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MG TD TF 1500 - 5/8 inch tube & hose for heater lines

I'm trying to puzzle out the various types of pipes, tubes and hoses to use for my heater system.
A 5/8 ID pipe carries 50% more water (and therefore 50%
heat) than a 1/2" ID pipe.
(Pipes and hoses measure by ID, tubes by OD, I believe).
Seems like the bigger I can possibly keep the ID, the more heat the system will deliver.
A lot of users have used 1/2 pipe to carry the water to the heater, but it seems like more heat will be transferred if I can keep to 5/8 tubing - and thin wall tubing if possible.
Has anybody done this in 5/8 throughout? I would like to avoid restricting the ID size, if at all possible, and keep it as large as I can.
Thanks!
Geoffrey M Baker

Geoff, does it really that that cold in Arizona? In the UK some house central heating systems use a microbore system where the pipes are only about 8mm dia, they deliver the same heat around the house as a more traditional 15mm so I'm wondering if your thoughts on 50% more heat for the bigger pipes are valid?
Cliff Harvey

When I had my heater installed I got more than enough heat usyng 1/2 inch pipes and hose.

Don H
Don Harmer

Cliff, it's just math. the volume of a 5/8 pipe is 50% bigger than a 1/2 pipe. So theoretically, it will transport 50% more volume of liquid, and therefore more heat. This is ESPECIALLY true in an unpressurized system with a low flow rate; because in a pressurized system you can use high pressure pumps to pump material faster through smaller diameter bores (probably much like your "microbore" system). In an unpressurized system, the bigger your pipes the more efficient the transfer.
Secondly, I'm thinking of tapping not off the hotter front of the engine (the elbow beneath the thermostat) but from the rear of the engine (because it is already tapped), which is not as hot, so again, it will let me get the most out of what I already have.
And no, it's not really that cold in AZ; but we do have milder winters so we can keep our cars operational year round, so we can drive on cold clear days when people in New England have long since locked their cars into heated garages. So from November to March, I can drive around very comfortably - but using the heater.

Cheapest bang for the buck in a heater system on an MG? I believe it lies in using the largest heater pipes you can install - 5/8 inch. The pipe connections on the heater core are 5/8, so if I can avoid reducing the tube and pipe diameter throughout the system, I will maximize heat transfer.
If you go down to 1/2 pipes anywhere in the system, you are reducing the potential efficiency by a full one-third.
Geoffrey M Baker

The pipes in the back of my Arnolt heater are 1/2". Are other folks seeing 5/8"?
Kevin McLemore

Kevin, my heater hoses all say 5/8. I assumed that the heater fitting would be the same!
If it's only 1/2 then certainly 5/8 pipe would be unnecessary.
I'll know when I pull it out of the car, but that won't be for a while, I'm still working on the convertible top (2nd dye coat today, then waterproofer).
Geoffrey M Baker

Chatting to a plumber today I explained about the subject of this thread and he told me that the bigger pipe shouldn't make any difference, the heat remains the same in the pipe whatever the diameter, he suggested a 'hotter' thermostat to increase the heat going to the heater matrix.
Cliff Harvey

Geoff, you might want to take a look at Bernoulli's Principle. It's the cross-sectional area of the pipe in the heater that will determine the flow. Bud
Bud Krueger

Bud, if we assume that the heating system is a simple loop, then the narrowest part of the loop will determine the amount of flow. If the narrowest part of the system is the crossection of the heater pipe, then that will determine it. If the narrowest part is a smaller fitting elsewhere, then that will determine it. Much like a resistor in a circuit, the amount of energy transferred is determined by the point of most resistance in the loop... in the case of plumbing, most resistance = smallest diameter.
So if the internal pipe diameter in my heater is only half an inch, there's no point in going larger elsewhere. (As I said earlier, I don't know what that diameter is, all I know for sure right now is I have hoses with a 5/8 inch ID running to the heater.) But if the internal diameter in my heater pipe is 5/8, I should keep the rest of the system 5/8. But if it is only half an inch at the heater, I will just run half inch pipe.
Cliff, your plumber needs to think about physics some more :). He is correct in that the "heat remains the same" but the larger the pipe the greater the flow, and the greater flow of hot water in the same period of time equates to more heat DELIVERED from point A to point B. We talk about our systems being "unpressurized" and that's not strictly true; the water pump impeller puts pressure on the system, just not much. So the water is moving around at the rate of several gallons per minute, and if it moves through bigger pipes, more water is being transferred over the same time, and because the medium of water is used to transfer heat, therefore more heat is being transferred, as well. This will remain true until the size of the pipe exceeds the pump's capacity to move the water.
At least, this is what I remember of my physics classes...
Geoffrey M Baker

Geof,
In the real world, there will be no noticable difference between 1/2" and 5/8" hoses.
If the water going into the heater is say 190F, the water returning to the engine via the 1/2" hose may be 185F and the water leaving the 5/8" hose may be 186F.
It passes through with a lot of heat, but the heat capacity of air is so low, in comparison, it really carries a very small percentage off.
As a steamfiter, I see 2" heating piping fed through a 1/4" control valve orifice! Go figure.
If you tap off the back of the head, you'll have a more significant handicap than 1/2" vs 5/8" hose discussion.

