycol. This has been a matter of debate between companies offering both systems. It's still a dilemma!

A decision is due soon. I'm tempted to give it a try... I guess you've decided against this option on your system?

 

[overlander matt]

I’m sure I saw a paper somewhere recently where for heating purposes solar thermal was still vastly more efficient. I’ll have a dig

I guess if you have the roof space, enough PV to run a heat source pump is the solution that provides the most comfort, but costs the most

It would be interesting to hear something in solar thermal's favour. It seems to be a dying technology - it's expensive given the need for the thermal store but if you already have that then it's more attractive if you think you might get 8 months of the year without using the gas boiler.

 

[fernicarry]

I guess you've decided against this option on your system?

I haven't decided against, just been kept busy with other things. I may finally get my own HW secondary return hooked up soonish as I'm now refitting the front shower room and I've rediscovered the pipework under the floor. Some serious rot, verging on structural, that needs to be addressed first.

I suppose at 2k thats still a lot more economical than PV of a comparable power output. Not sure I'd fancy a 15m trench though....

Even if you did an open loop wouldn't you still need glycol in the collector pipework since its outdoors and you need to prevent freezing?

 

[overlander matt]

I haven't decided against, just been kept busy with other things. I may finally get my own HW secondary return hooked up soonish as I'm now refitting the front shower room and I've rediscovered the pipework under the floor. Some serious rot, verging on structural, that needs to be addressed first.

I suppose at 2k thats still a lot more economical than PV of a comparable power output. Not sure I'd fancy a 15m trench though....

Even if you did an open loop wouldn't you still need glycol in the collector pipework since its outdoors and you need to prevent freezing?

That is the important question... I'm told that the controller's frost protection setting does enough to stop this by just pumping enough water through the collector to keep the temperature high enough to prevent freezing. The insulation is apparently the critical element in the collector to stop the water from freezing. You have to worry about a power failure and other potentially catastrophic outcomes - these are questions still to be answered!

All the best with your ongoing work there. It never seems to end.

 

[fernicarry]

I'm told that the controller's frost protection setting does enough to stop this by just pumping enough water through the collector to keep the temperature high enough to prevent freezing

That's too big a risk to take in my mind. Every winter I carefully drain the hose before the weather gets cold, then I use it again for something and forget to drain it afterwards, some time later it freezes and the plastic spray thing on the end splits. Gone through at least 4 of them now....

Admittedly buried pipes are much less likely to freeze. Our water main once froze but that was because it was exposed for some length by excavations to fix the drains. I suppose you could close off that loop and drain it in the winter since its not going to be doing a whole lot in terms of heat production.

I believe the temperature fluctuations in a solar loop are quite extreme. In a closed loop the pressure change is taken up by the expansion vessel, but in an open loop you could be causing your store to overflow a little and refill on a regular basis. This will draw in a certain amount of fresh oxygenated water. Of course the store is all copper and should be dosed as well, but I think its something to avoid if possible.

I'd be treating the solar input as a nice to have add-on that doesn't impact on the running of the main system.

It never seems to end

Indeed it does not.

 

[overlander matt]

It is going to be a learning experience. I'll have to keep a close eye on the system when the temperatures drop and will probably drain down the collector for the winter as you suggest. I'll keep you posted!

 

[overlander matt]

As promised, an update on the solar thermal collector installation... I had a patch of land a short distance (20m) from the house that conveniently fell outside the conservation area and curtilage of the listed building where we decided to house the collector. In many ways the distance from the house is not ideal and the location is slightly more south western than southern but that's all we had to work with. My biggest concern was the look of the collectors - I'm really not a fan so took the approach to improve the look of the area as much as possible which has added to the work by repointing the rather shabby wall behind and building a small brick plinth to support the two collectors. With a bit more work and a few plants, the area should look a smarter and fingers crossed the sun will come out eventually and we'll get some hot water. Since connecting up and switching the system on we've had 10 days of poor weather. Generally the 400L thermal store heats to around 45deg each day which is just about marginal for getting an acceptable shower. To top up the store to 55deg requires a 10-15min blast from the gas boiler. It will be interesting to see how it goes on a proper summer's day. I've got to complete the insulation at the thermal store end and tweak the control settings but it's nearly there!

 

[fernicarry]

Very nice.

 

[overlander matt]

Very nice.

Time will tell if it proves to be a success! So far we are compromising on our hot water use but also questioning whether we really needed to run the gas boiler as much as we did previously. I suspect there will be an evening top up of the thermal store required in the cooler weather before there is any central heating requirement.

 

[overlander matt]

An update on the solar thermal now that the sun has returned... I was initially a little concerned that the 40 tube solar collector was either not big enough for the 400L thermal store or there was too much heat loss on the long pipe run (25-30m) between the store and collector. On a sunny day like today the store has heated up to 60+deg (it might have gone higher had I not had a default setting on the controller limiting the maximum temperature in the store to 60deg) which satisfies our typical hot water needs (2 showers and a bath). So fundamentally, the system works but there will no doubt be a lot of tweaking to optimise it over the coming months.

Hopefully this might prove useful to anybody contemplating the solar thermal option.