By the way, I used the bypass fitting off the thermostat housing to feed our Arnolt heater and return back into the bypass branch on the pump suction side. Pretty simple.
JRN JIM

Wouldn't the determining factor here not be the diameter of the pipe but the efficiency of the heater to extract the heat from the fluid and transfer that heat into the ambient atmosphere? I wouldn't think that increasing the volume of hot fluid would have any impact on that heat transfer cycle?
Best regards
RodM
Rod Murray 54TF 3006

Glad to see this back up. Yesterday I had a look at the plumbing on my Arnolt heater and adapter. The i.d. of plumbing on both the heater and the adapter is slightly greater than 1/2", but less than 5/8". Bud
Bud Krueger

Meant to say that the o.d. of the plumbing is < 5/8". Bud
Bud Krueger

Bud, thanks for the info. When I get mine apart I'll measure it and then figure out if it's worth going to 5/8 for the slight increase in efficiency.
Rod, there are a million determining factors. How fast is the liquid moving? (A 6-vane impeller will presumably increase heat transfer by moving more water through the same pipes in the same time).
Certainly, the amount of square area of the radiating surface in the heater is important, but only because it gives a maximum theoretical amount of area for the heat transfer. Move hot water through it faster and it will deliver more heat (assuming ambient air temp is less than the water), until it reaches it's maximum ability to transfer heat. I very much doubt any current Arnolt heater is anywhere near its maximum heat transfer capacity, in any situation.
Suppose you install a pump that's twice as fast. It will not probably deliver twice the heat - but it will deliver more heat, so long as there is a difference between air temp and the water temp - and so long as the engine is generating more heat than the surrounding air temp.
Using a bigger pipe has the same effect - more water gets transferred, until the pipe is bigger than the pumps useful capacity to move the water.

When I get my heater off (and I'm at least one project away from that one anyway) then I'll figure out if it's worth it to go with bigger pipe.
Geoffrey M Baker

My heater piping is 5/8 OD, hoses 5/8 ID. PJ
Paul S Jennings

C'mon guys. The size of the hose doesn't determine the available heat, the engine and thermostat does. Larger pipe means that the same heat is dispersed over a larger volume of water, and the larger pipe means that the water is flowing slower than the most restrictive passage in the whole cooling/heating system.
Lew Palmer

This is NOT a simple problem.
I have been reading this thread for some days debating whether to chime in or not. Finally after two glasses of wine and pizza I am biting off a chunk.

If you do nothing else and the lager hose actually does increase the flow through the heather core. (Which I doubt it will) you can end up with less heat.
Do not confuse heat with temperature. The quantity of heat is usually called Q.
Starting from the radiator core, the amount of heat You get into the air, is the difference between the inlet water temperature (delta-T) and the flow of the water.
Thus if the water through the core is (say) 1 liter per minuet and the temperature difference is 10 degrees C between the inlet and outlet water, you have added (About) 500 calories to the air. (1 liter of water weighs 1000 gm, but a 50/50 mixture of antifreeze lowers the specific heat by about 50%).

Now if the pressure drop in the hose leading to the heater core is substantial, (I doubt this) then increasing the hose diameter will increase the pressure at the core forcing the fluid through the core faster.
If the water goes through the core faster it spends less time in the core and the delta-T may drop. This depends on the ambient temperature. If the difference in temperature between the hot water and the outside air is substantial, then delta-T is dependent on the airflow, the inlet temperature, the temperature of the air, the area of the core and the time the water spends in the core. Faster flow may lessen the time in the core resulting in a lower delta-T.

If the air temperature and the water temperature are closer then the higher exiting could cause a more efficient temperature exchange and the product of delta-T and the coolant flow might be constant or higher.

I would believe that the largest flow restriction would be in the heater core. In this case increasing the diameter of the inlet tubing would do little for the flow BUT it would give the inlet tubing a larger surface area. This larger area would loose more heat to the atmosphere and lower the temperature of the inlet coolant. This postulate would be most true if the core is the flow restriction. Then the coolant would spend the same amount of time in the larger inlet hose, which now has more surface area, causing it to cool off more.

If you need more heat the best way is to use a larger core.


Jim B.
JA Benjamin

It's easier to match the diameter of the heater shutoff valves on the cooling system, otherwise you will need step down hoses. My valves are 5/8ths, reason my piping is 5/8ths. Everything fits! PJ

Such as,


Paul S Jennings

This thread was discussed between 05/11/2014 and 13/11/2014

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